SPI kerneldoc

Various documentation updates for the SPI infrastructure, to clarify things
that may not have been clear, to cope with lack of editing, and fix
omissions.

Also, plug SPI into the kernel-api DocBook template, and fix all the
resulting glitches in document generation.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: "Randy.Dunlap" <rdunlap@xenotime.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/DocBook/kernel-api.tmpl

@@ -576,4 +576,67 @@ X!Idrivers/video/console/fonts.c
 !Edrivers/input/ff-core.c
 !Edrivers/input/ff-memless.c
   </chapter>
+
+  <chapter id="spi">
+      <title>Serial Peripheral Interface (SPI)</title>
+  <para>
+	SPI is the "Serial Peripheral Interface", widely used with
+	embedded systems because it is a simple and efficient
+	interface:  basically a multiplexed shift register.
+	Its three signal wires hold a clock (SCK, often in the range
+	of 1-20 MHz), a "Master Out, Slave In" (MOSI) data line, and
+	a "Master In, Slave Out" (MISO) data line.
+	SPI is a full duplex protocol; for each bit shifted out the
+	MOSI line (one per clock) another is shifted in on the MISO line.
+	Those bits are assembled into words of various sizes on the
+	way to and from system memory.
+	An additional chipselect line is usually active-low (nCS);
+	four signals are normally used for each peripheral, plus
+	sometimes an interrupt.
+  </para>
+  <para>
+	The SPI bus facilities listed here provide a generalized
+	interface to declare SPI busses and devices, manage them
+	according to the standard Linux driver model, and perform
+	input/output operations.
+	At this time, only "master" side interfaces are supported,
+	where Linux talks to SPI peripherals and does not implement
+	such a peripheral itself.
+	(Interfaces to support implementing SPI slaves would
+	necessarily look different.)
+  </para>
+  <para>
+	The programming interface is structured around two kinds of driver,
+	and two kinds of device.
+	A "Controller Driver" abstracts the controller hardware, which may
+	be as simple as a set of GPIO pins or as complex as a pair of FIFOs
+	connected to dual DMA engines on the other side of the SPI shift
+	register (maximizing throughput).  Such drivers bridge between
+	whatever bus they sit on (often the platform bus) and SPI, and
+	expose the SPI side of their device as a
+	<structname>struct spi_master</structname>.
+	SPI devices are children of that master, represented as a
+	<structname>struct spi_device</structname> and manufactured from
+	<structname>struct spi_board_info</structname> descriptors which
+	are usually provided by board-specific initialization code.
+	A <structname>struct spi_driver</structname> is called a
+	"Protocol Driver", and is bound to a spi_device using normal
+	driver model calls.
+  </para>
+  <para>
+	The I/O model is a set of queued messages.  Protocol drivers
+	submit one or more <structname>struct spi_message</structname>
+	objects, which are processed and completed asynchronously.
+	(There are synchronous wrappers, however.)  Messages are
+	built from one or more <structname>struct spi_transfer</structname>
+	objects, each of which wraps a full duplex SPI transfer.
+	A variety of protocol tweaking options are needed, because
+	different chips adopt very different policies for how they
+	use the bits transferred with SPI.
+  </para>
+!Iinclude/linux/spi/spi.h
+!Fdrivers/spi/spi.c spi_register_board_info
+!Edrivers/spi/spi.c
+  </chapter>
+
 </book>

Documentation/spi/spi-summary

@@ -8,7 +8,7 @@ What is SPI?
 The "Serial Peripheral Interface" (SPI) is a synchronous four wire serial
 link used to connect microcontrollers to sensors, memory, and peripherals.
 
-The three signal wires hold a clock (SCLK, often on the order of 10 MHz),
+The three signal wires hold a clock (SCK, often on the order of 10 MHz),
 and parallel data lines with "Master Out, Slave In" (MOSI) or "Master In,
 Slave Out" (MISO) signals.  (Other names are also used.)  There are four
 clocking modes through which data is exchanged; mode-0 and mode-3 are most
@@ -22,7 +22,7 @@ other signals, often including an interrupt to the master.
 
 Unlike serial busses like USB or SMBUS, even low level protocols for
 SPI slave functions are usually not interoperable between vendors
-(except for cases like SPI memory chips).
+(except for commodities like SPI memory chips).
 
   - SPI may be used for request/response style device protocols, as with
     touchscreen sensors and memory chips.
@@ -77,8 +77,9 @@ cards without needing a special purpose MMC/SD/SDIO controller.
 How do these driver programming interfaces work?
 ------------------------------------------------
 The <linux/spi/spi.h> header file includes kerneldoc, as does the
-main source code, and you should certainly read that.  This is just
-an overview, so you get the big picture before the details.
+main source code, and you should certainly read that chapter of the
+kernel API document.  This is just an overview, so you get the big
+picture before those details.
 
