1Usually, i2c devices are controlled by a kernel driver. But it is also
   2possible to access all devices on an adapter from userspace, through
   3the /dev interface. You need to load module i2c-dev for this.
   5Each registered i2c adapter gets a number, counting from 0. You can
   6examine /sys/class/i2c-dev/ to see what number corresponds to which adapter.
   7Alternatively, you can run "i2cdetect -l" to obtain a formatted list of all
   8i2c adapters present on your system at a given time. i2cdetect is part of
   9the i2c-tools package.
  11I2C device files are character device files with major device number 89
  12and a minor device number corresponding to the number assigned as
  13explained above. They should be called "i2c-%d" (i2c-0, i2c-1, ...,
  14i2c-10, ...). All 256 minor device numbers are reserved for i2c.
  17C example
  20So let's say you want to access an i2c adapter from a C program.
  21First, you need to include these two headers:
  23  #include <linux/i2c-dev.h>
  24  #include <i2c/smbus.h>
  26Now, you have to decide which adapter you want to access. You should
  27inspect /sys/class/i2c-dev/ or run "i2cdetect -l" to decide this.
  28Adapter numbers are assigned somewhat dynamically, so you can not
  29assume much about them. They can even change from one boot to the next.
  31Next thing, open the device file, as follows:
  33  int file;
  34  int adapter_nr = 2; /* probably dynamically determined */
  35  char filename[20];
  37  snprintf(filename, 19, "/dev/i2c-%d", adapter_nr);
  38  file = open(filename, O_RDWR);
  39  if (file < 0) {
  40    /* ERROR HANDLING; you can check errno to see what went wrong */
  41    exit(1);
  42  }
  44When you have opened the device, you must specify with what device
  45address you want to communicate:
  47  int addr = 0x40; /* The I2C address */
  49  if (ioctl(file, I2C_SLAVE, addr) < 0) {
  50    /* ERROR HANDLING; you can check errno to see what went wrong */
  51    exit(1);
  52  }
  54Well, you are all set up now. You can now use SMBus commands or plain
  55I2C to communicate with your device. SMBus commands are preferred if
  56the device supports them. Both are illustrated below.
  58  __u8 reg = 0x10; /* Device register to access */
  59  __s32 res;
  60  char buf[10];
  62  /* Using SMBus commands */
  63  res = i2c_smbus_read_word_data(file, reg);
  64  if (res < 0) {
  65    /* ERROR HANDLING: i2c transaction failed */
  66  } else {
  67    /* res contains the read word */
  68  }
  70  /*
  71   * Using I2C Write, equivalent of
  72   * i2c_smbus_write_word_data(file, reg, 0x6543)
  73   */
  74  buf[0] = reg;
  75  buf[1] = 0x43;
  76  buf[2] = 0x65;
  77  if (write(file, buf, 3) != 3) {
  78    /* ERROR HANDLING: i2c transaction failed */
  79  }
  81  /* Using I2C Read, equivalent of i2c_smbus_read_byte(file) */
  82  if (read(file, buf, 1) != 1) {
  83    /* ERROR HANDLING: i2c transaction failed */
  84  } else {
  85    /* buf[0] contains the read byte */
  86  }
  88Note that only a subset of the I2C and SMBus protocols can be achieved by
  89the means of read() and write() calls. In particular, so-called combined
  90transactions (mixing read and write messages in the same transaction)
  91aren't supported. For this reason, this interface is almost never used by
  92user-space programs.
  94IMPORTANT: because of the use of inline functions, you *have* to use
  95'-O' or some variation when you compile your program!
  98Full interface description
 101The following IOCTLs are defined:
 103ioctl(file, I2C_SLAVE, long addr)
 104  Change slave address. The address is passed in the 7 lower bits of the
 105  argument (except for 10 bit addresses, passed in the 10 lower bits in this
 106  case).
 108ioctl(file, I2C_TENBIT, long select)
 109  Selects ten bit addresses if select not equals 0, selects normal 7 bit
 110  addresses if select equals 0. Default 0.  This request is only valid
 111  if the adapter has I2C_FUNC_10BIT_ADDR.
 113ioctl(file, I2C_PEC, long select)
 114  Selects SMBus PEC (packet error checking) generation and verification
 115  if select not equals 0, disables if select equals 0. Default 0.
 116  Used only for SMBus transactions.  This request only has an effect if the
 117  the adapter has I2C_FUNC_SMBUS_PEC; it is still safe if not, it just
 118  doesn't have any effect.
