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Diffstat (limited to 'Documentation/i2c/functionality')
| -rw-r--r-- | Documentation/i2c/functionality | 135 |
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diff --git a/Documentation/i2c/functionality b/Documentation/i2c/functionality new file mode 100644 index 0000000..8a78a95 --- /dev/null +++ b/Documentation/i2c/functionality @@ -0,0 +1,135 @@ +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_REV_DIR_ADDR, + I2C_M_REV_DIR_ADDR and I2C_M_REV_DIR_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 i2c_detect +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 */ |
