160 lines
7.0 KiB
Plaintext
160 lines
7.0 KiB
Plaintext
|
|
What is udlfb?
|
|
===============
|
|
|
|
This is a driver for DisplayLink USB 2.0 era graphics chips.
|
|
|
|
DisplayLink chips provide simple hline/blit operations with some compression,
|
|
pairing that with a hardware framebuffer (16MB) on the other end of the
|
|
USB wire. That hardware framebuffer is able to drive the VGA, DVI, or HDMI
|
|
monitor with no CPU involvement until a pixel has to change.
|
|
|
|
The CPU or other local resource does all the rendering; optinally compares the
|
|
result with a local shadow of the remote hardware framebuffer to identify
|
|
the minimal set of pixels that have changed; and compresses and sends those
|
|
pixels line-by-line via USB bulk transfers.
|
|
|
|
Because of the efficiency of bulk transfers and a protocol on top that
|
|
does not require any acks - the effect is very low latency that
|
|
can support surprisingly high resolutions with good performance for
|
|
non-gaming and non-video applications.
|
|
|
|
Mode setting, EDID read, etc are other bulk or control transfers. Mode
|
|
setting is very flexible - able to set nearly arbitrary modes from any timing.
|
|
|
|
Advantages of USB graphics in general:
|
|
|
|
* Ability to add a nearly arbitrary number of displays to any USB 2.0
|
|
capable system. On Linux, number of displays is limited by fbdev interface
|
|
(FB_MAX is currently 32). Of course, all USB devices on the same
|
|
host controller share the same 480Mbs USB 2.0 interface.
|
|
|
|
Advantages of supporting DisplayLink chips with kernel framebuffer interface:
|
|
|
|
* The actual hardware functionality of DisplayLink chips matches nearly
|
|
one-to-one with the fbdev interface, making the driver quite small and
|
|
tight relative to the functionality it provides.
|
|
* X servers and other applications can use the standard fbdev interface
|
|
from user mode to talk to the device, without needing to know anything
|
|
about USB or DisplayLink's protocol at all. A "displaylink" X driver
|
|
and a slightly modified "fbdev" X driver are among those that already do.
|
|
|
|
Disadvantages:
|
|
|
|
* Fbdev's mmap interface assumes a real hardware framebuffer is mapped.
|
|
In the case of USB graphics, it is just an allocated (virtual) buffer.
|
|
Writes need to be detected and encoded into USB bulk transfers by the CPU.
|
|
Accurate damage/changed area notifications work around this problem.
|
|
In the future, hopefully fbdev will be enhanced with an small standard
|
|
interface to allow mmap clients to report damage, for the benefit
|
|
of virtual or remote framebuffers.
|
|
* Fbdev does not arbitrate client ownership of the framebuffer well.
|
|
* Fbcon assumes the first framebuffer it finds should be consumed for console.
|
|
* It's not clear what the future of fbdev is, given the rise of KMS/DRM.
|
|
|
|
How to use it?
|
|
==============
|
|
|
|
Udlfb, when loaded as a module, will match against all USB 2.0 generation
|
|
DisplayLink chips (Alex and Ollie family). It will then attempt to read the EDID
|
|
of the monitor, and set the best common mode between the DisplayLink device
|
|
and the monitor's capabilities.
|
|
|
|
If the DisplayLink device is successful, it will paint a "green screen" which
|
|
means that from a hardware and fbdev software perspective, everything is good.
|
|
|
|
At that point, a /dev/fb? interface will be present for user-mode applications
|
|
to open and begin writing to the framebuffer of the DisplayLink device using
|
|
standard fbdev calls. Note that if mmap() is used, by default the user mode
|
|
application must send down damage notifcations to trigger repaints of the
|
|
changed regions. Alternatively, udlfb can be recompiled with experimental
|
|
defio support enabled, to support a page-fault based detection mechanism
|
|
that can work without explicit notifcation.
