1655 lines
62 KiB
XML
1655 lines
62 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
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<book id="V4LGuide">
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<bookinfo>
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<title>Video4Linux Programming</title>
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<authorgroup>
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<author>
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<firstname>Alan</firstname>
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<surname>Cox</surname>
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<affiliation>
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<address>
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<email>alan@redhat.com</email>
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</address>
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</affiliation>
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</author>
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</authorgroup>
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<copyright>
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<year>2000</year>
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<holder>Alan Cox</holder>
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</copyright>
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<legalnotice>
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<para>
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This documentation is free software; you can redistribute
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it and/or modify it under the terms of the GNU General Public
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License as published by the Free Software Foundation; either
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version 2 of the License, or (at your option) any later
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version.
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</para>
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<para>
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This program is distributed in the hope that it will be
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useful, but WITHOUT ANY WARRANTY; without even the implied
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warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
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See the GNU General Public License for more details.
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</para>
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<para>
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You should have received a copy of the GNU General Public
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License along with this program; if not, write to the Free
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Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
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MA 02111-1307 USA
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</para>
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<para>
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For more details see the file COPYING in the source
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distribution of Linux.
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</para>
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</legalnotice>
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</bookinfo>
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<toc></toc>
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<chapter id="intro">
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<title>Introduction</title>
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<para>
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Parts of this document first appeared in Linux Magazine under a
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ninety day exclusivity.
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</para>
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<para>
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Video4Linux is intended to provide a common programming interface
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for the many TV and capture cards now on the market, as well as
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parallel port and USB video cameras. Radio, teletext decoders and
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vertical blanking data interfaces are also provided.
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</para>
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</chapter>
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<chapter id="radio">
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<title>Radio Devices</title>
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<para>
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There are a wide variety of radio interfaces available for PC's, and these
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are generally very simple to program. The biggest problem with supporting
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such devices is normally extracting documentation from the vendor.
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</para>
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<para>
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The radio interface supports a simple set of control ioctls standardised
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across all radio and tv interfaces. It does not support read or write, which
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are used for video streams. The reason radio cards do not allow you to read
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the audio stream into an application is that without exception they provide
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a connection on to a soundcard. Soundcards can be used to read the radio
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data just fine.
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</para>
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<sect1 id="registerradio">
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<title>Registering Radio Devices</title>
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<para>
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The Video4linux core provides an interface for registering devices. The
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first step in writing our radio card driver is to register it.
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</para>
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<programlisting>
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static struct video_device my_radio
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{
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"My radio",
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VID_TYPE_TUNER,
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radio_open.
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radio_close,
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NULL, /* no read */
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NULL, /* no write */
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NULL, /* no poll */
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radio_ioctl,
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NULL, /* no special init function */
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NULL /* no private data */
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};
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</programlisting>
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<para>
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This declares our video4linux device driver interface. The VID_TYPE_ value
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defines what kind of an interface we are, and defines basic capabilities.
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</para>
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<para>
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The only defined value relevant for a radio card is VID_TYPE_TUNER which
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indicates that the device can be tuned. Clearly our radio is going to have some
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way to change channel so it is tuneable.
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</para>
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<para>
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We declare an open and close routine, but we do not need read or write,
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which are used to read and write video data to or from the card itself. As
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we have no read or write there is no poll function.
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</para>
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<para>
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The private initialise function is run when the device is registered. In
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this driver we've already done all the work needed. The final pointer is a
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private data pointer that can be used by the device driver to attach and
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retrieve private data structures. We set this field "priv" to NULL for
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the moment.
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</para>
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<para>
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Having the structure defined is all very well but we now need to register it
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with the kernel.
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</para>
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<programlisting>
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static int io = 0x320;
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int __init myradio_init(struct video_init *v)
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{
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if(!request_region(io, MY_IO_SIZE, "myradio"))
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{
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printk(KERN_ERR
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"myradio: port 0x%03X is in use.\n", io);
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return -EBUSY;
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}
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if(video_device_register(&my_radio, VFL_TYPE_RADIO)==-1) {
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release_region(io, MY_IO_SIZE);
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return -EINVAL;
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}
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return 0;
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}
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</programlisting>
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<para>
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The first stage of the initialisation, as is normally the case, is to check
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that the I/O space we are about to fiddle with doesn't belong to some other
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driver. If it is we leave well alone. If the user gives the address of the
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wrong device then we will spot this. These policies will generally avoid
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crashing the machine.
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</para>
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<para>
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Now we ask the Video4Linux layer to register the device for us. We hand it
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our carefully designed video_device structure and also tell it which group
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of devices we want it registered with. In this case VFL_TYPE_RADIO.
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</para>
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<para>
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The types available are
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</para>
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<table frame="all" id="Device_Types"><title>Device Types</title>
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<tgroup cols="3" align="left">
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<tbody>
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<row>
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<entry>VFL_TYPE_RADIO</entry><entry>/dev/radio{n}</entry><entry>
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Radio devices are assigned in this block. As with all of these
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selections the actual number assignment is done by the video layer
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accordijng to what is free.</entry>
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</row><row>
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<entry>VFL_TYPE_GRABBER</entry><entry>/dev/video{n}</entry><entry>
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Video capture devices and also -- counter-intuitively for the name --
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hardware video playback devices such as MPEG2 cards.</entry>
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</row><row>
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<entry>VFL_TYPE_VBI</entry><entry>/dev/vbi{n}</entry><entry>
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The VBI devices capture the hidden lines on a television picture
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that carry further information like closed caption data, teletext
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(primarily in Europe) and now Intercast and the ATVEC internet
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television encodings.</entry>
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</row><row>
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<entry>VFL_TYPE_VTX</entry><entry>/dev/vtx[n}</entry><entry>
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VTX is 'Videotext' also known as 'Teletext'. This is a system for
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sending numbered, 40x25, mostly textual page images over the hidden
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lines. Unlike the /dev/vbi interfaces, this is for 'smart' decoder
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chips. (The use of the word smart here has to be taken in context,
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the smartest teletext chips are fairly dumb pieces of technology).
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</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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We are most definitely a radio.
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</para>
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<para>
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Finally we allocate our I/O space so that nobody treads on us and return 0
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to signify general happiness with the state of the universe.
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</para>
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</sect1>
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<sect1 id="openradio">
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<title>Opening And Closing The Radio</title>
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<para>
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The functions we declared in our video_device are mostly very simple.
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Firstly we can drop in what is basically standard code for open and close.
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</para>
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<programlisting>
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static int users = 0;
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static int radio_open(struct video_device *dev, int flags)
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{
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if(users)
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return -EBUSY;
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users++;
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return 0;
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}
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</programlisting>
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<para>
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At open time we need to do nothing but check if someone else is also using
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the radio card. If nobody is using it we make a note that we are using it,
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then we ensure that nobody unloads our driver on us.
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</para>
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<programlisting>
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static int radio_close(struct video_device *dev)
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{
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users--;
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}
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</programlisting>
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<para>
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At close time we simply need to reduce the user count and allow the module
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to become unloadable.
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</para>
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<para>
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If you are sharp you will have noticed neither the open nor the close
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routines attempt to reset or change the radio settings. This is intentional.
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It allows an application to set up the radio and exit. It avoids a user
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having to leave an application running all the time just to listen to the
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radio.
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</para>
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</sect1>
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<sect1 id="ioctlradio">
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<title>The Ioctl Interface</title>
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<para>
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This leaves the ioctl routine, without which the driver will not be
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terribly useful to anyone.