 120ioctl(file, I2C_FUNCS, unsigned long *funcs)
 121  Gets the adapter functionality and puts it in *funcs.
 123ioctl(file, I2C_RDWR, struct i2c_rdwr_ioctl_data *msgset)
 124  Do combined read/write transaction without stop in between.
 125  Only valid if the adapter has I2C_FUNC_I2C.  The argument is
 126  a pointer to a
 128  struct i2c_rdwr_ioctl_data {
 129      struct i2c_msg *msgs;  /* ptr to array of simple messages */
 130      int nmsgs;             /* number of messages to exchange */
 131  }
 133  The msgs[] themselves contain further pointers into data buffers.
 134  The function will write or read data to or from that buffers depending
 135  on whether the I2C_M_RD flag is set in a particular message or not.
 136  The slave address and whether to use ten bit address mode has to be
 137  set in each message, overriding the values set with the above ioctl's.
 139ioctl(file, I2C_SMBUS, struct i2c_smbus_ioctl_data *args)
 140  If possible, use the provided i2c_smbus_* methods described below instead
 141  of issuing direct ioctls.
 143You can do plain i2c transactions by using read(2) and write(2) calls.
 144You do not need to pass the address byte; instead, set it through
 145ioctl I2C_SLAVE before you try to access the device.
 147You can do SMBus level transactions (see documentation file smbus-protocol
 148for details) through the following functions:
 149  __s32 i2c_smbus_write_quick(int file, __u8 value);
 150  __s32 i2c_smbus_read_byte(int file);
 151  __s32 i2c_smbus_write_byte(int file, __u8 value);
 152  __s32 i2c_smbus_read_byte_data(int file, __u8 command);
 153  __s32 i2c_smbus_write_byte_data(int file, __u8 command, __u8 value);
 154  __s32 i2c_smbus_read_word_data(int file, __u8 command);
 155  __s32 i2c_smbus_write_word_data(int file, __u8 command, __u16 value);
 156  __s32 i2c_smbus_process_call(int file, __u8 command, __u16 value);
 157  __s32 i2c_smbus_read_block_data(int file, __u8 command, __u8 *values);
 158  __s32 i2c_smbus_write_block_data(int file, __u8 command, __u8 length,
 159                                   __u8 *values);
 160All these transactions return -1 on failure; you can read errno to see
 161what happened. The 'write' transactions return 0 on success; the
 162'read' transactions return the read value, except for read_block, which
 163returns the number of values read. The block buffers need not be longer
 164than 32 bytes.
 166The above functions are made available by linking against the libi2c library,
 167which is provided by the i2c-tools project.  See:
 171Implementation details
 174For the interested, here's the code flow which happens inside the kernel
 175when you use the /dev interface to I2C:
 1771* Your program opens /dev/i2c-N and calls ioctl() on it, as described in
 178section "C example" above.
 1802* These open() and ioctl() calls are handled by the i2c-dev kernel
 181driver: see i2c-dev.c:i2cdev_open() and i2c-dev.c:i2cdev_ioctl(),
 182respectively. You can think of i2c-dev as a generic I2C chip driver
 183that can be programmed from user-space.
 1853* Some ioctl() calls are for administrative tasks and are handled by
 186i2c-dev directly. Examples include I2C_SLAVE (set the address of the
 187device you want to access) and I2C_PEC (enable or disable SMBus error
 188checking on future transactions.)
 1904* Other ioctl() calls are converted to in-kernel function calls by
 191i2c-dev. Examples include I2C_FUNCS, which queries the I2C adapter
 192functionality using i2c.h:i2c_get_functionality(), and I2C_SMBUS, which
 193performs an SMBus transaction using i2c-core-smbus.c:i2c_smbus_xfer().
 195The i2c-dev driver is responsible for checking all the parameters that
 196come from user-space for validity. After this point, there is no
 197difference between these calls that came from user-space through i2c-dev
 198and calls that would have been performed by kernel I2C chip drivers
 199directly. This means that I2C bus drivers don't need to implement
 200anything special to support access from user-space.
 2025* These i2c.h functions are wrappers to the actual implementation of
 203your I2C bus driver. Each adapter must declare callback functions
 204implementing these standard calls. i2c.h:i2c_get_functionality() calls
 205i2c_adapter.algo->functionality(), while
 206i2c-core-smbus.c:i2c_smbus_xfer() calls either
 207adapter.algo->smbus_xfer() if it is implemented, or if not,
 208i2c-core-smbus.c:i2c_smbus_xfer_emulated() which in turn calls
 211After your I2C bus driver has processed these requests, execution runs
 212up the call chain, with almost no processing done, except by i2c-dev to
 213package the returned data, if any, in suitable format for the ioctl.