|
|
|
|
The most common client of udlfb is xf86-video-displaylink or a modified
|
|
xf86-video-fbdev X server. These servers have no real DisplayLink specific
|
|
code. They write to the standard framebuffer interface and rely on udlfb
|
|
to do its thing. The one extra feature they have is the ability to report
|
|
rectangles from the X DAMAGE protocol extension down to udlfb via udlfb's
|
|
damage interface (which will hopefully be standardized for all virtual
|
|
framebuffers that need damage info). These damage notifications allow
|
|
udlfb to efficiently process the changed pixels.
|
|
|
|
Module Options
|
|
==============
|
|
|
|
Special configuration for udlfb is usually unnecessary. There are a few
|
|
options, however.
|
|
|
|
From the command line, pass options to modprobe
|
|
modprobe udlfb fb_defio=0 console=1 shadow=1
|
|
|
|
Or modify options on the fly at /sys/module/udlfb/parameters directory via
|
|
sudo nano fb_defio
|
|
change the parameter in place, and save the file.
|
|
|
|
Unplug/replug USB device to apply with new settings
|
|
|
|
Or for permanent option, create file like /etc/modprobe.d/udlfb.conf with text
|
|
options udlfb fb_defio=0 console=1 shadow=1
|
|
|
|
Accepted boolean options:
|
|
|
|
fb_defio Make use of the fb_defio (CONFIG_FB_DEFERRED_IO) kernel
|
|
module to track changed areas of the framebuffer by page faults.
|
|
Standard fbdev applications that use mmap but that do not
|
|
report damage, should be able to work with this enabled.
|
|
Disable when running with X server that supports reporting
|
|
changed regions via ioctl, as this method is simpler,
|
|
more stable, and higher performance.
|
|
default: fb_defio=1
|
|
|
|
console Allow fbcon to attach to udlfb provided framebuffers.
|
|
Can be disabled if fbcon and other clients
|
|
(e.g. X with --shared-vt) are in conflict.
|
|
default: console=1
|
|
|
|
shadow Allocate a 2nd framebuffer to shadow what's currently across
|
|
the USB bus in device memory. If any pixels are unchanged,
|
|
do not transmit. Spends host memory to save USB transfers.
|
|
Enabled by default. Only disable on very low memory systems.
|
|
default: shadow=1
|
|
|
|
Sysfs Attributes
|
|
================
|
|
|
|
Udlfb creates several files in /sys/class/graphics/fb?
|
|
Where ? is the sequential framebuffer id of the particular DisplayLink device
|
|
|
|
edid If a valid EDID blob is written to this file (typically
|
|
by a udev rule), then udlfb will use this EDID as a
|
|
backup in case reading the actual EDID of the monitor
|
|
attached to the DisplayLink device fails. This is
|
|
especially useful for fixed panels, etc. that cannot
|
|
communicate their capabilities via EDID. Reading
|
|
this file returns the current EDID of the attached
|
|
monitor (or last backup value written). This is
|
|
useful to get the EDID of the attached monitor,
|
|
which can be passed to utilities like parse-edid.
|
|
|
|
metrics_bytes_rendered 32-bit count of pixel bytes rendered
|
|
|
|
metrics_bytes_identical 32-bit count of how many of those bytes were found to be
|
|
unchanged, based on a shadow framebuffer check
|
|
|
|
metrics_bytes_sent 32-bit count of how many bytes were transferred over
|
|
USB to communicate the resulting changed pixels to the
|
|
hardware. Includes compression and protocol overhead
|
|
|
|
metrics_cpu_kcycles_used 32-bit count of CPU cycles used in processing the
|
|
above pixels (in thousands of cycles).
|
|
|
|
metrics_reset Write-only. Any write to this file resets all metrics
|
|
above to zero. Note that the 32-bit counters above
|
|
roll over very quickly. To get reliable results, design
|
|
performance tests to start and finish in a very short
|
|
period of time (one minute or less is safe).
|
|
|
|
--
|
|
Bernie Thompson <bernie@plugable.com>
|