functionality.rst (6504B)
1======================= 2I2C/SMBus Functionality 3======================= 4 5INTRODUCTION 6------------ 7 8Because not every I2C or SMBus adapter implements everything in the 9I2C specifications, a client can not trust that everything it needs 10is implemented when it is given the option to attach to an adapter: 11the client needs some way to check whether an adapter has the needed 12functionality. 13 14 15FUNCTIONALITY CONSTANTS 16----------------------- 17 18For the most up-to-date list of functionality constants, please check 19<uapi/linux/i2c.h>! 20 21 =============================== ============================================== 22 I2C_FUNC_I2C Plain i2c-level commands (Pure SMBus 23 adapters typically can not do these) 24 I2C_FUNC_10BIT_ADDR Handles the 10-bit address extensions 25 I2C_FUNC_PROTOCOL_MANGLING Knows about the I2C_M_IGNORE_NAK, 26 I2C_M_REV_DIR_ADDR and I2C_M_NO_RD_ACK 27 flags (which modify the I2C protocol!) 28 I2C_FUNC_NOSTART Can skip repeated start sequence 29 I2C_FUNC_SMBUS_QUICK Handles the SMBus write_quick command 30 I2C_FUNC_SMBUS_READ_BYTE Handles the SMBus read_byte command 31 I2C_FUNC_SMBUS_WRITE_BYTE Handles the SMBus write_byte command 32 I2C_FUNC_SMBUS_READ_BYTE_DATA Handles the SMBus read_byte_data command 33 I2C_FUNC_SMBUS_WRITE_BYTE_DATA Handles the SMBus write_byte_data command 34 I2C_FUNC_SMBUS_READ_WORD_DATA Handles the SMBus read_word_data command 35 I2C_FUNC_SMBUS_WRITE_WORD_DATA Handles the SMBus write_byte_data command 36 I2C_FUNC_SMBUS_PROC_CALL Handles the SMBus process_call command 37 I2C_FUNC_SMBUS_READ_BLOCK_DATA Handles the SMBus read_block_data command 38 I2C_FUNC_SMBUS_WRITE_BLOCK_DATA Handles the SMBus write_block_data command 39 I2C_FUNC_SMBUS_READ_I2C_BLOCK Handles the SMBus read_i2c_block_data command 40 I2C_FUNC_SMBUS_WRITE_I2C_BLOCK Handles the SMBus write_i2c_block_data command 41 =============================== ============================================== 42 43A few combinations of the above flags are also defined for your convenience: 44 45 ========================= ====================================== 46 I2C_FUNC_SMBUS_BYTE Handles the SMBus read_byte 47 and write_byte commands 48 I2C_FUNC_SMBUS_BYTE_DATA Handles the SMBus read_byte_data 49 and write_byte_data commands 50 I2C_FUNC_SMBUS_WORD_DATA Handles the SMBus read_word_data 51 and write_word_data commands 52 I2C_FUNC_SMBUS_BLOCK_DATA Handles the SMBus read_block_data 53 and write_block_data commands 54 I2C_FUNC_SMBUS_I2C_BLOCK Handles the SMBus read_i2c_block_data 55 and write_i2c_block_data commands 56 I2C_FUNC_SMBUS_EMUL Handles all SMBus commands that can be 57 emulated by a real I2C adapter (using 58 the transparent emulation layer) 59 ========================= ====================================== 60 61In kernel versions prior to 3.5 I2C_FUNC_NOSTART was implemented as 62part of I2C_FUNC_PROTOCOL_MANGLING. 63 64 65ADAPTER IMPLEMENTATION 66---------------------- 67 68When you write a new adapter driver, you will have to implement a 69function callback ``functionality``. Typical implementations are given 70below. 71 72A typical SMBus-only adapter would list all the SMBus transactions it 73supports. This example comes from the i2c-piix4 driver:: 74 75 static u32 piix4_func(struct i2c_adapter *adapter) 76 { 77 return I2C_FUNC_SMBUS_QUICK | I2C_FUNC_SMBUS_BYTE | 78 I2C_FUNC_SMBUS_BYTE_DATA | I2C_FUNC_SMBUS_WORD_DATA | 79 I2C_FUNC_SMBUS_BLOCK_DATA; 80 } 81 82A typical full-I2C adapter would use the following (from the i2c-pxa 83driver):: 84 85 static u32 i2c_pxa_functionality(struct i2c_adapter *adap) 86 { 87 return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL; 88 } 89 90I2C_FUNC_SMBUS_EMUL includes all the SMBus transactions (with the 91addition of I2C block transactions) which i2c-core can emulate using 92I2C_FUNC_I2C without any help from the adapter driver. The idea is 93to let the client drivers check for the support of SMBus functions 94without having to care whether the said functions are implemented in 95hardware by the adapter, or emulated in software by i2c-core on top 96of an I2C adapter. 97 98 99CLIENT CHECKING 100--------------- 101 102Before a client tries to attach to an adapter, or even do tests to check 103whether one of the devices it supports is present on an adapter, it should 104check whether the needed functionality is present. The typical way to do 105this is (from the lm75 driver):: 106 107 static int lm75_detect(...) 108 { 109 (...) 110 if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA | 111 I2C_FUNC_SMBUS_WORD_DATA)) 112 goto exit; 113 (...) 114 } 115 116Here, the lm75 driver checks if the adapter can do both SMBus byte data 117and SMBus word data transactions. If not, then the driver won't work on 118this adapter and there's no point in going on. If the check above is 119successful, then the driver knows that it can call the following 120functions: i2c_smbus_read_byte_data(), i2c_smbus_write_byte_data(), 121i2c_smbus_read_word_data() and i2c_smbus_write_word_data(). As a rule of 122thumb, the functionality constants you test for with 123i2c_check_functionality() should match exactly the i2c_smbus_* functions 124which you driver is calling. 125 126Note that the check above doesn't tell whether the functionalities are 127implemented in hardware by the underlying adapter or emulated in 128software by i2c-core. Client drivers don't have to care about this, as 129i2c-core will transparently implement SMBus transactions on top of I2C 130adapters. 131 132 133CHECKING THROUGH /DEV 134--------------------- 135 136If you try to access an adapter from a userspace program, you will have 137to use the /dev interface. You will still have to check whether the 138functionality you need is supported, of course. This is done using 139the I2C_FUNCS ioctl. An example, adapted from the i2cdetect program, is 140below:: 141 142 int file; 143 if (file = open("/dev/i2c-0", O_RDWR) < 0) { 144 /* Some kind of error handling */ 145 exit(1); 146 } 147 if (ioctl(file, I2C_FUNCS, &funcs) < 0) { 148 /* Some kind of error handling */ 149 exit(1); 150 } 151 if (!(funcs & I2C_FUNC_SMBUS_QUICK)) { 152 /* Oops, the needed functionality (SMBus write_quick function) is 153 not available! */ 154 exit(1); 155 } 156 /* Now it is safe to use the SMBus write_quick command */