Documentation/i2c/functionality

   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 */