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</para>
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<programlisting>
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static int radio_ioctl(struct video_device *dev, unsigned int cmd, void *arg)
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{
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switch(cmd)
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{
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case VIDIOCGCAP:
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{
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struct video_capability v;
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v.type = VID_TYPE_TUNER;
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v.channels = 1;
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v.audios = 1;
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v.maxwidth = 0;
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v.minwidth = 0;
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v.maxheight = 0;
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v.minheight = 0;
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strcpy(v.name, "My Radio");
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if(copy_to_user(arg, &v, sizeof(v)))
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return -EFAULT;
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return 0;
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}
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</programlisting>
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<para>
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VIDIOCGCAP is the first ioctl all video4linux devices must support. It
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allows the applications to find out what sort of a card they have found and
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to figure out what they want to do about it. The fields in the structure are
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</para>
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<table frame="all" id="video_capability_fields"><title>struct video_capability fields</title>
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<tgroup cols="2" align="left">
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<tbody>
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<row>
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<entry>name</entry><entry>The device text name. This is intended for the user.</entry>
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</row><row>
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<entry>channels</entry><entry>The number of different channels you can tune on
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this card. It could even by zero for a card that has
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no tuning capability. For our simple FM radio it is 1.
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An AM/FM radio would report 2.</entry>
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</row><row>
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<entry>audios</entry><entry>The number of audio inputs on this device. For our
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radio there is only one audio input.</entry>
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</row><row>
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<entry>minwidth,minheight</entry><entry>The smallest size the card is capable of capturing
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images in. We set these to zero. Radios do not
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capture pictures</entry>
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</row><row>
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<entry>maxwidth,maxheight</entry><entry>The largest image size the card is capable of
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capturing. For our radio we report 0.
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</entry>
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</row><row>
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<entry>type</entry><entry>This reports the capabilities of the device, and
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matches the field we filled in in the struct
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video_device when registering.</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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Having filled in the fields, we use copy_to_user to copy the structure into
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the users buffer. If the copy fails we return an EFAULT to the application
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so that it knows it tried to feed us garbage.
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</para>
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<para>
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The next pair of ioctl operations select which tuner is to be used and let
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the application find the tuner properties. We have only a single FM band
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tuner in our example device.
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</para>
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<programlisting>
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case VIDIOCGTUNER:
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{
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struct video_tuner v;
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if(copy_from_user(&v, arg, sizeof(v))!=0)
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return -EFAULT;
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if(v.tuner)
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return -EINVAL;
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v.rangelow=(87*16000);
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v.rangehigh=(108*16000);
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v.flags = VIDEO_TUNER_LOW;
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v.mode = VIDEO_MODE_AUTO;
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v.signal = 0xFFFF;
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strcpy(v.name, "FM");
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if(copy_to_user(&v, arg, sizeof(v))!=0)
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return -EFAULT;
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return 0;
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}
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|
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</programlisting>
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<para>
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The VIDIOCGTUNER ioctl allows applications to query a tuner. The application
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sets the tuner field to the tuner number it wishes to query. The query does
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not change the tuner that is being used, it merely enquires about the tuner
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in question.
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</para>
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<para>
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We have exactly one tuner so after copying the user buffer to our temporary
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structure we complain if they asked for a tuner other than tuner 0.
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</para>
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<para>
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The video_tuner structure has the following fields
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</para>
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<table frame="all" id="video_tuner_fields"><title>struct video_tuner fields</title>
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<tgroup cols="2" align="left">
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<tbody>
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<row>
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<entry>int tuner</entry><entry>The number of the tuner in question</entry>
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</row><row>
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<entry>char name[32]</entry><entry>A text description of this tuner. "FM" will do fine.
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This is intended for the application.</entry>
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</row><row>
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<entry>u32 flags</entry>
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<entry>Tuner capability flags</entry>
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</row>
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<row>
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<entry>u16 mode</entry><entry>The current reception mode</entry>
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</row><row>
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<entry>u16 signal</entry><entry>The signal strength scaled between 0 and 65535. If
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a device cannot tell the signal strength it should
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report 65535. Many simple cards contain only a
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signal/no signal bit. Such cards will report either
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0 or 65535.</entry>
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</row><row>
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<entry>u32 rangelow, rangehigh</entry><entry>
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The range of frequencies supported by the radio
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or TV. It is scaled according to the VIDEO_TUNER_LOW
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flag.</entry>
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</row>
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</tbody>
|
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</tgroup>
|
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</table>
|
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|
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<table frame="all" id="video_tuner_flags"><title>struct video_tuner flags</title>
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<tgroup cols="2" align="left">
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<tbody>
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<row>
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<entry>VIDEO_TUNER_PAL</entry><entry>A PAL TV tuner</entry>
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</row><row>
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<entry>VIDEO_TUNER_NTSC</entry><entry>An NTSC (US) TV tuner</entry>
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</row><row>
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<entry>VIDEO_TUNER_SECAM</entry><entry>A SECAM (French) TV tuner</entry>
|
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</row><row>
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<entry>VIDEO_TUNER_LOW</entry><entry>
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The tuner frequency is scaled in 1/16th of a KHz
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steps. If not it is in 1/16th of a MHz steps
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</entry>
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</row><row>
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<entry>VIDEO_TUNER_NORM</entry><entry>The tuner can set its format</entry>
|
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</row><row>
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<entry>VIDEO_TUNER_STEREO_ON</entry><entry>The tuner is currently receiving a stereo signal</entry>
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</row>
|
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</tbody>
|
|
</tgroup>
|
|
</table>
|
|
|
|
<table frame="all" id="video_tuner_modes"><title>struct video_tuner modes</title>
|
|
<tgroup cols="2" align="left">
|
|
<tbody>
|
|
<row>
|
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<entry>VIDEO_MODE_PAL</entry><entry>PAL Format</entry>
|
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</row><row>
|
|
<entry>VIDEO_MODE_NTSC</entry><entry>NTSC Format (USA)</entry>
|
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</row><row>
|
|
<entry>VIDEO_MODE_SECAM</entry><entry>French Format</entry>
|
|
</row><row>
|
|
<entry>VIDEO_MODE_AUTO</entry><entry>A device that does not need to do
|
|
TV format switching</entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</table>
|
|
<para>
|
|
The settings for the radio card are thus fairly simple. We report that we
|
|
are a tuner called "FM" for FM radio. In order to get the best tuning
|
|
resolution we report VIDEO_TUNER_LOW and select tuning to 1/16th of KHz. Its
|
|
unlikely our card can do that resolution but it is a fair bet the card can
|
|
do better than 1/16th of a MHz. VIDEO_TUNER_LOW is appropriate to almost all
|
|
radio usage.
|
|
</para>
|
|
<para>
|
|
We report that the tuner automatically handles deciding what format it is
|
|
receiving - true enough as it only handles FM radio. Our example card is
|
|
also incapable of detecting stereo or signal strengths so it reports a
|
|
strength of 0xFFFF (maximum) and no stereo detected.
|
|
</para>
|
|
<para>
|
|
To finish off we set the range that can be tuned to be 87-108Mhz, the normal
|
|
FM broadcast radio range. It is important to find out what the card is
|
|
actually capable of tuning. It is easy enough to simply use the FM broadcast
|
|
range. Unfortunately if you do this you will discover the FM broadcast
|
|
ranges in the USA, Europe and Japan are all subtly different and some users
|
|
cannot receive all the stations they wish.
|
|
</para>
|
|
<para>
|
|
The application also needs to be able to set the tuner it wishes to use. In
|
|
our case, with a single tuner this is rather simple to arrange.
