Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 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 <uapi/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 and I2C_M_NO_RD_ACK 22 flags (which modify the I2C protocol!) 23 I2C_FUNC_NOSTART Can skip repeated start sequence 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 that can be 50 emulated by a real I2C adapter (using 51 the transparent emulation layer) 52 53 In kernel versions prior to 3.5 I2C_FUNC_NOSTART was implemented as 54 part of I2C_FUNC_PROTOCOL_MANGLING. 55 56 57 ADAPTER IMPLEMENTATION 58 ---------------------- 59 60 When you write a new adapter driver, you will have to implement a 61 function callback `functionality'. Typical implementations are given 62 below. 63 64 A typical SMBus-only adapter would list all the SMBus transactions it 65 supports. This example comes from the i2c-piix4 driver: 66 67 static u32 piix4_func(struct i2c_adapter *adapter) 68 { 69 return I2C_FUNC_SMBUS_QUICK | I2C_FUNC_SMBUS_BYTE | 70 I2C_FUNC_SMBUS_BYTE_DATA | I2C_FUNC_SMBUS_WORD_DATA | 71 I2C_FUNC_SMBUS_BLOCK_DATA; 72 } 73 74 A typical full-I2C adapter would use the following (from the i2c-pxa 75 driver): 76 77 static u32 i2c_pxa_functionality(struct i2c_adapter *adap) 78 { 79 return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL; 80 } 81 82 I2C_FUNC_SMBUS_EMUL includes all the SMBus transactions (with the 83 addition of I2C block transactions) which i2c-core can emulate using 84 I2C_FUNC_I2C without any help from the adapter driver. The idea is 85 to let the client drivers check for the support of SMBus functions 86 without having to care whether the said functions are implemented in 87 hardware by the adapter, or emulated in software by i2c-core on top 88 of an I2C adapter. 89 90 91 CLIENT CHECKING 92 --------------- 93 94 Before a client tries to attach to an adapter, or even do tests to check 95 whether one of the devices it supports is present on an adapter, it should 96 check whether the needed functionality is present. The typical way to do 97 this is (from the lm75 driver): 98 99 static int lm75_detect(...) 100 { 101 (...) 102 if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA | 103 I2C_FUNC_SMBUS_WORD_DATA)) 104 goto exit; 105 (...) 106 } 107 108 Here, the lm75 driver checks if the adapter can do both SMBus byte data 109 and SMBus word data transactions. If not, then the driver won't work on 110 this adapter and there's no point in going on. If the check above is 111 successful, then the driver knows that it can call the following 112 functions: i2c_smbus_read_byte_data(), i2c_smbus_write_byte_data(), 113 i2c_smbus_read_word_data() and i2c_smbus_write_word_data(). As a rule of 114 thumb, the functionality constants you test for with 115 i2c_check_functionality() should match exactly the i2c_smbus_* functions 116 which you driver is calling. 117 118 Note that the check above doesn't tell whether the functionalities are 119 implemented in hardware by the underlying adapter or emulated in 120 software by i2c-core. Client drivers don't have to care about this, as 121 i2c-core will transparently implement SMBus transactions on top of I2C 122 adapters. 123 124 125 CHECKING THROUGH /DEV 126 --------------------- 127 128 If you try to access an adapter from a userspace program, you will have 129 to use the /dev interface. You will still have to check whether the 130 functionality you need is supported, of course. This is done using 131 the I2C_FUNCS ioctl. An example, adapted from the i2cdetect program, is 132 below: 133 134 int file; 135 if (file = open("/dev/i2c-0", O_RDWR) < 0) { 136 /* Some kind of error handling */ 137 exit(1); 138 } 139 if (ioctl(file, I2C_FUNCS, &funcs) < 0) { 140 /* Some kind of error handling */ 141 exit(1); 142 } 143 if (!(funcs & I2C_FUNC_SMBUS_QUICK)) { 144 /* Oops, the needed functionality (SMBus write_quick function) is 145 not available! */ 146 exit(1); 147 } 148 /* Now it is safe to use the SMBus write_quick command */