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 =================
 SPI userspace API
 =================
 
 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 spidev driver contains lists of SPI devices that are supported for
 the different hardware topology representations.
 
 The following are the SPI device tables supported by the spidev driver:
 
     - struct spi_device_id spidev_spi_ids[]: list of devices that can be
       bound when these are defined using a struct spi_board_info with a
       .modalias field matching one of the entries in the table.
 
     - struct of_device_id spidev_dt_ids[]: list of devices that can be
       bound when these are defined using a Device Tree node that has a
       compatible string matching one of the entries in the table.
 
     - struct acpi_device_id spidev_acpi_ids[]: list of devices that can
       be bound when these are defined using a ACPI device object with a
       _HID matching one of the entries in the table.
 
 You are encouraged to add an entry for your SPI device name to relevant
 tables, if these don't already have an entry for the device. To do that,
 post a patch for spidev to the linux-spi@vger.kernel.org mailing list.
 
 It used to be supported to define an SPI device using the "spidev" name.
 For example, as .modalias = "spidev" or compatible = "spidev".  But this
 is no longer supported by the Linux kernel and instead a real SPI device
 name as listed in one of the tables must be used.
 
 Not having a real SPI device name will lead to an error being printed and
 the spidev driver failing to probe.
 
 Sysfs also supports userspace driven binding/unbinding of drivers to
 devices that do not bind automatically using one of the tables above.
 To make the spidev driver bind to such a device, use the following:
 
     echo spidev > /sys/bus/spi/devices/spiB.C/driver_override
     echo spiB.C > /sys/bus/spi/drivers/spidev/bind
 
 When the spidev driver is bound to a SPI device, the sysfs node for the
 device will include a child device node with a "dev" attribute that will
 be understood by udev or mdev (udev replacement from 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.
         Note that this request is limited to SPI mode flags that fit in a
         single byte.
 
     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
         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.

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