Based on kernel version 2.6.26. Page generated on 2008-07-16 21:12 EST.
1 INTRODUCTION 2 ------------ 3 4 Because not every I2C or SMBus adapter implements everything in the 5 I2C specifications, a client can not trust that everything it needs 6 is implemented when it is given the option to attach to an adapter: 7 the client needs some way to check whether an adapter has the needed 8 functionality. 9 10 11 FUNCTIONALITY CONSTANTS 12 ----------------------- 13 14 For 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_IGNORE_NAK, 21 I2C_M_REV_DIR_ADDR, I2C_M_NOSTART and 22 I2C_M_NO_RD_ACK flags (which modify the 23 I2C protocol!) 24 I2C_FUNC_SMBUS_QUICK Handles the SMBus write_quick command 25 I2C_FUNC_SMBUS_READ_BYTE Handles the SMBus read_byte command 26 I2C_FUNC_SMBUS_WRITE_BYTE Handles the SMBus write_byte command 27 I2C_FUNC_SMBUS_READ_BYTE_DATA Handles the SMBus read_byte_data command 28 I2C_FUNC_SMBUS_WRITE_BYTE_DATA Handles the SMBus write_byte_data command 29 I2C_FUNC_SMBUS_READ_WORD_DATA Handles the SMBus read_word_data command 30 I2C_FUNC_SMBUS_WRITE_WORD_DATA Handles the SMBus write_byte_data command 31 I2C_FUNC_SMBUS_PROC_CALL Handles the SMBus process_call command 32 I2C_FUNC_SMBUS_READ_BLOCK_DATA Handles the SMBus read_block_data command 33 I2C_FUNC_SMBUS_WRITE_BLOCK_DATA Handles the SMBus write_block_data command 34 I2C_FUNC_SMBUS_READ_I2C_BLOCK Handles the SMBus read_i2c_block_data command 35 I2C_FUNC_SMBUS_WRITE_I2C_BLOCK Handles the SMBus write_i2c_block_data command 36 37 A few combinations of the above flags are also defined for your convenience: 38 39 I2C_FUNC_SMBUS_BYTE Handles the SMBus read_byte 40 and write_byte commands 41 I2C_FUNC_SMBUS_BYTE_DATA Handles the SMBus read_byte_data 42 and write_byte_data commands 43 I2C_FUNC_SMBUS_WORD_DATA Handles the SMBus read_word_data 44 and write_word_data commands 45 I2C_FUNC_SMBUS_BLOCK_DATA Handles the SMBus read_block_data 46 and write_block_data commands 47 I2C_FUNC_SMBUS_I2C_BLOCK Handles the SMBus read_i2c_block_data 48 and write_i2c_block_data commands 49 I2C_FUNC_SMBUS_EMUL Handles all SMBus commands than can be 50 emulated by a real I2C adapter (using 51 the transparent emulation layer) 52 53 54 ALGORITHM/ADAPTER IMPLEMENTATION 55 -------------------------------- 56 57 When you write a new algorithm driver, you will have to implement a 58 function callback `functionality', that gets an i2c_adapter structure 59 pointer as its only parameter: 60 61 struct i2c_algorithm { 62 /* Many other things of course; check <linux/i2c.h>! */ 63 u32 (*functionality) (struct i2c_adapter *); 64 } 65 66 A typically implementation is given below, from i2c-algo-bit.c: 67 68 static u32 bit_func(struct i2c_adapter *adap) 69 { 70 return I2C_FUNC_SMBUS_EMUL | I2C_FUNC_10BIT_ADDR | 71 I2C_FUNC_PROTOCOL_MANGLING; 72 } 73 74 75 76 CLIENT CHECKING 77 --------------- 78 79 Before a client tries to attach to an adapter, or even do tests to check 80 whether one of the devices it supports is present on an adapter, it should 81 check whether the needed functionality is present. There are two functions 82 defined which should be used instead of calling the functionality hook 83 in the algorithm structure directly: 84 85 /* Return the functionality mask */ 86 extern u32 i2c_get_functionality (struct i2c_adapter *adap); 87 88 /* Return 1 if adapter supports everything we need, 0 if not. */ 89 extern int i2c_check_functionality (struct i2c_adapter *adap, u32 func); 90 91 This is a typical way to use these functions (from the writing-clients 92 document): 93 int foo_detect_client(struct i2c_adapter *adapter, int address, 94 unsigned short flags, int kind) 95 { 96 /* Define needed variables */ 97 98 /* As the very first action, we check whether the adapter has the 99 needed functionality: we need the SMBus read_word_data, 100 write_word_data and write_byte functions in this example. */ 101 if (!i2c_check_functionality(adapter,I2C_FUNC_SMBUS_WORD_DATA | 102 I2C_FUNC_SMBUS_WRITE_BYTE)) 103 goto ERROR0; 104 105 /* Now we can do the real detection */ 106 107 ERROR0: 108 /* Return an error */ 109 } 110 111 112 113 CHECKING THROUGH /DEV 114 --------------------- 115 116 If you try to access an adapter from a userspace program, you will have 117 to use the /dev interface. You will still have to check whether the 118 functionality you need is supported, of course. This is done using 119 the I2C_FUNCS ioctl. An example, adapted from the lm_sensors i2cdetect 120 program, is below: 121 122 int file; 123 if (file = open("/dev/i2c-0",O_RDWR) < 0) { 124 /* Some kind of error handling */ 125 exit(1); 126 } 127 if (ioctl(file,I2C_FUNCS,&funcs) < 0) { 128 /* Some kind of error handling */ 129 exit(1); 130 } 131 if (! (funcs & I2C_FUNC_SMBUS_QUICK)) { 132 /* Oops, the needed functionality (SMBus write_quick function) is 133 not available! */ 134 exit(1); 135 } 136 /* Now it is safe to use the SMBus write_quick command */