|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.
|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
|(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
|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.
|Note that this request is limited to SPI mode flags that fit in a
|SPI_IOC_RD_MODE32, SPI_IOC_WR_MODE32 ... pass a pointer to a uin32_t
|which will return (RD) or assign (WR) the full SPI transfer mode,
|not limited to the bits that fit in one byte.
|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
|- At this time there is no async I/O support; everything is purely
|- 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.