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-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_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 than 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 */