root/i2c/trunk/doc/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
<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_NOSTART
                                  flags (which modify the i2c protocol!)
  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 than can be
                                  emulated by a real I2C adapter (using
                                  the transparent emulation layer)


ALGORITHM/ADAPTER IMPLEMENTATION
--------------------------------

When you write a new algorithm driver, you will have to implement a
function callback `functionality', that gets an i2c_adapter structure
pointer as its only parameter:

  struct i2c_algorithm {
        /* Many other things of course; check <linux/i2c.h>! */
        u32 (*functionality) (struct i2c_adapter *);
  }

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

  static u32 bit_func(struct i2c_adapter *adap)
  {
        return I2C_FUNC_SMBUS_EMUL | I2C_FUNC_10BIT_ADDR | 
               I2C_FUNC_PROTOCOL_MANGLING;
  }



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. There are two functions
defined which should be used instead of calling the functionality hook
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
document):
  int foo_detect_client(struct i2c_adapter *adapter, int address, 
                          unsigned short flags, int kind)
  {
        /* Define needed variables */

        /* As the very first action, we check whether the adapter has the
           needed functionality: we need the SMBus read_word_data,
           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 */
  }



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 lm_sensors 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 */