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 <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 */