[linux-2.6-block.git] / Documentation / i2c / 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 */
Commit	Line	Data
	1	INTRODUCTION
	2	------------
	3
	4	Because not every I2C or SMBus adapter implements everything in the
	5	I2C specifications, a client can not trust that everything it needs
	6	is implemented when it is given the option to attach to an adapter:
	7	the client needs some way to check whether an adapter has the needed
	8	functionality.
	9
	10
	11	FUNCTIONALITY CONSTANTS
	12	-----------------------
	13
	14	For the most up-to-date list of functionality constants, please check
	15	<uapi/linux/i2c.h>!
	16
	17	I2C_FUNC_I2C Plain i2c-level commands (Pure SMBus
	18	adapters typically can not do these)
	19	I2C_FUNC_10BIT_ADDR Handles the 10-bit address extensions
	20	I2C_FUNC_PROTOCOL_MANGLING Knows about the I2C_M_IGNORE_NAK,
	21	I2C_M_REV_DIR_ADDR and I2C_M_NO_RD_ACK
	22	flags (which modify the I2C protocol!)
	23	I2C_FUNC_NOSTART Can skip repeated start sequence
	24	I2C_FUNC_SMBUS_QUICK Handles the SMBus write_quick command
	25	I2C_FUNC_SMBUS_READ_BYTE Handles the SMBus read_byte command
	26	I2C_FUNC_SMBUS_WRITE_BYTE Handles the SMBus write_byte command
	27	I2C_FUNC_SMBUS_READ_BYTE_DATA Handles the SMBus read_byte_data command
	28	I2C_FUNC_SMBUS_WRITE_BYTE_DATA Handles the SMBus write_byte_data command
	29	I2C_FUNC_SMBUS_READ_WORD_DATA Handles the SMBus read_word_data command
	30	I2C_FUNC_SMBUS_WRITE_WORD_DATA Handles the SMBus write_byte_data command
	31	I2C_FUNC_SMBUS_PROC_CALL Handles the SMBus process_call command
	32	I2C_FUNC_SMBUS_READ_BLOCK_DATA Handles the SMBus read_block_data command
	33	I2C_FUNC_SMBUS_WRITE_BLOCK_DATA Handles the SMBus write_block_data command
	34	I2C_FUNC_SMBUS_READ_I2C_BLOCK Handles the SMBus read_i2c_block_data command
	35	I2C_FUNC_SMBUS_WRITE_I2C_BLOCK Handles the SMBus write_i2c_block_data command
	36
	37	A few combinations of the above flags are also defined for your convenience:
	38
	39	I2C_FUNC_SMBUS_BYTE Handles the SMBus read_byte
	40	and write_byte commands
	41	I2C_FUNC_SMBUS_BYTE_DATA Handles the SMBus read_byte_data
	42	and write_byte_data commands
	43	I2C_FUNC_SMBUS_WORD_DATA Handles the SMBus read_word_data
	44	and write_word_data commands
	45	I2C_FUNC_SMBUS_BLOCK_DATA Handles the SMBus read_block_data
	46	and write_block_data commands
	47	I2C_FUNC_SMBUS_I2C_BLOCK Handles the SMBus read_i2c_block_data
	48	and write_i2c_block_data commands
	49	I2C_FUNC_SMBUS_EMUL Handles all SMBus commands that can be
	50	emulated by a real I2C adapter (using
	51	the transparent emulation layer)
	52
	53	In kernel versions prior to 3.5 I2C_FUNC_NOSTART was implemented as
	54	part of I2C_FUNC_PROTOCOL_MANGLING.
	55
	56
	57	ADAPTER IMPLEMENTATION
	58	----------------------
	59
	60	When you write a new adapter driver, you will have to implement a
	61	function callback `functionality'. Typical implementations are given
	62	below.
	63
	64	A typical SMBus-only adapter would list all the SMBus transactions it
	65	supports. This example comes from the i2c-piix4 driver:
	66
	67	static u32 piix4_func(struct i2c_adapter *adapter)
	68	{
	69	return I2C_FUNC_SMBUS_QUICK \| I2C_FUNC_SMBUS_BYTE \|
	70	I2C_FUNC_SMBUS_BYTE_DATA \| I2C_FUNC_SMBUS_WORD_DATA \|
	71	I2C_FUNC_SMBUS_BLOCK_DATA;
	72	}
	73
	74	A typical full-I2C adapter would use the following (from the i2c-pxa
	75	driver):
	76
	77	static u32 i2c_pxa_functionality(struct i2c_adapter *adap)
	78	{
	79	return I2C_FUNC_I2C \| I2C_FUNC_SMBUS_EMUL;
	80	}
	81
	82	I2C_FUNC_SMBUS_EMUL includes all the SMBus transactions (with the
	83	addition of I2C block transactions) which i2c-core can emulate using
	84	I2C_FUNC_I2C without any help from the adapter driver. The idea is
	85	to let the client drivers check for the support of SMBus functions
	86	without having to care whether the said functions are implemented in
	87	hardware by the adapter, or emulated in software by i2c-core on top
	88	of an I2C adapter.
	89
	90
	91	CLIENT CHECKING
	92	---------------
	93
	94	Before a client tries to attach to an adapter, or even do tests to check
	95	whether one of the devices it supports is present on an adapter, it should
	96	check whether the needed functionality is present. The typical way to do
	97	this is (from the lm75 driver):
	98
	99	static int lm75_detect(...)
	100	{
	101	(...)
	102	if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA \|
	103	I2C_FUNC_SMBUS_WORD_DATA))
	104	goto exit;
	105	(...)
	106	}
	107
	108	Here, the lm75 driver checks if the adapter can do both SMBus byte data
	109	and SMBus word data transactions. If not, then the driver won't work on
	110	this adapter and there's no point in going on. If the check above is
	111	successful, then the driver knows that it can call the following
	112	functions: i2c_smbus_read_byte_data(), i2c_smbus_write_byte_data(),
	113	i2c_smbus_read_word_data() and i2c_smbus_write_word_data(). As a rule of
	114	thumb, the functionality constants you test for with
	115	i2c_check_functionality() should match exactly the i2c_smbus_* functions
	116	which you driver is calling.
	117
	118	Note that the check above doesn't tell whether the functionalities are
	119	implemented in hardware by the underlying adapter or emulated in
	120	software by i2c-core. Client drivers don't have to care about this, as
	121	i2c-core will transparently implement SMBus transactions on top of I2C
	122	adapters.
	123
	124
	125	CHECKING THROUGH /DEV
	126	---------------------
	127
	128	If you try to access an adapter from a userspace program, you will have
	129	to use the /dev interface. You will still have to check whether the
	130	functionality you need is supported, of course. This is done using
	131	the I2C_FUNCS ioctl. An example, adapted from the i2cdetect program, is
	132	below:
	133
	134	int file;
	135	if (file = open("/dev/i2c-0", O_RDWR) < 0) {
	136	/* Some kind of error handling */
	137	exit(1);
	138	}
	139	if (ioctl(file, I2C_FUNCS, &funcs) < 0) {
	140	/* Some kind of error handling */
	141	exit(1);
	142	}
	143	if (!(funcs & I2C_FUNC_SMBUS_QUICK)) {
	144	/* Oops, the needed functionality (SMBus write_quick function) is
	145	not available! */
	146	exit(1);
	147	}
	148	/* Now it is safe to use the SMBus write_quick command */