 SPI requests always go into I/O queues.  Requests for a given SPI device
 are always executed in FIFO order, and complete asynchronously through
@@ -88,7 +89,7 @@ a command and then reading its response.
 
 There are two types of SPI driver, here called:
 
-  Controller drivers ... these are often built in to System-On-Chip
+  Controller drivers ... controllers may be built in to System-On-Chip
 	processors, and often support both Master and Slave roles.
 	These drivers touch hardware registers and may use DMA.
 	Or they can be PIO bitbangers, needing just GPIO pins.
@@ -108,18 +109,18 @@ those two types of driver.  At this writing, Linux has no slave side
 programming interface.
 
 There is a minimal core of SPI programming interfaces, focussing on
-using driver model to connect controller and protocol drivers using
+using the driver model to connect controller and protocol drivers using
 device tables provided by board specific initialization code.  SPI
 shows up in sysfs in several locations:
 
-   /sys/devices/.../CTLR/spiB.C ... spi_device for on bus "B",
+   /sys/devices/.../CTLR/spiB.C ... spi_device on bus "B",
 	chipselect C, accessed through CTLR.
 
    /sys/devices/.../CTLR/spiB.C/modalias ... identifies the driver
 	that should be used with this device (for hotplug/coldplug)
 
    /sys/bus/spi/devices/spiB.C ... symlink to the physical
-   	spiB-C device
+   	spiB.C device
 
    /sys/bus/spi/drivers/D ... driver for one or more spi*.* devices
 
@@ -240,7 +241,7 @@ The board_info should provide enough information to let the system work
 without the chip's driver being loaded.  The most troublesome aspect of
 that is likely the SPI_CS_HIGH bit in the spi_device.mode field, since
 sharing a bus with a device that interprets chipselect "backwards" is
-not possible.
+not possible until the infrastructure knows how to deselect it.
 
 Then your board initialization code would register that table with the SPI
 infrastructure, so that it's available later when the SPI master controller
@@ -268,16 +269,14 @@ board info based on the board that was hotplugged.  Of course, you'd later
 call at least spi_unregister_device() when that board is removed.
 
 When Linux includes support for MMC/SD/SDIO/DataFlash cards through SPI, those
-configurations will also be dynamic.  Fortunately, those devices all support
-basic device identification probes, so that support should hotplug normally.
+configurations will also be dynamic.  Fortunately, such devices all support
+basic device identification probes, so they should hotplug normally.
 
 
 How do I write an "SPI Protocol Driver"?
 ----------------------------------------
-All SPI drivers are currently kernel drivers.  A userspace driver API
-would just be another kernel driver, probably offering some lowlevel
-access through aio_read(), aio_write(), and ioctl() calls and using the
-standard userspace sysfs mechanisms to bind to a given SPI device.
+Most SPI drivers are currently kernel drivers, but there's also support
+for userspace drivers.  Here we talk only about kernel drivers.
 
 SPI protocol drivers somewhat resemble platform device drivers:
 
@@ -319,7 +318,8 @@ might look like this unless you're creating a class_device:
 
 As soon as it enters probe(), the driver may issue I/O requests to
 the SPI device using "struct spi_message".  When remove() returns,
-the driver guarantees that it won't submit any more such messages.
+or after probe() fails, the driver guarantees that it won't submit
+any more such messages.
 
   - An spi_message is a sequence of protocol operations, executed
     as one atomic sequence.  SPI driver controls include:
@@ -368,7 +368,8 @@ the driver guarantees that it won't submit any more such messages.
 Some drivers may need to modify spi_device characteristics like the
 transfer mode, wordsize, or clock rate.  This is done with spi_setup(),
 which would normally be called from probe() before the first I/O is
-done to the device.
+done to the device.  However, that can also be called at any time
+that no message is pending for that device.
 
 While "spi_device" would be the bottom boundary of the driver, the
 upper boundaries might include sysfs (especially for sensor readings),
@@ -445,11 +446,15 @@ SPI MASTER METHODS
 	This sets up the device clock rate, SPI mode, and word sizes.
 	Drivers may change the defaults provided by board_info, and then
 	call spi_setup(spi) to invoke this routine.  It may sleep.
+	Unless each SPI slave has its own configuration registers, don't
+	change them right away ... otherwise drivers could corrupt I/O
+	that's in progress for other SPI devices.
 
     master->transfer(struct spi_device *spi, struct spi_message *message)
     	This must not sleep.  Its responsibility is arrange that the
-	transfer happens and its complete() callback is issued; the two
-	will normally happen later, after other transfers complete.
+	transfer happens and its complete() callback is issued.  The two
+	will normally happen later, after other transfers complete, and
+	if the controller is idle it will need to be kickstarted.
 
