linux/Documentation/i2c/functionality
<<
>>
Prefs
   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 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
  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
  53In kernel versions prior to 3.5 I2C_FUNC_NOSTART was implemented as
  54part of I2C_FUNC_PROTOCOL_MANGLING.
  55
  56
  57ADAPTER IMPLEMENTATION
  58----------------------
  59
  60When you write a new adapter driver, you will have to implement a
  61function callback `functionality'. Typical implementations are given
  62below.
  63
  64A typical SMBus-only adapter would list all the SMBus transactions it
  65supports. 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
  74A typical full-I2C adapter would use the following (from the i2c-pxa
  75driver):
  76
  77  static u32 i2c_pxa_functionality(struct i2c_adapter *adap)
  78  {
  79        return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL;
  80  }
  81
  82I2C_FUNC_SMBUS_EMUL includes all the SMBus transactions (with the
  83addition of I2C block transactions) which i2c-core can emulate using
  84I2C_FUNC_I2C without any help from the adapter driver. The idea is
  85to let the client drivers check for the support of SMBus functions
  86without having to care whether the said functions are implemented in
  87hardware by the adapter, or emulated in software by i2c-core on top
  88of an I2C adapter.
  89
  90
  91CLIENT CHECKING
  92---------------
  93
  94Before a client tries to attach to an adapter, or even do tests to check
  95whether one of the devices it supports is present on an adapter, it should
  96check whether the needed functionality is present. The typical way to do
  97this 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
 108Here, the lm75 driver checks if the adapter can do both SMBus byte data
 109and SMBus word data transactions. If not, then the driver won't work on
 110this adapter and there's no point in going on. If the check above is
 111successful, then the driver knows that it can call the following
 112functions: i2c_smbus_read_byte_data(), i2c_smbus_write_byte_data(),
 113i2c_smbus_read_word_data() and i2c_smbus_write_word_data(). As a rule of
 114thumb, the functionality constants you test for with
 115i2c_check_functionality() should match exactly the i2c_smbus_* functions
 116which you driver is calling.
 117
 118Note that the check above doesn't tell whether the functionalities are
 119implemented in hardware by the underlying adapter or emulated in
 120software by i2c-core. Client drivers don't have to care about this, as
 121i2c-core will transparently implement SMBus transactions on top of I2C
 122adapters.
 123
 124
 125CHECKING THROUGH /DEV
 126---------------------
 127
 128If you try to access an adapter from a userspace program, you will have
 129to use the /dev interface. You will still have to check whether the
 130functionality you need is supported, of course. This is done using
 131the I2C_FUNCS ioctl. An example, adapted from the i2cdetect program, is
 132below:
 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 */
 149
lxr.linux.no kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.