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, I2C_M_NOSTART and

I2C_M_NO_RD_ACK 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)

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 */