|
|
</para>
|
|
<programlisting>
|
|
|
|
case VIDIOCSTUNER:
|
|
{
|
|
struct video_tuner v;
|
|
if(copy_from_user(&v, arg, sizeof(v)))
|
|
return -EFAULT;
|
|
if(v.tuner != 0)
|
|
return -EINVAL;
|
|
return 0;
|
|
}
|
|
|
|
</programlisting>
|
|
<para>
|
|
We copy the user supplied structure into kernel memory so we can examine it.
|
|
If the user has selected a tuner other than zero we reject the request. If
|
|
they wanted tuner 0 then, surprisingly enough, that is the current tuner already.
|
|
</para>
|
|
<para>
|
|
The next two ioctls we need to provide are to get and set the frequency of
|
|
the radio. These both use an unsigned long argument which is the frequency.
|
|
The scale of the frequency depends on the VIDEO_TUNER_LOW flag as I
|
|
mentioned earlier on. Since we have VIDEO_TUNER_LOW set this will be in
|
|
1/16ths of a KHz.
|
|
</para>
|
|
<programlisting>
|
|
|
|
static unsigned long current_freq;
|
|
|
|
|
|
|
|
case VIDIOCGFREQ:
|
|
if(copy_to_user(arg, &current_freq,
|
|
sizeof(unsigned long))
|
|
return -EFAULT;
|
|
return 0;
|
|
|
|
</programlisting>
|
|
<para>
|
|
Querying the frequency in our case is relatively simple. Our radio card is
|
|
too dumb to let us query the signal strength so we remember our setting if
|
|
we know it. All we have to do is copy it to the user.
|
|
</para>
|
|
<programlisting>
|
|
|
|
|
|
case VIDIOCSFREQ:
|
|
{
|
|
u32 freq;
|
|
if(copy_from_user(arg, &freq,
|
|
sizeof(unsigned long))!=0)
|
|
return -EFAULT;
|
|
if(hardware_set_freq(freq)<0)
|
|
return -EINVAL;
|
|
current_freq = freq;
|
|
return 0;
|
|
}
|
|
|
|
</programlisting>
|
|
<para>
|
|
Setting the frequency is a little more complex. We begin by copying the
|
|
desired frequency into kernel space. Next we call a hardware specific routine
|
|
to set the radio up. This might be as simple as some scaling and a few
|
|
writes to an I/O port. For most radio cards it turns out a good deal more
|
|
complicated and may involve programming things like a phase locked loop on
|
|
the card. This is what documentation is for.
|
|
</para>
|
|
<para>
|
|
The final set of operations we need to provide for our radio are the
|
|
volume controls. Not all radio cards can even do volume control. After all
|
|
there is a perfectly good volume control on the sound card. We will assume
|
|
our radio card has a simple 4 step volume control.
|
|
</para>
|
|
<para>
|
|
There are two ioctls with audio we need to support
|
|
</para>
|
|
<programlisting>
|
|
|
|
static int current_volume=0;
|
|
|
|
case VIDIOCGAUDIO:
|
|
{
|
|
struct video_audio v;
|
|
if(copy_from_user(&v, arg, sizeof(v)))
|
|
return -EFAULT;
|
|
if(v.audio != 0)
|
|
return -EINVAL;
|
|
v.volume = 16384*current_volume;
|
|
v.step = 16384;
|
|
strcpy(v.name, "Radio");
|
|
v.mode = VIDEO_SOUND_MONO;
|
|
v.balance = 0;
|
|
v.base = 0;
|
|
v.treble = 0;
|
|
|
|
if(copy_to_user(arg. &v, sizeof(v)))
|
|
return -EFAULT;
|
|
return 0;
|
|
}
|
|
|
|
</programlisting>
|
|
<para>
|
|
Much like the tuner we start by copying the user structure into kernel
|
|
space. Again we check if the user has asked for a valid audio input. We have
|
|
only input 0 and we punt if they ask for another input.
|
|
</para>
|
|
<para>
|
|
Then we fill in the video_audio structure. This has the following format
|
|
</para>
|
|
<table frame="all" id="video_audio_fields"><title>struct video_audio fields</title>
|
|
<tgroup cols="2" align="left">
|
|
<tbody>
|
|
<row>
|
|
<entry>audio</entry><entry>The input the user wishes to query</entry>
|
|
</row><row>
|
|
<entry>volume</entry><entry>The volume setting on a scale of 0-65535</entry>
|
|
</row><row>
|
|
<entry>base</entry><entry>The base level on a scale of 0-65535</entry>
|
|
</row><row>
|
|
<entry>treble</entry><entry>The treble level on a scale of 0-65535</entry>
|
|
</row><row>
|
|
<entry>flags</entry><entry>The features this audio device supports
|
|
</entry>
|
|
</row><row>
|
|
<entry>name</entry><entry>A text name to display to the user. We picked
|
|
"Radio" as it explains things quite nicely.</entry>
|
|
</row><row>
|
|
<entry>mode</entry><entry>The current reception mode for the audio
|
|
|
|
We report MONO because our card is too stupid to know if it is in
|
|
mono or stereo.
|
|
</entry>
|
|
</row><row>
|
|
<entry>balance</entry><entry>The stereo balance on a scale of 0-65535, 32768 is
|
|
middle.</entry>
|
|
</row><row>
|
|
<entry>step</entry><entry>The step by which the volume control jumps. This is
|
|
used to help make it easy for applications to set
|
|
slider behaviour.</entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</table>
|
|
|
|
<table frame="all" id="video_audio_flags"><title>struct video_audio flags</title>
|
|
<tgroup cols="2" align="left">
|
|
<tbody>
|
|
<row>
|
|
<entry>VIDEO_AUDIO_MUTE</entry><entry>The audio is currently muted. We
|
|
could fake this in our driver but we
|
|
choose not to bother.</entry>
|
|
</row><row>
|
|
<entry>VIDEO_AUDIO_MUTABLE</entry><entry>The input has a mute option</entry>
|
|
</row><row>
|
|
<entry>VIDEO_AUDIO_TREBLE</entry><entry>The input has a treble control</entry>
|
|
</row><row>
|
|
<entry>VIDEO_AUDIO_BASS</entry><entry>The input has a base control</entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</table>
|
|
|
|
<table frame="all" id="video_audio_modes"><title>struct video_audio modes</title>
|
|
<tgroup cols="2" align="left">
|
|
<tbody>
|
|
<row>
|
|
<entry>VIDEO_SOUND_MONO</entry><entry>Mono sound</entry>
|
|
</row><row>
|
|
<entry>VIDEO_SOUND_STEREO</entry><entry>Stereo sound</entry>
|
|
</row><row>
|
|
<entry>VIDEO_SOUND_LANG1</entry><entry>Alternative language 1 (TV specific)</entry>
|
|
</row><row>
|
|
<entry>VIDEO_SOUND_LANG2</entry><entry>Alternative language 2 (TV specific)</entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</table>
|
|
<para>
|
|
Having filled in the structure we copy it back to user space.
|
|
</para>
|
|
<para>
|
|
The VIDIOCSAUDIO ioctl allows the user to set the audio parameters in the
|
|
video_audio structure. The driver does its best to honour the request.
