144 lines
6.5 KiB
Plaintext
144 lines
6.5 KiB
Plaintext
SPI devices have a limited userspace API, supporting basic half-duplex
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read() and write() access to SPI slave devices. Using ioctl() requests,
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full duplex transfers and device I/O configuration are also available.
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#include <fcntl.h>
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#include <unistd.h>
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#include <sys/ioctl.h>
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#include <linux/types.h>
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#include <linux/spi/spidev.h>
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Some reasons you might want to use this programming interface include:
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* Prototyping in an environment that's not crash-prone; stray pointers
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in userspace won't normally bring down any Linux system.
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* Developing simple protocols used to talk to microcontrollers acting
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as SPI slaves, which you may need to change quite often.
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Of course there are drivers that can never be written in userspace, because
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they need to access kernel interfaces (such as IRQ handlers or other layers
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of the driver stack) that are not accessible to userspace.
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DEVICE CREATION, DRIVER BINDING
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===============================
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The simplest way to arrange to use this driver is to just list it in the
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spi_board_info for a device as the driver it should use: the "modalias"
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entry is "spidev", matching the name of the driver exposing this API.
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Set up the other device characteristics (bits per word, SPI clocking,
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chipselect polarity, etc) as usual, so you won't always need to override
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them later.
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(Sysfs also supports userspace driven binding/unbinding of drivers to
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devices. That mechanism might be supported here in the future.)
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When you do that, the sysfs node for the SPI device will include a child
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device node with a "dev" attribute that will be understood by udev or mdev.
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(Larger systems will have "udev". Smaller ones may configure "mdev" into
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busybox; it's less featureful, but often enough.) For a SPI device with
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chipselect C on bus B, you should see:
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/dev/spidevB.C ... character special device, major number 153 with
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a dynamically chosen minor device number. This is the node
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that userspace programs will open, created by "udev" or "mdev".
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/sys/devices/.../spiB.C ... as usual, the SPI device node will
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be a child of its SPI master controller.
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/sys/class/spidev/spidevB.C ... created when the "spidev" driver
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binds to that device. (Directory or symlink, based on whether
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or not you enabled the "deprecated sysfs files" Kconfig option.)
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Do not try to manage the /dev character device special file nodes by hand.
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That's error prone, and you'd need to pay careful attention to system
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security issues; udev/mdev should already be configured securely.
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If you unbind the "spidev" driver from that device, those two "spidev" nodes
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(in sysfs and in /dev) should automatically be removed (respectively by the
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kernel and by udev/mdev). You can unbind by removing the "spidev" driver
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module, which will affect all devices using this driver. You can also unbind
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by having kernel code remove the SPI device, probably by removing the driver
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for its SPI controller (so its spi_master vanishes).
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Since this is a standard Linux device driver -- even though it just happens
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to expose a low level API to userspace -- it can be associated with any number
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of devices at a time. Just provide one spi_board_info record for each such
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SPI device, and you'll get a /dev device node for each device.
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BASIC CHARACTER DEVICE API
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==========================
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Normal open() and close() operations on /dev/spidevB.D files work as you
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would expect.
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Standard read() and write() operations are obviously only half-duplex, and
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the chipselect is deactivated between those operations. Full-duplex access,
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and composite operation without chipselect de-activation, is available using
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the SPI_IOC_MESSAGE(N) request.
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Several ioctl() requests let your driver read or override the device's current
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settings for data transfer parameters:
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SPI_IOC_RD_MODE, SPI_IOC_WR_MODE ... pass a pointer to a byte which will
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return (RD) or assign (WR) the SPI transfer mode. Use the constants
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SPI_MODE_0..SPI_MODE_3; or if you prefer you can combine SPI_CPOL
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(clock polarity, idle high iff this is set) or SPI_CPHA (clock phase,
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sample on trailing edge iff this is set) flags.
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SPI_IOC_RD_LSB_FIRST, SPI_IOC_WR_LSB_FIRST ... pass a pointer to a byte
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which will return (RD) or assign (WR) the bit justification used to
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transfer SPI words. Zero indicates MSB-first; other values indicate
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the less common LSB-first encoding. In both cases the specified value
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is right-justified in each word, so that unused (TX) or undefined (RX)
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bits are in the MSBs.
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SPI_IOC_RD_BITS_PER_WORD, SPI_IOC_WR_BITS_PER_WORD ... pass a pointer to
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a byte which will return (RD) or assign (WR) the number of bits in
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each SPI transfer word. The value zero signifies eight bits.
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SPI_IOC_RD_MAX_SPEED_HZ, SPI_IOC_WR_MAX_SPEED_HZ ... pass a pointer to a
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u32 which will return (RD) or assign (WR) the maximum SPI transfer
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speed, in Hz. The controller can't necessarily assign that specific
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clock speed.
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NOTES:
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- At this time there is no async I/O support; everything is purely
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synchronous.
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- There's currently no way to report the actual bit rate used to
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shift data to/from a given device.
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- From userspace, you can't currently change the chip select polarity;
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that could corrupt transfers to other devices sharing the SPI bus.
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Each SPI device is deselected when it's not in active use, allowing
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other drivers to talk to other devices.
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- There's a limit on the number of bytes each I/O request can transfer
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to the SPI device. It defaults to one page, but that can be changed
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using a module parameter.
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- Because SPI has no low-level transfer acknowledgement, you usually
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won't see any I/O errors when talking to a non-existent device.
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FULL DUPLEX CHARACTER DEVICE API
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================================
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See the spidev_fdx.c sample program for one example showing the use of the
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full duplex programming interface. (Although it doesn't perform a full duplex
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transfer.) The model is the same as that used in the kernel spi_sync()
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request; the individual transfers offer the same capabilities as are
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available to kernel drivers (except that it's not asynchronous).
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The example shows one half-duplex RPC-style request and response message.
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These requests commonly require that the chip not be deselected between
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the request and response. Several such requests could be chained into
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a single kernel request, even allowing the chip to be deselected after
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each response. (Other protocol options include changing the word size
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and bitrate for each transfer segment.)
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To make a full duplex request, provide both rx_buf and tx_buf for the
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same transfer. It's even OK if those are the same buffer.
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