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Commit 88b28328 authored by Jean Delvare's avatar Jean Delvare Committed by Jean Delvare
Browse files

i2c: Improve the functionality documentation 



Attempt to make the documentation about the I2C/SMBus functionality
checking API clearer.

Signed-off-by: default avatarJean Delvare <khali@linux-fr.org>
parent 1a31a88f

Documentation/i2c/functionality

+52 −43
Original line number Original line Diff line number Diff line
@@ -51,26 +51,38 @@ A few combinations of the above flags are also defined for your convenience: 
                                  the transparent emulation layer)

                                  the transparent emulation layer)









ALGORITHM/ADAPTER IMPLEMENTATION

ADAPTER IMPLEMENTATION

--------------------------------

----------------------





When you write a new algorithm driver, you will have to implement a

When you write a new adapter driver, you will have to implement a

function callback `functionality', that gets an i2c_adapter structure

function callback `functionality'. Typical implementations are given

pointer as its only parameter:

below.





  struct i2c_algorithm {

A typical SMBus-only adapter would list all the SMBus transactions it

	/* Many other things of course; check <linux/i2c.h>! */

supports. This example comes from the i2c-piix4 driver:

	u32 (*functionality) (struct i2c_adapter *);



  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 typically implementation is given below, from i2c-algo-bit.c:

A typical full-I2C adapter would use the following (from the i2c-pxa

driver):





  static u32 bit_func(struct i2c_adapter *adap)

  static u32 i2c_pxa_functionality(struct i2c_adapter *adap)

  {

  {

	return I2C_FUNC_SMBUS_EMUL | I2C_FUNC_10BIT_ADDR | 

	return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL;

	       I2C_FUNC_PROTOCOL_MANGLING;

  }

  }





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

CLIENT CHECKING
@@ -78,36 +90,33 @@ CLIENT CHECKING 




Before a client tries to attach to an adapter, or even do tests to check

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

whether one of the devices it supports is present on an adapter, it should

check whether the needed functionality is present. There are two functions

check whether the needed functionality is present. The typical way to do

defined which should be used instead of calling the functionality hook

this is (from the lm75 driver):

in the algorithm structure directly:



  /* Return the functionality mask */

  extern u32 i2c_get_functionality (struct i2c_adapter *adap);



  /* Return 1 if adapter supports everything we need, 0 if not. */

  extern int i2c_check_functionality (struct i2c_adapter *adap, u32 func);





This is a typical way to use these functions (from the writing-clients

  static int lm75_detect(...)

document):

  int foo_detect_client(struct i2c_adapter *adapter, int address, 

                          unsigned short flags, int kind)

  {

  {

	/* Define needed variables */

	(...)



	if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA |

	/* As the very first action, we check whether the adapter has the

				     I2C_FUNC_SMBUS_WORD_DATA))

	   needed functionality: we need the SMBus read_word_data,

		goto exit;

           write_word_data and write_byte functions in this example. */

	(...)

	if (!i2c_check_functionality(adapter,I2C_FUNC_SMBUS_WORD_DATA |

	                                     I2C_FUNC_SMBUS_WRITE_BYTE))

		goto ERROR0;



	/* Now we can do the real detection */



	ERROR0:

		/* Return an error */

  }

  }





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

CHECKING THROUGH /DEV
@@ -116,8 +125,8 @@ CHECKING THROUGH /DEV 
If you try to access an adapter from a userspace program, you will have

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

to use the /dev interface. You will still have to check whether the

functionality you need is supported, of course. This is done using

functionality you need is supported, of course. This is done using

the I2C_FUNCS ioctl. An example, adapted from the lm_sensors i2cdetect

the I2C_FUNCS ioctl. An example, adapted from the i2cdetect program, is

program, is below:

below:





  int file;

  int file;

  if (file = open("/dev/i2c-0", O_RDWR) < 0) {

  if (file = open("/dev/i2c-0", O_RDWR) < 0) {