linux/Documentation/i2c/functionality.rst
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   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 */
 157