|
|
</para>
|
|
<programlisting>
|
|
|
|
case VIDIOCSAUDIO:
|
|
{
|
|
struct video_audio v;
|
|
if(copy_from_user(&v, arg, sizeof(v)))
|
|
return -EFAULT;
|
|
if(v.audio)
|
|
return -EINVAL;
|
|
current_volume = v/16384;
|
|
hardware_set_volume(current_volume);
|
|
return 0;
|
|
}
|
|
|
|
</programlisting>
|
|
<para>
|
|
In our case there is very little that the user can set. The volume is
|
|
basically the limit. Note that we could pretend to have a mute feature
|
|
by rewriting this to
|
|
</para>
|
|
<programlisting>
|
|
|
|
case VIDIOCSAUDIO:
|
|
{
|
|
struct video_audio v;
|
|
if(copy_from_user(&v, arg, sizeof(v)))
|
|
return -EFAULT;
|
|
if(v.audio)
|
|
return -EINVAL;
|
|
current_volume = v/16384;
|
|
if(v.flags&VIDEO_AUDIO_MUTE)
|
|
hardware_set_volume(0);
|
|
else
|
|
hardware_set_volume(current_volume);
|
|
current_muted = v.flags &
|
|
VIDEO_AUDIO_MUTE;
|
|
return 0;
|
|
}
|
|
|
|
</programlisting>
|
|
<para>
|
|
This with the corresponding changes to the VIDIOCGAUDIO code to report the
|
|
state of the mute flag we save and to report the card has a mute function,
|
|
will allow applications to use a mute facility with this card. It is
|
|
questionable whether this is a good idea however. User applications can already
|
|
fake this themselves and kernel space is precious.
|
|
</para>
|
|
<para>
|
|
We now have a working radio ioctl handler. So we just wrap up the function
|
|
</para>
|
|
<programlisting>
|
|
|
|
|
|
}
|
|
return -ENOIOCTLCMD;
|
|
}
|
|
|
|
</programlisting>
|
|
<para>
|
|
and pass the Video4Linux layer back an error so that it knows we did not
|
|
understand the request we got passed.
|
|
</para>
|
|
</sect1>
|
|
<sect1 id="modradio">
|
|
<title>Module Wrapper</title>
|
|
<para>
|
|
Finally we add in the usual module wrapping and the driver is done.
|
|
</para>
|
|
<programlisting>
|
|
|
|
#ifndef MODULE
|
|
|
|
static int io = 0x300;
|
|
|
|
#else
|
|
|
|
static int io = -1;
|
|
|
|
#endif
|
|
|
|
MODULE_AUTHOR("Alan Cox");
|
|
MODULE_DESCRIPTION("A driver for an imaginary radio card.");
|
|
module_param(io, int, 0444);
|
|
MODULE_PARM_DESC(io, "I/O address of the card.");
|
|
|
|
static int __init init(void)
|
|
{
|
|
if(io==-1)
|
|
{
|
|
printk(KERN_ERR
|
|
"You must set an I/O address with io=0x???\n");
|
|
return -EINVAL;
|
|
}
|
|
return myradio_init(NULL);
|
|
}
|
|
|
|
static void __exit cleanup(void)
|
|
{
|
|
video_unregister_device(&my_radio);
|
|
release_region(io, MY_IO_SIZE);
|
|
}
|
|
|
|
module_init(init);
|
|
module_exit(cleanup);
|
|
|
|
</programlisting>
|
|
<para>
|
|
In this example we set the IO base by default if the driver is compiled into
|
|
the kernel: you can still set it using "my_radio.irq" if this file is called <filename>my_radio.c</filename>. For the module we require the
|
|
user sets the parameter. We set io to a nonsense port (-1) so that we can
|
|
tell if the user supplied an io parameter or not.
|
|
</para>
|
|
<para>
|
|
We use MODULE_ defines to give an author for the card driver and a
|
|
description. We also use them to declare that io is an integer and it is the
|
|
address of the card, and can be read by anyone from sysfs.
|
|
</para>
|
|
<para>
|
|
The clean-up routine unregisters the video_device we registered, and frees
|
|
up the I/O space. Note that the unregister takes the actual video_device
|
|
structure as its argument. Unlike the file operations structure which can be
|
|
shared by all instances of a device a video_device structure as an actual
|
|
instance of the device. If you are registering multiple radio devices you
|
|
need to fill in one structure per device (most likely by setting up a
|
|
template and copying it to each of the actual device structures).
|
|
</para>
|
|
</sect1>
|
|
</chapter>
|
|
<chapter id="Video_Capture_Devices">
|
|
<title>Video Capture Devices</title>
|
|
<sect1 id="introvid">
|
|
<title>Video Capture Device Types</title>
|
|
<para>
|
|
The video capture devices share the same interfaces as radio devices. In
|
|
order to explain the video capture interface I will use the example of a
|
|
camera that has no tuners or audio input. This keeps the example relatively
|
|
clean. To get both combine the two driver examples.
|
|
</para>
|
|
<para>
|
|
Video capture devices divide into four categories. A little technology
|
|
backgrounder. Full motion video even at television resolution (which is
|
|
actually fairly low) is pretty resource-intensive. You are continually
|
|
passing megabytes of data every second from the capture card to the display.
|
|
several alternative approaches have emerged because copying this through the
|
|
processor and the user program is a particularly bad idea .
|
|
</para>
|
|
<para>
|
|
The first is to add the television image onto the video output directly.
|
|
This is also how some 3D cards work. These basic cards can generally drop the
|
|
video into any chosen rectangle of the display. Cards like this, which
|
|
include most mpeg1 cards that used the feature connector, aren't very
|
|
friendly in a windowing environment. They don't understand windows or
|
|
clipping. The video window is always on the top of the display.
|
|
</para>
|
|
<para>
|
|
Chroma keying is a technique used by cards to get around this. It is an old
|
|
television mixing trick where you mark all the areas you wish to replace
|
|
with a single clear colour that isn't used in the image - TV people use an
|
|
incredibly bright blue while computing people often use a particularly
|
|
virulent purple. Bright blue occurs on the desktop. Anyone with virulent
|
|
purple windows has another problem besides their TV overlay.
|
|
</para>
|
|
<para>
|
|
The third approach is to copy the data from the capture card to the video
|
|
card, but to do it directly across the PCI bus. This relieves the processor
|
|
from doing the work but does require some smartness on the part of the video
|
|
capture chip, as well as a suitable video card. Programming this kind of
|
|
card and more so debugging it can be extremely tricky. There are some quite
|
|
complicated interactions with the display and you may also have to cope with
|
|
various chipset bugs that show up when PCI cards start talking to each
|
|
other.
|
|
</para>
|
|
<para>
|
|
To keep our example fairly simple we will assume a card that supports
|
|
overlaying a flat rectangular image onto the frame buffer output, and which
|
|
can also capture stuff into processor memory.
|
|
</para>
|
|
</sect1>
|
|
<sect1 id="regvid">
|
|
<title>Registering Video Capture Devices</title>
|
|
<para>
|
|
This time we need to add more functions for our camera device.
|
|
</para>
|
|
<programlisting>
|
|
static struct video_device my_camera
|
|
{
|
|
"My Camera",
|
|
VID_TYPE_OVERLAY|VID_TYPE_SCALES|\
|
|
VID_TYPE_CAPTURE|VID_TYPE_CHROMAKEY,
|
|
camera_open.
|
|
camera_close,
|
|
camera_read, /* no read */
|
|
NULL, /* no write */
|
|
camera_poll, /* no poll */
|
|
camera_ioctl,
|
|
NULL, /* no special init function */
|
|
NULL /* no private data */
|
|
};
|
|
</programlisting>
|
|
<para>
|
|
We need a read() function which is used for capturing data from
|
|
the card, and we need a poll function so that a driver can wait for the next
|
|
frame to be captured.
