spi: docs: convert to ReST and add it to the kABI bookset

While there's one file there with briefily describes the uAPI,
the documentation was written just like most subsystems: focused
on kernel developers. So, add it together with driver-api books.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com> # for iio
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/spi/spidev.rst

@@ -0,0 +1,163 @@
+=================
+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 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.
+	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.