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