|
|
</para>
|
|
<para>
|
|
We use the extra video capability flags that did not apply to the
|
|
radio interface. The video related flags are
|
|
</para>
|
|
<table frame="all" id="Capture_Capabilities"><title>Capture Capabilities</title>
|
|
<tgroup cols="2" align="left">
|
|
<tbody>
|
|
<row>
|
|
<entry>VID_TYPE_CAPTURE</entry><entry>We support image capture</entry>
|
|
</row><row>
|
|
<entry>VID_TYPE_TELETEXT</entry><entry>A teletext capture device (vbi{n])</entry>
|
|
</row><row>
|
|
<entry>VID_TYPE_OVERLAY</entry><entry>The image can be directly overlaid onto the
|
|
frame buffer</entry>
|
|
</row><row>
|
|
<entry>VID_TYPE_CHROMAKEY</entry><entry>Chromakey can be used to select which parts
|
|
of the image to display</entry>
|
|
</row><row>
|
|
<entry>VID_TYPE_CLIPPING</entry><entry>It is possible to give the board a list of
|
|
rectangles to draw around. </entry>
|
|
</row><row>
|
|
<entry>VID_TYPE_FRAMERAM</entry><entry>The video capture goes into the video memory
|
|
and actually changes it. Applications need
|
|
to know this so they can clean up after the
|
|
card</entry>
|
|
</row><row>
|
|
<entry>VID_TYPE_SCALES</entry><entry>The image can be scaled to various sizes,
|
|
rather than being a single fixed size.</entry>
|
|
</row><row>
|
|
<entry>VID_TYPE_MONOCHROME</entry><entry>The capture will be monochrome. This isn't a
|
|
complete answer to the question since a mono
|
|
camera on a colour capture card will still
|
|
produce mono output.</entry>
|
|
</row><row>
|
|
<entry>VID_TYPE_SUBCAPTURE</entry><entry>The card allows only part of its field of
|
|
view to be captured. This enables
|
|
applications to avoid copying all of a large
|
|
image into memory when only some section is
|
|
relevant.</entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</table>
|
|
<para>
|
|
We set VID_TYPE_CAPTURE so that we are seen as a capture card,
|
|
VID_TYPE_CHROMAKEY so the application knows it is time to draw in virulent
|
|
purple, and VID_TYPE_SCALES because we can be resized.
|
|
</para>
|
|
<para>
|
|
Our setup is fairly similar. This time we also want an interrupt line
|
|
for the 'frame captured' signal. Not all cards have this so some of them
|
|
cannot handle poll().
|
|
</para>
|
|
<programlisting>
|
|
|
|
|
|
static int io = 0x320;
|
|
static int irq = 11;
|
|
|
|
int __init mycamera_init(struct video_init *v)
|
|
{
|
|
if(!request_region(io, MY_IO_SIZE, "mycamera"))
|
|
{
|
|
printk(KERN_ERR
|
|
"mycamera: port 0x%03X is in use.\n", io);
|
|
return -EBUSY;
|
|
}
|
|
|
|
if(video_device_register(&my_camera,
|
|
VFL_TYPE_GRABBER)==-1) {
|
|
release_region(io, MY_IO_SIZE);
|
|
return -EINVAL;
|
|
}
|
|
return 0;
|
|
}
|
|
|
|
</programlisting>
|
|
<para>
|
|
This is little changed from the needs of the radio card. We specify
|
|
VFL_TYPE_GRABBER this time as we want to be allocated a /dev/video name.
|
|
</para>
|
|
</sect1>
|
|
<sect1 id="opvid">
|
|
<title>Opening And Closing The Capture Device</title>
|
|
<programlisting>
|
|
|
|
|
|
static int users = 0;
|
|
|
|
static int camera_open(struct video_device *dev, int flags)
|
|
{
|
|
if(users)
|
|
return -EBUSY;
|
|
if(request_irq(irq, camera_irq, 0, "camera", dev)<0)
|
|
return -EBUSY;
|
|
users++;
|
|
return 0;
|
|
}
|
|
|
|
|
|
static int camera_close(struct video_device *dev)
|
|
{
|
|
users--;
|
|
free_irq(irq, dev);
|
|
}
|
|
</programlisting>
|
|
<para>
|
|
The open and close routines are also quite similar. The only real change is
|
|
that we now request an interrupt for the camera device interrupt line. If we
|
|
cannot get the interrupt we report EBUSY to the application and give up.
|
|
</para>
|
|
</sect1>
|
|
<sect1 id="irqvid">
|
|
<title>Interrupt Handling</title>
|
|
<para>
|
|
Our example handler is for an ISA bus device. If it was PCI you would be
|
|
able to share the interrupt and would have set IRQF_SHARED to indicate a
|
|
shared IRQ. We pass the device pointer as the interrupt routine argument. We
|
|
don't need to since we only support one card but doing this will make it
|
|
easier to upgrade the driver for multiple devices in the future.
|
|
</para>
|
|
<para>
|
|
Our interrupt routine needs to do little if we assume the card can simply
|
|
queue one frame to be read after it captures it.
|
|
</para>
|
|
<programlisting>
|
|
|
|
|
|
static struct wait_queue *capture_wait;
|
|
static int capture_ready = 0;
|
|
|
|
static void camera_irq(int irq, void *dev_id,
|
|
struct pt_regs *regs)
|
|
{
|
|
capture_ready=1;
|
|
wake_up_interruptible(&capture_wait);
|
|
}
|
|
</programlisting>
|
|
<para>
|
|
The interrupt handler is nice and simple for this card as we are assuming
|
|
the card is buffering the frame for us. This means we have little to do but
|
|
wake up anybody interested. We also set a capture_ready flag, as we may
|
|
capture a frame before an application needs it. In this case we need to know
|
|
that a frame is ready. If we had to collect the frame on the interrupt life
|
|
would be more complex.
|
|
</para>
|
|
<para>
|
|
The two new routines we need to supply are camera_read which returns a
|
|
frame, and camera_poll which waits for a frame to become ready.
|
|
</para>
|
|
<programlisting>
|
|
|
|
|
|
static int camera_poll(struct video_device *dev,
|
|
struct file *file, struct poll_table *wait)
|
|
{
|
|
poll_wait(file, &capture_wait, wait);
|
|
if(capture_read)
|
|
return POLLIN|POLLRDNORM;
|
|
return 0;
|
|
}
|
|
|
|
</programlisting>
|
|
<para>
|
|
Our wait queue for polling is the capture_wait queue. This will cause the
|
|
task to be woken up by our camera_irq routine. We check capture_read to see
|
|
if there is an image present and if so report that it is readable.
|
|
</para>
|
|
</sect1>
|
|
<sect1 id="rdvid">
|
|
<title>Reading The Video Image</title>
|
|
<programlisting>
|
|
|
|
|
|
static long camera_read(struct video_device *dev, char *buf,
|
|
unsigned long count)
|
|
{
|
|
struct wait_queue wait = { current, NULL };
|
|
u8 *ptr;
|
|
int len;
|
|
int i;
|
|
|
|
add_wait_queue(&capture_wait, &wait);
|
|
|
|
while(!capture_ready)
|
|
{
|
|
if(file->flags&O_NDELAY)
|
|
{
|
|
remove_wait_queue(&capture_wait, &wait);
|
|
current->state = TASK_RUNNING;
|
|
return -EWOULDBLOCK;
|
|
}
|
|
if(signal_pending(current))
|
|
{
|
|
remove_wait_queue(&capture_wait, &wait);
|
|
current->state = TASK_RUNNING;
|
|
return -ERESTARTSYS;
|
|
}
|
|
schedule();
|
|
current->state = TASK_INTERRUPTIBLE;
|
|
}
|
|
remove_wait_queue(&capture_wait, &wait);
|
|
current->state = TASK_RUNNING;
|
|
|
|
</programlisting>
|
|
<para>
|
|
The first thing we have to do is to ensure that the application waits until
|
|
the next frame is ready. The code here is almost identical to the mouse code
|
|
we used earlier in this chapter. It is one of the common building blocks of
|
|
Linux device driver code and probably one which you will find occurs in any
|
|
drivers you write.
