docs: i2c: convert to ReST and add to driver-api bookset

Convert each file at I2C subsystem, renaming them to .rst and
adding to the driver-api book.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Wolfram Sang <wsa@the-dreams.de>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 156 insertions, 0 deletions

diff --git a/Documentation/i2c/functionality.rst b/Documentation/i2c/functionality.rst
new file mode 100644
index 000000000000..377507c56162
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+++ b/Documentation/i2c/functionality.rst
@@ -0,0 +1,156 @@
+=======================
+I2C/SMBus Functionality
+=======================
+
+INTRODUCTION
+------------
+
+Because not every I2C or SMBus adapter implements everything in the
+I2C specifications, a client can not trust that everything it needs
+is implemented when it is given the option to attach to an adapter:
+the client needs some way to check whether an adapter has the needed
+functionality.
+
+
+FUNCTIONALITY CONSTANTS
+-----------------------
+
+For the most up-to-date list of functionality constants, please check
+<uapi/linux/i2c.h>!
+
+  =============================== ==============================================
+  I2C_FUNC_I2C                    Plain i2c-level commands (Pure SMBus
+                                  adapters typically can not do these)
+  I2C_FUNC_10BIT_ADDR             Handles the 10-bit address extensions
+  I2C_FUNC_PROTOCOL_MANGLING      Knows about the I2C_M_IGNORE_NAK,
+                                  I2C_M_REV_DIR_ADDR and I2C_M_NO_RD_ACK
+                                  flags (which modify the I2C protocol!)
+  I2C_FUNC_NOSTART                Can skip repeated start sequence
+  I2C_FUNC_SMBUS_QUICK            Handles the SMBus write_quick command
+  I2C_FUNC_SMBUS_READ_BYTE        Handles the SMBus read_byte command
+  I2C_FUNC_SMBUS_WRITE_BYTE       Handles the SMBus write_byte command
+  I2C_FUNC_SMBUS_READ_BYTE_DATA   Handles the SMBus read_byte_data command
+  I2C_FUNC_SMBUS_WRITE_BYTE_DATA  Handles the SMBus write_byte_data command
+  I2C_FUNC_SMBUS_READ_WORD_DATA   Handles the SMBus read_word_data command
+  I2C_FUNC_SMBUS_WRITE_WORD_DATA  Handles the SMBus write_byte_data command
+  I2C_FUNC_SMBUS_PROC_CALL        Handles the SMBus process_call command
+  I2C_FUNC_SMBUS_READ_BLOCK_DATA  Handles the SMBus read_block_data command
+  I2C_FUNC_SMBUS_WRITE_BLOCK_DATA Handles the SMBus write_block_data command
+  I2C_FUNC_SMBUS_READ_I2C_BLOCK   Handles the SMBus read_i2c_block_data command
+  I2C_FUNC_SMBUS_WRITE_I2C_BLOCK  Handles the SMBus write_i2c_block_data command
+  =============================== ==============================================
+
+A few combinations of the above flags are also defined for your convenience:
+
+  =========================       ======================================
+  I2C_FUNC_SMBUS_BYTE             Handles the SMBus read_byte
+                                  and write_byte commands
+  I2C_FUNC_SMBUS_BYTE_DATA        Handles the SMBus read_byte_data
+                                  and write_byte_data commands
+  I2C_FUNC_SMBUS_WORD_DATA        Handles the SMBus read_word_data
+                                  and write_word_data commands
+  I2C_FUNC_SMBUS_BLOCK_DATA       Handles the SMBus read_block_data
+                                  and write_block_data commands
+  I2C_FUNC_SMBUS_I2C_BLOCK        Handles the SMBus read_i2c_block_data
+                                  and write_i2c_block_data commands
+  I2C_FUNC_SMBUS_EMUL             Handles all SMBus commands that can be
+                                  emulated by a real I2C adapter (using
+                                  the transparent emulation layer)
+  =========================       ======================================
+
+In kernel versions prior to 3.5 I2C_FUNC_NOSTART was implemented as
+part of I2C_FUNC_PROTOCOL_MANGLING.
+
+
+ADAPTER IMPLEMENTATION
+----------------------
+
+When you write a new adapter driver, you will have to implement a
+function callback ``functionality``. Typical implementations are given
+below.
+
+A typical SMBus-only adapter would list all the SMBus transactions it
+supports. This example comes from the i2c-piix4 driver::
+
+  static u32 piix4_func(struct i2c_adapter *adapter)
+  {
+	return I2C_FUNC_SMBUS_QUICK | I2C_FUNC_SMBUS_BYTE |
+	       I2C_FUNC_SMBUS_BYTE_DATA | I2C_FUNC_SMBUS_WORD_DATA |
+	       I2C_FUNC_SMBUS_BLOCK_DATA;
+  }
+
+A typical full-I2C adapter would use the following (from the i2c-pxa
+driver)::
+
+  static u32 i2c_pxa_functionality(struct i2c_adapter *adap)
+  {
+	return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL;
+  }
+
+I2C_FUNC_SMBUS_EMUL includes all the SMBus transactions (with the
+addition of I2C block transactions) which i2c-core can emulate using
+I2C_FUNC_I2C without any help from the adapter driver. The idea is
+to let the client drivers check for the support of SMBus functions
+without having to care whether the said functions are implemented in
+hardware by the adapter, or emulated in software by i2c-core on top
+of an I2C adapter.
+
+
+CLIENT CHECKING
+---------------
+
+Before a client tries to attach to an adapter, or even do tests to check
+whether one of the devices it supports is present on an adapter, it should
+check whether the needed functionality is present. The typical way to do
+this is (from the lm75 driver)::
+
+  static int lm75_detect(...)
+  {
+	(...)
+	if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA |
+				     I2C_FUNC_SMBUS_WORD_DATA))
+		goto exit;
+	(...)
+  }
+
+Here, the lm75 driver checks if the adapter can do both SMBus byte data
+and SMBus word data transactions. If not, then the driver won't work on
+this adapter and there's no point in going on. If the check above is
+successful, then the driver knows that it can call the following
+functions: i2c_smbus_read_byte_data(), i2c_smbus_write_byte_data(),
+i2c_smbus_read_word_data() and i2c_smbus_write_word_data(). As a rule of
+thumb, the functionality constants you test for with
+i2c_check_functionality() should match exactly the i2c_smbus_* functions
+which you driver is calling.
+
+Note that the check above doesn't tell whether the functionalities are
+implemented in hardware by the underlying adapter or emulated in
+software by i2c-core. Client drivers don't have to care about this, as
+i2c-core will transparently implement SMBus transactions on top of I2C
+adapters.
+
+
+CHECKING THROUGH /DEV
+---------------------
+
+If you try to access an adapter from a userspace program, you will have
+to use the /dev interface. You will still have to check whether the
+functionality you need is supported, of course. This is done using
+the I2C_FUNCS ioctl. An example, adapted from the i2cdetect program, is
+below::
+
+  int file;
+  if (file = open("/dev/i2c-0", O_RDWR) < 0) {
+	/* Some kind of error handling */
+	exit(1);
+  }
+  if (ioctl(file, I2C_FUNCS, &funcs) < 0) {
+	/* Some kind of error handling */
+	exit(1);
+  }
+  if (!(funcs & I2C_FUNC_SMBUS_QUICK)) {
+	/* Oops, the needed functionality (SMBus write_quick function) is
+           not available! */
+	exit(1);
+  }
+  /* Now it is safe to use the SMBus write_quick command */