linux/Documentation/i2c/dev-interface.rst
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   1============================================
   2Implementing I2C device drivers in userspace
   3============================================
   4
   5Usually, I2C devices are controlled by a kernel driver. But it is also
   6possible to access all devices on an adapter from userspace, through
   7the /dev interface. You need to load module i2c-dev for this.
   8
   9Each registered I2C adapter gets a number, counting from 0. You can
  10examine /sys/class/i2c-dev/ to see what number corresponds to which adapter.
  11Alternatively, you can run "i2cdetect -l" to obtain a formatted list of all
  12I2C adapters present on your system at a given time. i2cdetect is part of
  13the i2c-tools package.
  14
  15I2C device files are character device files with major device number 89
  16and a minor device number corresponding to the number assigned as
  17explained above. They should be called "i2c-%d" (i2c-0, i2c-1, ...,
  18i2c-10, ...). All 256 minor device numbers are reserved for I2C.
  19
  20
  21C example
  22=========
  23
  24So let's say you want to access an I2C adapter from a C program.
  25First, you need to include these two headers::
  26
  27  #include <linux/i2c-dev.h>
  28  #include <i2c/smbus.h>
  29
  30Now, you have to decide which adapter you want to access. You should
  31inspect /sys/class/i2c-dev/ or run "i2cdetect -l" to decide this.
  32Adapter numbers are assigned somewhat dynamically, so you can not
  33assume much about them. They can even change from one boot to the next.
  34
  35Next thing, open the device file, as follows::
  36
  37  int file;
  38  int adapter_nr = 2; /* probably dynamically determined */
  39  char filename[20];
  40
  41  snprintf(filename, 19, "/dev/i2c-%d", adapter_nr);
  42  file = open(filename, O_RDWR);
  43  if (file < 0) {
  44    /* ERROR HANDLING; you can check errno to see what went wrong */
  45    exit(1);
  46  }
  47
  48When you have opened the device, you must specify with what device
  49address you want to communicate::
  50
  51  int addr = 0x40; /* The I2C address */
  52
  53  if (ioctl(file, I2C_SLAVE, addr) < 0) {
  54    /* ERROR HANDLING; you can check errno to see what went wrong */
  55    exit(1);
  56  }
  57
  58Well, you are all set up now. You can now use SMBus commands or plain
  59I2C to communicate with your device. SMBus commands are preferred if
  60the device supports them. Both are illustrated below::
  61
  62  __u8 reg = 0x10; /* Device register to access */
  63  __s32 res;
  64  char buf[10];
  65
  66  /* Using SMBus commands */
  67  res = i2c_smbus_read_word_data(file, reg);
  68  if (res < 0) {
  69    /* ERROR HANDLING: I2C transaction failed */
  70  } else {
  71    /* res contains the read word */
  72  }
  73
  74  /*
  75   * Using I2C Write, equivalent of
  76   * i2c_smbus_write_word_data(file, reg, 0x6543)
  77   */
  78  buf[0] = reg;
  79  buf[1] = 0x43;
  80  buf[2] = 0x65;
  81  if (write(file, buf, 3) != 3) {
  82    /* ERROR HANDLING: I2C transaction failed */
  83  }
  84
  85  /* Using I2C Read, equivalent of i2c_smbus_read_byte(file) */
  86  if (read(file, buf, 1) != 1) {
  87    /* ERROR HANDLING: I2C transaction failed */
  88  } else {
  89    /* buf[0] contains the read byte */
  90  }
  91
  92Note that only a subset of the I2C and SMBus protocols can be achieved by
  93the means of read() and write() calls. In particular, so-called combined
  94transactions (mixing read and write messages in the same transaction)
  95aren't supported. For this reason, this interface is almost never used by
  96user-space programs.
  97
  98IMPORTANT: because of the use of inline functions, you *have* to use
  99'-O' or some variation when you compile your program!
 100
 101
 102Full interface description
 103==========================
 104
 105The following IOCTLs are defined:
 106
 107``ioctl(file, I2C_SLAVE, long addr)``
 108  Change slave address. The address is passed in the 7 lower bits of the
 109  argument (except for 10 bit addresses, passed in the 10 lower bits in this
 110  case).
 111
 112``ioctl(file, I2C_TENBIT, long select)``
 113  Selects ten bit addresses if select not equals 0, selects normal 7 bit
 114  addresses if select equals 0. Default 0.  This request is only valid
 115  if the adapter has I2C_FUNC_10BIT_ADDR.
 116
 117``ioctl(file, I2C_PEC, long select)``
 118  Selects SMBus PEC (packet error checking) generation and verification
 119  if select not equals 0, disables if select equals 0. Default 0.
 120  Used only for SMBus transactions.  This request only has an effect if the
 121  the adapter has I2C_FUNC_SMBUS_PEC; it is still safe if not, it just
 122  doesn't have any effect.