|
|
</para>
|
|
<para>
|
|
We wait for a frame to be ready, or for a signal to interrupt our waiting. If a
|
|
signal occurs we need to return from the system call so that the signal can
|
|
be sent to the application itself. We also check to see if the user actually
|
|
wanted to avoid waiting - ie if they are using non-blocking I/O and have other things
|
|
to get on with.
|
|
</para>
|
|
<para>
|
|
Next we copy the data from the card to the user application. This is rarely
|
|
as easy as our example makes out. We will add capture_w, and capture_h here
|
|
to hold the width and height of the captured image. We assume the card only
|
|
supports 24bit RGB for now.
|
|
</para>
|
|
<programlisting>
|
|
|
|
|
|
|
|
capture_ready = 0;
|
|
|
|
ptr=(u8 *)buf;
|
|
len = capture_w * 3 * capture_h; /* 24bit RGB */
|
|
|
|
if(len>count)
|
|
len=count; /* Doesn't all fit */
|
|
|
|
for(i=0; i<len; i++)
|
|
{
|
|
put_user(inb(io+IMAGE_DATA), ptr);
|
|
ptr++;
|
|
}
|
|
|
|
hardware_restart_capture();
|
|
|
|
return i;
|
|
}
|
|
|
|
</programlisting>
|
|
<para>
|
|
For a real hardware device you would try to avoid the loop with put_user().
|
|
Each call to put_user() has a time overhead checking whether the accesses to user
|
|
space are allowed. It would be better to read a line into a temporary buffer
|
|
then copy this to user space in one go.
|
|
</para>
|
|
<para>
|
|
Having captured the image and put it into user space we can kick the card to
|
|
get the next frame acquired.
|
|
</para>
|
|
</sect1>
|
|
<sect1 id="iocvid">
|
|
<title>Video Ioctl Handling</title>
|
|
<para>
|
|
As with the radio driver the major control interface is via the ioctl()
|
|
function. Video capture devices support the same tuner calls as a radio
|
|
device and also support additional calls to control how the video functions
|
|
are handled. In this simple example the card has no tuners to avoid making
|
|
the code complex.
|
|
</para>
|
|
<programlisting>
|
|
|
|
|
|
|
|
static int camera_ioctl(struct video_device *dev, unsigned int cmd, void *arg)
|
|
{
|
|
switch(cmd)
|
|
{
|
|
case VIDIOCGCAP:
|
|
{
|
|
struct video_capability v;
|
|
v.type = VID_TYPE_CAPTURE|\
|
|
VID_TYPE_CHROMAKEY|\
|
|
VID_TYPE_SCALES|\
|
|
VID_TYPE_OVERLAY;
|
|
v.channels = 1;
|
|
v.audios = 0;
|
|
v.maxwidth = 640;
|
|
v.minwidth = 16;
|
|
v.maxheight = 480;
|
|
v.minheight = 16;
|
|
strcpy(v.name, "My Camera");
|
|
if(copy_to_user(arg, &v, sizeof(v)))
|
|
return -EFAULT;
|
|
return 0;
|
|
}
|
|
|
|
|
|
</programlisting>
|
|
<para>
|
|
The first ioctl we must support and which all video capture and radio
|
|
devices are required to support is VIDIOCGCAP. This behaves exactly the same
|
|
as with a radio device. This time, however, we report the extra capabilities
|
|
we outlined earlier on when defining our video_dev structure.
|
|
</para>
|
|
<para>
|
|
We now set the video flags saying that we support overlay, capture,
|
|
scaling and chromakey. We also report size limits - our smallest image is
|
|
16x16 pixels, our largest is 640x480.
|
|
</para>
|
|
<para>
|
|
To keep things simple we report no audio and no tuning capabilities at all.
|
|
</para>
|
|
<programlisting>
|
|
|
|
case VIDIOCGCHAN:
|
|
{
|
|
struct video_channel v;
|
|
if(copy_from_user(&v, arg, sizeof(v)))
|
|
return -EFAULT;
|
|
if(v.channel != 0)
|
|
return -EINVAL;
|
|
v.flags = 0;
|
|
v.tuners = 0;
|
|
v.type = VIDEO_TYPE_CAMERA;
|
|
v.norm = VIDEO_MODE_AUTO;
|
|
strcpy(v.name, "Camera Input");break;
|
|
if(copy_to_user(&v, arg, sizeof(v)))
|
|
return -EFAULT;
|
|
return 0;
|
|
}
|
|
|
|
|
|
</programlisting>
|
|
<para>
|
|
This follows what is very much the standard way an ioctl handler looks
|
|
in Linux. We copy the data into a kernel space variable and we check that the
|
|
request is valid (in this case that the input is 0). Finally we copy the
|
|
camera info back to the user.
|
|
</para>
|
|
<para>
|
|
The VIDIOCGCHAN ioctl allows a user to ask about video channels (that is
|
|
inputs to the video card). Our example card has a single camera input. The
|
|
fields in the structure are
|
|
</para>
|
|
<table frame="all" id="video_channel_fields"><title>struct video_channel fields</title>
|
|
<tgroup cols="2" align="left">
|
|
<tbody>
|
|
<row>
|
|
|
|
<entry>channel</entry><entry>The channel number we are selecting</entry>
|
|
</row><row>
|
|
<entry>name</entry><entry>The name for this channel. This is intended
|
|
to describe the port to the user.
|
|
Appropriate names are therefore things like
|
|
"Camera" "SCART input"</entry>
|
|
</row><row>
|
|
<entry>flags</entry><entry>Channel properties</entry>
|
|
</row><row>
|
|
<entry>type</entry><entry>Input type</entry>
|
|
</row><row>
|
|
<entry>norm</entry><entry>The current television encoding being used
|
|
if relevant for this channel.
|
|
</entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</table>
|
|
<table frame="all" id="video_channel_flags"><title>struct video_channel flags</title>
|
|
<tgroup cols="2" align="left">
|
|
<tbody>
|
|
<row>
|
|
<entry>VIDEO_VC_TUNER</entry><entry>Channel has a tuner.</entry>
|
|
</row><row>
|
|
<entry>VIDEO_VC_AUDIO</entry><entry>Channel has audio.</entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</table>
|
|
<table frame="all" id="video_channel_types"><title>struct video_channel types</title>
|
|
<tgroup cols="2" align="left">
|
|
<tbody>
|
|
<row>
|
|
<entry>VIDEO_TYPE_TV</entry><entry>Television input.</entry>
|
|
</row><row>
|
|
<entry>VIDEO_TYPE_CAMERA</entry><entry>Fixed camera input.</entry>
|
|
</row><row>
|
|
<entry>0</entry><entry>Type is unknown.</entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</table>
|
|
<table frame="all" id="video_channel_norms"><title>struct video_channel norms</title>
|
|
<tgroup cols="2" align="left">
|
|
<tbody>
|
|
<row>
|
|
<entry>VIDEO_MODE_PAL</entry><entry>PAL encoded Television</entry>
|
|
</row><row>
|
|
<entry>VIDEO_MODE_NTSC</entry><entry>NTSC (US) encoded Television</entry>
|
|
</row><row>
|
|
<entry>VIDEO_MODE_SECAM</entry><entry>SECAM (French) Television </entry>
|
|
</row><row>
|
|
<entry>VIDEO_MODE_AUTO</entry><entry>Automatic switching, or format does not
|
|
matter</entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</table>
|
|
<para>
|
|
The corresponding VIDIOCSCHAN ioctl allows a user to change channel and to
|
|
request the norm is changed - for example to switch between a PAL or an NTSC
|
|
format camera.
