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_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 37A 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 54ADAPTER IMPLEMENTATION 55---------------------- 56 57When you write a new adapter driver, you will have to implement a 58function callback `functionality'. Typical implementations are given 59below. 60 61A typical SMBus-only adapter would list all the SMBus transactions it 62supports. This example comes from the i2c-piix4 driver: 63 64 static u32 piix4_func(struct i2c_adapter *adapter) 65 { 66 return I2C_FUNC_SMBUS_QUICK | I2C_FUNC_SMBUS_BYTE | 67 I2C_FUNC_SMBUS_BYTE_DATA | I2C_FUNC_SMBUS_WORD_DATA | 68 I2C_FUNC_SMBUS_BLOCK_DATA; 69 } 70 71A typical full-I2C adapter would use the following (from the i2c-pxa 72driver): 73 74 static u32 i2c_pxa_functionality(struct i2c_adapter *adap) 75 { 76 return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL; 77 } 78 79I2C_FUNC_SMBUS_EMUL includes all the SMBus transactions (with the 80addition of I2C block transactions) which i2c-core can emulate using 81I2C_FUNC_I2C without any help from the adapter driver. The idea is 82to let the client drivers check for the support of SMBus functions 83without having to care whether the said functions are implemented in 84hardware by the adapter, or emulated in software by i2c-core on top 85of an I2C adapter. 86 87 88CLIENT CHECKING 89--------------- 90 91Before a client tries to attach to an adapter, or even do tests to check 92whether one of the devices it supports is present on an adapter, it should 93check whether the needed functionality is present. The typical way to do 94this is (from the lm75 driver): 95 96 static int lm75_detect(...) 97 { 98 (...) 99 if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA | 100 I2C_FUNC_SMBUS_WORD_DATA)) 101 goto exit; 102 (...) 103 } 104 105Here, the lm75 driver checks if the adapter can do both SMBus byte data 106and SMBus word data transactions. If not, then the driver won't work on 107this adapter and there's no point in going on. If the check above is 108successful, then the driver knows that it can call the following 109functions: i2c_smbus_read_byte_data(), i2c_smbus_write_byte_data(), 110i2c_smbus_read_word_data() and i2c_smbus_write_word_data(). As a rule of 111thumb, the functionality constants you test for with 112i2c_check_functionality() should match exactly the i2c_smbus_* functions 113which you driver is calling. 114 115Note that the check above doesn't tell whether the functionalities are 116implemented in hardware by the underlying adapter or emulated in 117software by i2c-core. Client drivers don't have to care about this, as 118i2c-core will transparently implement SMBus transactions on top of I2C 119adapters. 120 121 122CHECKING THROUGH /DEV 123--------------------- 124 125If you try to access an adapter from a userspace program, you will have 126to use the /dev interface. You will still have to check whether the 127functionality you need is supported, of course. This is done using 128the I2C_FUNCS ioctl. An example, adapted from the i2cdetect program, is 129below: 130 131 int file; 132 if (file = open("/dev/i2c-0", O_RDWR) < 0) { 133 /* Some kind of error handling */ 134 exit(1); 135 } 136 if (ioctl(file, I2C_FUNCS, &funcs) < 0) { 137 /* Some kind of error handling */ 138 exit(1); 139 } 140 if (!(funcs & I2C_FUNC_SMBUS_QUICK)) { 141 /* Oops, the needed functionality (SMBus write_quick function) is 142 not available! */ 143 exit(1); 144 } 145 /* Now it is safe to use the SMBus write_quick command */ 146