     master->cleanup(struct spi_device *spi)
 	Your controller driver may use spi_device.controller_state to hold

drivers/spi/spi.c

@@ -152,6 +152,11 @@ static void spi_drv_shutdown(struct device *dev)
 	sdrv->shutdown(to_spi_device(dev));
 }
 
+/**
+ * spi_register_driver - register a SPI driver
+ * @sdrv: the driver to register
+ * Context: can sleep
+ */
 int spi_register_driver(struct spi_driver *sdrv)
 {
 	sdrv->driver.bus = &spi_bus_type;
@@ -183,7 +188,13 @@ static LIST_HEAD(board_list);
 static DECLARE_MUTEX(board_lock);
 
 
-/* On typical mainboards, this is purely internal; and it's not needed
+/**
+ * spi_new_device - instantiate one new SPI device
+ * @master: Controller to which device is connected
+ * @chip: Describes the SPI device
+ * Context: can sleep
+ *
+ * On typical mainboards, this is purely internal; and it's not needed
  * after board init creates the hard-wired devices.  Some development
  * platforms may not be able to use spi_register_board_info though, and
  * this is exported so that for example a USB or parport based adapter
@@ -251,7 +262,12 @@ fail:
 }
 EXPORT_SYMBOL_GPL(spi_new_device);
 
-/*
+/**
+ * spi_register_board_info - register SPI devices for a given board
+ * @info: array of chip descriptors
+ * @n: how many descriptors are provided
+ * Context: can sleep
+ *
  * Board-specific early init code calls this (probably during arch_initcall)
  * with segments of the SPI device table.  Any device nodes are created later,
  * after the relevant parent SPI controller (bus_num) is defined.  We keep
@@ -337,9 +353,10 @@ static struct class spi_master_class = {
 /**
  * spi_alloc_master - allocate SPI master controller
  * @dev: the controller, possibly using the platform_bus
- * @size: how much driver-private data to preallocate; the pointer to this
+ * @size: how much zeroed driver-private data to allocate; the pointer to this
  *	memory is in the class_data field of the returned class_device,
  *	accessible with spi_master_get_devdata().
+ * Context: can sleep
  *
  * This call is used only by SPI master controller drivers, which are the
  * only ones directly touching chip registers.  It's how they allocate
@@ -375,6 +392,7 @@ EXPORT_SYMBOL_GPL(spi_alloc_master);
 /**
  * spi_register_master - register SPI master controller
  * @master: initialized master, originally from spi_alloc_master()
+ * Context: can sleep
  *
  * SPI master controllers connect to their drivers using some non-SPI bus,
  * such as the platform bus.  The final stage of probe() in that code
@@ -437,6 +455,7 @@ static int __unregister(struct device *dev, void *unused)
 /**
  * spi_unregister_master - unregister SPI master controller
  * @master: the master being unregistered
+ * Context: can sleep
  *
  * This call is used only by SPI master controller drivers, which are the
  * only ones directly touching chip registers.
@@ -455,6 +474,7 @@ EXPORT_SYMBOL_GPL(spi_unregister_master);
 /**
  * spi_busnum_to_master - look up master associated with bus_num
  * @bus_num: the master's bus number
+ * Context: can sleep
  *
  * This call may be used with devices that are registered after
  * arch init time.  It returns a refcounted pointer to the relevant
@@ -492,6 +512,7 @@ static void spi_complete(void *arg)
  * spi_sync - blocking/synchronous SPI data transfers
  * @spi: device with which data will be exchanged
  * @message: describes the data transfers
+ * Context: can sleep
  *
  * This call may only be used from a context that may sleep.  The sleep
  * is non-interruptible, and has no timeout.  Low-overhead controller
@@ -508,7 +529,7 @@ static void spi_complete(void *arg)
  *
  * The return value is a negative error code if the message could not be
  * submitted, else zero.  When the value is zero, then message->status is
- * also defined:  it's the completion code for the transfer, either zero
+ * also defined;  it's the completion code for the transfer, either zero
  * or a negative error code from the controller driver.
  */
 int spi_sync(struct spi_device *spi, struct spi_message *message)
@@ -538,6 +559,7 @@ static u8	*buf;
  * @n_tx: size of txbuf, in bytes
  * @rxbuf: buffer into which data will be read
  * @n_rx: size of rxbuf, in bytes (need not be dma-safe)
+ * Context: can sleep
  *
  * This performs a half duplex MicroWire style transaction with the
  * device, sending txbuf and then reading rxbuf.  The return value
@@ -545,7 +567,8 @@ static u8	*buf;
  * This call may only be used from a context that may sleep.
  *
  * Parameters to this routine are always copied using a small buffer;
- * performance-sensitive or bulk transfer code should instead use
+ * portable code should never use this for more than 32 bytes.
+ * Performance-sensitive or bulk transfer code should instead use
  * spi_{async,sync}() calls with dma-safe buffers.
  */
 int spi_write_then_read(struct spi_device *spi,