|
|
</para>
|
|
<programlisting>
|
|
|
|
|
|
case VIDIOCSCHAN:
|
|
{
|
|
struct video_channel v;
|
|
if(copy_from_user(&v, arg, sizeof(v)))
|
|
return -EFAULT;
|
|
if(v.channel != 0)
|
|
return -EINVAL;
|
|
if(v.norm != VIDEO_MODE_AUTO)
|
|
return -EINVAL;
|
|
return 0;
|
|
}
|
|
|
|
|
|
</programlisting>
|
|
<para>
|
|
The implementation of this call in our driver is remarkably easy. Because we
|
|
are assuming fixed format hardware we need only check that the user has not
|
|
tried to change anything.
|
|
</para>
|
|
<para>
|
|
The user also needs to be able to configure and adjust the picture they are
|
|
seeing. This is much like adjusting a television set. A user application
|
|
also needs to know the palette being used so that it knows how to display
|
|
the image that has been captured. The VIDIOCGPICT and VIDIOCSPICT ioctl
|
|
calls provide this information.
|
|
</para>
|
|
<programlisting>
|
|
|
|
|
|
case VIDIOCGPICT
|
|
{
|
|
struct video_picture v;
|
|
v.brightness = hardware_brightness();
|
|
v.hue = hardware_hue();
|
|
v.colour = hardware_saturation();
|
|
v.contrast = hardware_brightness();
|
|
/* Not settable */
|
|
v.whiteness = 32768;
|
|
v.depth = 24; /* 24bit */
|
|
v.palette = VIDEO_PALETTE_RGB24;
|
|
if(copy_to_user(&v, arg,
|
|
sizeof(v)))
|
|
return -EFAULT;
|
|
return 0;
|
|
}
|
|
|
|
|
|
</programlisting>
|
|
<para>
|
|
The brightness, hue, color, and contrast provide the picture controls that
|
|
are akin to a conventional television. Whiteness provides additional
|
|
control for greyscale images. All of these values are scaled between 0-65535
|
|
and have 32768 as the mid point setting. The scaling means that applications
|
|
do not have to worry about the capability range of the hardware but can let
|
|
it make a best effort attempt.
|
|
</para>
|
|
<para>
|
|
Our depth is 24, as this is in bits. We will be returning RGB24 format. This
|
|
has one byte of red, then one of green, then one of blue. This then repeats
|
|
for every other pixel in the image. The other common formats the interface
|
|
defines are
|
|
</para>
|
|
<table frame="all" id="Framebuffer_Encodings"><title>Framebuffer Encodings</title>
|
|
<tgroup cols="2" align="left">
|
|
<tbody>
|
|
<row>
|
|
<entry>GREY</entry><entry>Linear greyscale. This is for simple cameras and the
|
|
like</entry>
|
|
</row><row>
|
|
<entry>RGB565</entry><entry>The top 5 bits hold 32 red levels, the next six bits
|
|
hold green and the low 5 bits hold blue. </entry>
|
|
</row><row>
|
|
<entry>RGB555</entry><entry>The top bit is clear. The red green and blue levels
|
|
each occupy five bits.</entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</table>
|
|
<para>
|
|
Additional modes are support for YUV capture formats. These are common for
|
|
TV and video conferencing applications.
|
|
</para>
|
|
<para>
|
|
The VIDIOCSPICT ioctl allows a user to set some of the picture parameters.
|
|
Exactly which ones are supported depends heavily on the card itself. It is
|
|
possible to support many modes and effects in software. In general doing
|
|
this in the kernel is a bad idea. Video capture is a performance-sensitive
|
|
application and the programs can often do better if they aren't being
|
|
'helped' by an overkeen driver writer. Thus for our device we will report
|
|
RGB24 only and refuse to allow a change.
|
|
</para>
|
|
<programlisting>
|
|
|
|
|
|
case VIDIOCSPICT:
|
|
{
|
|
struct video_picture v;
|
|
if(copy_from_user(&v, arg, sizeof(v)))
|
|
return -EFAULT;
|
|
if(v.depth!=24 ||
|
|
v.palette != VIDEO_PALETTE_RGB24)
|
|
return -EINVAL;
|
|
set_hardware_brightness(v.brightness);
|
|
set_hardware_hue(v.hue);
|
|
set_hardware_saturation(v.colour);
|
|
set_hardware_brightness(v.contrast);
|
|
return 0;
|
|
}
|
|
|
|
|
|
</programlisting>
|
|
<para>
|
|
We check the user has not tried to change the palette or the depth. We do
|
|
not want to carry out some of the changes and then return an error. This may
|
|
confuse the application which will be assuming no change occurred.
|
|
</para>
|
|
<para>
|
|
In much the same way as you need to be able to set the picture controls to
|
|
get the right capture images, many cards need to know what they are
|
|
displaying onto when generating overlay output. In some cases getting this
|
|
wrong even makes a nasty mess or may crash the computer. For that reason
|
|
the VIDIOCSBUF ioctl used to set up the frame buffer information may well
|
|
only be usable by root.
|
|
</para>
|
|
<para>
|
|
We will assume our card is one of the old ISA devices with feature connector
|
|
and only supports a couple of standard video modes. Very common for older
|
|
cards although the PCI devices are way smarter than this.
|
|
</para>
|
|
<programlisting>
|
|
|
|
|
|
static struct video_buffer capture_fb;
|
|
|
|
case VIDIOCGFBUF:
|
|
{
|
|
if(copy_to_user(arg, &capture_fb,
|
|
sizeof(capture_fb)))
|
|
return -EFAULT;
|
|
return 0;
|
|
|
|
}
|
|
|
|
|
|
</programlisting>
|
|
<para>
|
|
We keep the frame buffer information in the format the ioctl uses. This
|
|
makes it nice and easy to work with in the ioctl calls.
|
|
</para>
|
|
<programlisting>
|
|
|
|
case VIDIOCSFBUF:
|
|
{
|
|
struct video_buffer v;
|
|
|
|
if(!capable(CAP_SYS_ADMIN))
|
|
return -EPERM;
|
|
|
|
if(copy_from_user(&v, arg, sizeof(v)))
|
|
return -EFAULT;
|
|
if(v.width!=320 && v.width!=640)
|
|
return -EINVAL;
|
|
if(v.height!=200 && v.height!=240
|
|
&& v.height!=400
|
|
&& v.height !=480)
|
|
return -EINVAL;
|
|
memcpy(&capture_fb, &v, sizeof(v));
|
|
hardware_set_fb(&v);
|
|
return 0;
|
|
}
|
|
|
|
|
|
|
|
</programlisting>
|
|
<para>
|
|
The capable() function checks a user has the required capability. The Linux
|
|
operating system has a set of about 30 capabilities indicating privileged
|
|
access to services. The default set up gives the superuser (uid 0) all of
|
|
them and nobody else has any.
