1INTRODUCTION 2------------ 3 4Because not every I2C or SMBus adapter implements everything in the 5I2C specifications, a client can not trust that everything it needs 6is implemented when it is given the option to attach to an adapter: 7the client needs some way to check whether an adapter has the needed 8functionality. 9 10 11FUNCTIONALITY CONSTANTS 12----------------------- 13 14For the most up-to-date list of functionality constants, please check 15<linux/i2c.h>! 16 17 I2C_FUNC_I2C Plain i2c-level commands (Pure SMBus 18 adapters typically can not do these) 19 I2C_FUNC_10BIT_ADDR Handles the 10-bit address extensions 20 I2C_FUNC_PROTOCOL_MANGLING Knows about the I2C_M_REV_DIR_ADDR, 21 I2C_M_REV_DIR_ADDR and I2C_M_REV_DIR_NOSTART 22 flags (which modify the i2c protocol!) 23 I2C_FUNC_SMBUS_QUICK Handles the SMBus write_quick command 24 I2C_FUNC_SMBUS_READ_BYTE Handles the SMBus read_byte command 25 I2C_FUNC_SMBUS_WRITE_BYTE Handles the SMBus write_byte command 26 I2C_FUNC_SMBUS_READ_BYTE_DATA Handles the SMBus read_byte_data command 27 I2C_FUNC_SMBUS_WRITE_BYTE_DATA Handles the SMBus write_byte_data command 28 I2C_FUNC_SMBUS_READ_WORD_DATA Handles the SMBus read_word_data command 29 I2C_FUNC_SMBUS_WRITE_WORD_DATA Handles the SMBus write_byte_data command 30 I2C_FUNC_SMBUS_PROC_CALL Handles the SMBus process_call command 31 I2C_FUNC_SMBUS_READ_BLOCK_DATA Handles the SMBus read_block_data command 32 I2C_FUNC_SMBUS_WRITE_BLOCK_DATA Handles the SMBus write_block_data command 33 I2C_FUNC_SMBUS_READ_I2C_BLOCK Handles the SMBus read_i2c_block_data command 34 I2C_FUNC_SMBUS_WRITE_I2C_BLOCK Handles the SMBus write_i2c_block_data command 35 36A few combinations of the above flags are also defined for your convenience: 37 38 I2C_FUNC_SMBUS_BYTE Handles the SMBus read_byte 39 and write_byte commands 40 I2C_FUNC_SMBUS_BYTE_DATA Handles the SMBus read_byte_data 41 and write_byte_data commands 42 I2C_FUNC_SMBUS_WORD_DATA Handles the SMBus read_word_data 43 and write_word_data commands 44 I2C_FUNC_SMBUS_BLOCK_DATA Handles the SMBus read_block_data 45 and write_block_data commands 46 I2C_FUNC_SMBUS_I2C_BLOCK Handles the SMBus read_i2c_block_data 47 and write_i2c_block_data commands 48 I2C_FUNC_SMBUS_EMUL Handles all SMBus commands than can be 49 emulated by a real I2C adapter (using 50 the transparent emulation layer) 51 52 53ALGORITHM/ADAPTER IMPLEMENTATION 54-------------------------------- 55 56When you write a new algorithm driver, you will have to implement a 57function callback `functionality', that gets an i2c_adapter structure 58pointer as its only parameter: 59 60 struct i2c_algorithm { 61 /* Many other things of course; check <linux/i2c.h>! */ 62 u32 (*functionality) (struct i2c_adapter *); 63 } 64 65A typically implementation is given below, from i2c-algo-bit.c: 66 67 static u32 bit_func(struct i2c_adapter *adap) 68 { 69 return I2C_FUNC_SMBUS_EMUL | I2C_FUNC_10BIT_ADDR | 70 I2C_FUNC_PROTOCOL_MANGLING; 71 } 72 73 74 75CLIENT CHECKING 76--------------- 77 78Before a client tries to attach to an adapter, or even do tests to check 79whether one of the devices it supports is present on an adapter, it should 80check whether the needed functionality is present. There are two functions 81defined which should be used instead of calling the functionality hook 82in the algorithm structure directly: 83 84 /* Return the functionality mask */ 85 extern u32 i2c_get_functionality (struct i2c_adapter *adap); 86 87 /* Return 1 if adapter supports everything we need, 0 if not. */ 88 extern int i2c_check_functionality (struct i2c_adapter *adap, u32 func); 89 90This is a typical way to use these functions (from the writing-clients 91document): 92 int foo_detect_client(struct i2c_adapter *adapter, int address, 93 unsigned short flags, int kind) 94 { 95 /* Define needed variables */ 96 97 /* As the very first action, we check whether the adapter has the 98 needed functionality: we need the SMBus read_word_data, 99 write_word_data and write_byte functions in this example. */ 100 if (!i2c_check_functionality(adapter,I2C_FUNC_SMBUS_WORD_DATA | 101 I2C_FUNC_SMBUS_WRITE_BYTE)) 102 goto ERROR0; 103 104 /* Now we can do the real detection */ 105 106 ERROR0: 107 /* Return an error */ 108 } 109 110 111 112CHECKING THROUGH /DEV 113--------------------- 114 115If you try to access an adapter from a userspace program, you will have 116to use the /dev interface. You will still have to check whether the 117functionality you need is supported, of course. This is done using 118the I2C_FUNCS ioctl. An example, adapted from the lm_sensors i2c_detect 119program, is below: 120 121 int file; 122 if (file = open("/dev/i2c-0",O_RDWR) < 0) { 123 /* Some kind of error handling */ 124 exit(1); 125 } 126 if (ioctl(file,I2C_FUNCS,&funcs) < 0) { 127 /* Some kind of error handling */ 128 exit(1); 129 } 130 if (! (funcs & I2C_FUNC_SMBUS_QUICK)) { 131 /* Oops, the needed functionality (SMBus write_quick function) is 132 not available! */ 133 exit(1); 134 } 135 /* Now it is safe to use the SMBus write_quick command */ 136