 123
 124``ioctl(file, I2C_FUNCS, unsigned long *funcs)``
 125  Gets the adapter functionality and puts it in ``*funcs``.
 126
 127``ioctl(file, I2C_RDWR, struct i2c_rdwr_ioctl_data *msgset)``
 128  Do combined read/write transaction without stop in between.
 129  Only valid if the adapter has I2C_FUNC_I2C.  The argument is
 130  a pointer to a::
 131
 132    struct i2c_rdwr_ioctl_data {
 133      struct i2c_msg *msgs;  /* ptr to array of simple messages */
 134      int nmsgs;             /* number of messages to exchange */
 135    }
 136
 137  The msgs[] themselves contain further pointers into data buffers.
 138  The function will write or read data to or from that buffers depending
 139  on whether the I2C_M_RD flag is set in a particular message or not.
 140  The slave address and whether to use ten bit address mode has to be
 141  set in each message, overriding the values set with the above ioctl's.
 142
 143``ioctl(file, I2C_SMBUS, struct i2c_smbus_ioctl_data *args)``
 144  If possible, use the provided ``i2c_smbus_*`` methods described below instead
 145  of issuing direct ioctls.
 146
 147You can do plain I2C transactions by using read(2) and write(2) calls.
 148You do not need to pass the address byte; instead, set it through
 149ioctl I2C_SLAVE before you try to access the device.
 150
 151You can do SMBus level transactions (see documentation file smbus-protocol
 152for details) through the following functions::
 153
 154  __s32 i2c_smbus_write_quick(int file, __u8 value);
 155  __s32 i2c_smbus_read_byte(int file);
 156  __s32 i2c_smbus_write_byte(int file, __u8 value);
 157  __s32 i2c_smbus_read_byte_data(int file, __u8 command);
 158  __s32 i2c_smbus_write_byte_data(int file, __u8 command, __u8 value);
 159  __s32 i2c_smbus_read_word_data(int file, __u8 command);
 160  __s32 i2c_smbus_write_word_data(int file, __u8 command, __u16 value);
 161  __s32 i2c_smbus_process_call(int file, __u8 command, __u16 value);
 162  __s32 i2c_smbus_block_process_call(int file, __u8 command, __u8 length,
 163                                     __u8 *values);
 164  __s32 i2c_smbus_read_block_data(int file, __u8 command, __u8 *values);
 165  __s32 i2c_smbus_write_block_data(int file, __u8 command, __u8 length,
 166                                   __u8 *values);
 167
 168All these transactions return -1 on failure; you can read errno to see
 169what happened. The 'write' transactions return 0 on success; the
 170'read' transactions return the read value, except for read_block, which
 171returns the number of values read. The block buffers need not be longer
 172than 32 bytes.
 173
 174The above functions are made available by linking against the libi2c library,
 175which is provided by the i2c-tools project.  See:
 176https://git.kernel.org/pub/scm/utils/i2c-tools/i2c-tools.git/.
 177
 178
 179Implementation details
 180======================
 181
 182For the interested, here's the code flow which happens inside the kernel
 183when you use the /dev interface to I2C:
 184
 1851) Your program opens /dev/i2c-N and calls ioctl() on it, as described in
 186   section "C example" above.
 187
 1882) These open() and ioctl() calls are handled by the i2c-dev kernel
 189   driver: see i2c-dev.c:i2cdev_open() and i2c-dev.c:i2cdev_ioctl(),
 190   respectively. You can think of i2c-dev as a generic I2C chip driver
 191   that can be programmed from user-space.
 192
 1933) Some ioctl() calls are for administrative tasks and are handled by
 194   i2c-dev directly. Examples include I2C_SLAVE (set the address of the
 195   device you want to access) and I2C_PEC (enable or disable SMBus error
 196   checking on future transactions.)
 197
 1984) Other ioctl() calls are converted to in-kernel function calls by
 199   i2c-dev. Examples include I2C_FUNCS, which queries the I2C adapter
 200   functionality using i2c.h:i2c_get_functionality(), and I2C_SMBUS, which
 201   performs an SMBus transaction using i2c-core-smbus.c:i2c_smbus_xfer().
 202
 203   The i2c-dev driver is responsible for checking all the parameters that
 204   come from user-space for validity. After this point, there is no
 205   difference between these calls that came from user-space through i2c-dev
 206   and calls that would have been performed by kernel I2C chip drivers
 207   directly. This means that I2C bus drivers don't need to implement
 208   anything special to support access from user-space.
 209
 2105) These i2c.h functions are wrappers to the actual implementation of
 211   your I2C bus driver. Each adapter must declare callback functions
 212   implementing these standard calls. i2c.h:i2c_get_functionality() calls
 213   i2c_adapter.algo->functionality(), while
 214   i2c-core-smbus.c:i2c_smbus_xfer() calls either
 215   adapter.algo->smbus_xfer() if it is implemented, or if not,
 216   i2c-core-smbus.c:i2c_smbus_xfer_emulated() which in turn calls
 217   i2c_adapter.algo->master_xfer().
 218
 219After your I2C bus driver has processed these requests, execution runs
 220up the call chain, with almost no processing done, except by i2c-dev to
 221package the returned data, if any, in suitable format for the ioctl.
 222