|
|
</para>
|
|
<para>
|
|
We check that the user has the SYS_ADMIN capability, that is they are
|
|
allowed to operate as the machine administrator. We don't want anyone but
|
|
the administrator making a mess of the display.
|
|
</para>
|
|
<para>
|
|
Next we check for standard PC video modes (320 or 640 wide with either
|
|
EGA or VGA depths). If the mode is not a standard video mode we reject it as
|
|
not supported by our card. If the mode is acceptable we save it so that
|
|
VIDIOCFBUF will give the right answer next time it is called. The
|
|
hardware_set_fb() function is some undescribed card specific function to
|
|
program the card for the desired mode.
|
|
</para>
|
|
<para>
|
|
Before the driver can display an overlay window it needs to know where the
|
|
window should be placed, and also how large it should be. If the card
|
|
supports clipping it needs to know which rectangles to omit from the
|
|
display. The video_window structure is used to describe the way the image
|
|
should be displayed.
|
|
</para>
|
|
<table frame="all" id="video_window_fields"><title>struct video_window fields</title>
|
|
<tgroup cols="2" align="left">
|
|
<tbody>
|
|
<row>
|
|
<entry>width</entry><entry>The width in pixels of the desired image. The card
|
|
may use a smaller size if this size is not available</entry>
|
|
</row><row>
|
|
<entry>height</entry><entry>The height of the image. The card may use a smaller
|
|
size if this size is not available.</entry>
|
|
</row><row>
|
|
<entry>x</entry><entry> The X position of the top left of the window. This
|
|
is in pixels relative to the left hand edge of the
|
|
picture. Not all cards can display images aligned on
|
|
any pixel boundary. If the position is unsuitable
|
|
the card adjusts the image right and reduces the
|
|
width.</entry>
|
|
</row><row>
|
|
<entry>y</entry><entry> The Y position of the top left of the window. This
|
|
is counted in pixels relative to the top edge of the
|
|
picture. As with the width if the card cannot
|
|
display starting on this line it will adjust the
|
|
values.</entry>
|
|
</row><row>
|
|
<entry>chromakey</entry><entry>The colour (expressed in RGB32 format) for the
|
|
chromakey colour if chroma keying is being used. </entry>
|
|
</row><row>
|
|
<entry>clips</entry><entry>An array of rectangles that must not be drawn
|
|
over.</entry>
|
|
</row><row>
|
|
<entry>clipcount</entry><entry>The number of clips in this array.</entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</table>
|
|
<para>
|
|
Each clip is a struct video_clip which has the following fields
|
|
</para>
|
|
<table frame="all" id="video_clip_fields"><title>video_clip fields</title>
|
|
<tgroup cols="2" align="left">
|
|
<tbody>
|
|
<row>
|
|
<entry>x, y</entry><entry>Co-ordinates relative to the display</entry>
|
|
</row><row>
|
|
<entry>width, height</entry><entry>Width and height in pixels</entry>
|
|
</row><row>
|
|
<entry>next</entry><entry>A spare field for the application to use</entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</table>
|
|
<para>
|
|
The driver is required to ensure it always draws in the area requested or a smaller area, and that it never draws in any of the areas that are clipped.
|
|
This may well mean it has to leave alone. small areas the application wished to be
|
|
drawn.
|
|
</para>
|
|
<para>
|
|
Our example card uses chromakey so does not have to address most of the
|
|
clipping. We will add a video_window structure to our global variables to
|
|
remember our parameters, as we did with the frame buffer.
|
|
</para>
|
|
<programlisting>
|
|
|
|
|
|
case VIDIOCGWIN:
|
|
{
|
|
if(copy_to_user(arg, &capture_win,
|
|
sizeof(capture_win)))
|
|
return -EFAULT;
|
|
return 0;
|
|
}
|
|
|
|
|
|
case VIDIOCSWIN:
|
|
{
|
|
struct video_window v;
|
|
if(copy_from_user(&v, arg, sizeof(v)))
|
|
return -EFAULT;
|
|
if(v.width > 640 || v.height > 480)
|
|
return -EINVAL;
|
|
if(v.width < 16 || v.height < 16)
|
|
return -EINVAL;
|
|
hardware_set_key(v.chromakey);
|
|
hardware_set_window(v);
|
|
memcpy(&capture_win, &v, sizeof(v));
|
|
capture_w = v.width;
|
|
capture_h = v.height;
|
|
return 0;
|
|
}
|
|
|
|
|
|
</programlisting>
|
|
<para>
|
|
Because we are using Chromakey our setup is fairly simple. Mostly we have to
|
|
check the values are sane and load them into the capture card.
|
|
</para>
|
|
<para>
|
|
With all the setup done we can now turn on the actual capture/overlay. This
|
|
is done with the VIDIOCCAPTURE ioctl. This takes a single integer argument
|
|
where 0 is on and 1 is off.
|
|
</para>
|
|
<programlisting>
|
|
|
|
|
|
case VIDIOCCAPTURE:
|
|
{
|
|
int v;
|
|
if(get_user(v, (int *)arg))
|
|
return -EFAULT;
|
|
if(v==0)
|
|
hardware_capture_off();
|
|
else
|
|
{
|
|
if(capture_fb.width == 0
|
|
|| capture_w == 0)
|
|
return -EINVAL;
|
|
hardware_capture_on();
|
|
}
|
|
return 0;
|
|
}
|
|
|
|
|
|
</programlisting>
|
|
<para>
|
|
We grab the flag from user space and either enable or disable according to
|
|
its value. There is one small corner case we have to consider here. Suppose
|
|
that the capture was requested before the video window or the frame buffer
|
|
had been set up. In those cases there will be unconfigured fields in our
|
|
card data, as well as unconfigured hardware settings. We check for this case and
|
|
return an error if the frame buffer or the capture window width is zero.
|
|
</para>
|
|
<programlisting>
|
|
|
|
|
|
default:
|
|
return -ENOIOCTLCMD;
|
|
}
|
|
}
|
|
</programlisting>
|
|
<para>
|
|
|
|
We don't need to support any other ioctls, so if we get this far, it is time
|
|
to tell the video layer that we don't now what the user is talking about.
|
|
</para>
|
|
</sect1>
|
|
<sect1 id="endvid">
|
|
<title>Other Functionality</title>
|
|
<para>
|
|
The Video4Linux layer supports additional features, including a high
|
|
performance mmap() based capture mode and capturing part of the image.
|
|
These features are out of the scope of the book. You should however have enough
|
|
example code to implement most simple video4linux devices for radio and TV
|
|
cards.
|
|
</para>
|
|
</sect1>
|
|
</chapter>
|
|
<chapter id="bugs">
|
|
<title>Known Bugs And Assumptions</title>
|
|
<para>
|
|
<variablelist>
|
|
<varlistentry><term>Multiple Opens</term>
|
|
<listitem>
|
|
<para>
|
|
The driver assumes multiple opens should not be allowed. A driver
|
|
can work around this but not cleanly.
|
|
</para>
|
|
</listitem></varlistentry>
|
|
|
|
<varlistentry><term>API Deficiencies</term>
|
|
<listitem>
|
|
<para>
|
|
The existing API poorly reflects compression capable devices. There
|
|
are plans afoot to merge V4L, V4L2 and some other ideas into a
|
|
better interface.
|
|
</para>
|
|
</listitem></varlistentry>
|
|
</variablelist>
|
|
|
|
</para>
|
|
</chapter>
|
|
|
|
<chapter id="pubfunctions">
|
|
<title>Public Functions Provided</title>
|
|
!Edrivers/media/video/videodev.c
|
|
</chapter>
|
|
|
|
</book>
|