Usually, i2c devices are controlled by a kernel driver. But it is also
possible to access all devices on an adapter from userspace, through
the /dev interface. You need to load module i2c-dev for this.

Each registered i2c adapter gets a number, counting from 0. You can
examine /sys/class/i2c-dev/ to see what number corresponds to which adapter.
Alternatively, you can run "i2cdetect -l" to obtain a formated list of all
i2c adapters present on your system at a given time. i2cdetect is part of
the i2c-tools package.

I2C device files are character device files with major device number 89
and a minor device number corresponding to the number assigned as 
explained above. They should be called "i2c-%d" (i2c-0, i2c-1, ..., 
i2c-10, ...). All 256 minor device numbers are reserved for i2c.


C example
=========

So let's say you want to access an i2c adapter from a C program. The
first thing to do is "#include <linux/i2c-dev.h>". Please note that
there are two files named "i2c-dev.h" out there, one is distributed
with the Linux kernel and is meant to be included from kernel
driver code, the other one is distributed with i2c-tools and is
meant to be included from user-space programs. You obviously want
the second one here.

Now, you have to decide which adapter you want to access. You should
inspect /sys/class/i2c-dev/ or run "i2cdetect -l" to decide this.
Adapter numbers are assigned somewhat dynamically, so you can not
assume much about them. They can even change from one boot to the next.

Next thing, open the device file, as follows:

  int file;
  int adapter_nr = 2; /* probably dynamically determined */
  char filename[20];
  
  snprintf(filename, 19, "/dev/i2c-%d", adapter_nr);
  file = open(filename, O_RDWR);
  if (file < 0) {
    /* ERROR HANDLING; you can check errno to see what went wrong */
    exit(1);
  }

When you have opened the device, you must specify with what device
address you want to communicate:

  int addr = 0x40; /* The I2C address */

  if (ioctl(file, I2C_SLAVE, addr) < 0) {
    /* ERROR HANDLING; you can check errno to see what went wrong */
    exit(1);
  }

Well, you are all set up now. You can now use SMBus commands or plain
I2C to communicate with your device. SMBus commands are preferred if
the device supports them. Both are illustrated below.

  __u8 reg = 0x10; /* Device register to access */
  __s32 res;
  char buf[10];

  /* Using SMBus commands */
  res = i2c_smbus_read_word_data(file, reg);
  if (res < 0) {
    /* ERROR HANDLING: i2c transaction failed */
  } else {
    /* res contains the read word */
  }

  /* Using I2C Write, equivalent of 
     i2c_smbus_write_word_data(file, reg, 0x6543) */
  buf[0] = reg;
  buf[1] = 0x43;
  buf[2] = 0x65;
  if (write(file, buf, 3) != 3) {
    /* ERROR HANDLING: i2c transaction failed */
  }

  /* Using I2C Read, equivalent of i2c_smbus_read_byte(file) */
  if (read(file, buf, 1) != 1) {
    /* ERROR HANDLING: i2c transaction failed */
  } else {
    /* buf[0] contains the read byte */
  }

Note that only a subset of the I2C and SMBus protocols can be achieved by
the means of read() and write() calls. In particular, so-called combined
transactions (mixing read and write messages in the same transaction)
aren't supported. For this reason, this interface is almost never used by
user-space programs.

IMPORTANT: because of the use of inline functions, you *have* to use
'-O' or some variation when you compile your program!


Full interface description
==========================

The following IOCTLs are defined:

ioctl(file, I2C_SLAVE, long addr)
  Change slave address. The address is passed in the 7 lower bits of the
  argument (except for 10 bit addresses, passed in the 10 lower bits in this
  case).

ioctl(file, I2C_TENBIT, long select)
  Selects ten bit addresses if select not equals 0, selects normal 7 bit
  addresses if select equals 0. Default 0.  This request is only valid
  if the adapter has I2C_FUNC_10BIT_ADDR.

ioctl(file, I2C_PEC, long select)
  Selects SMBus PEC (packet error checking) generation and verification
  if select not equals 0, disables if select equals 0. Default 0.
  Used only for SMBus transactions.  This request only has an effect if the
  the adapter has I2C_FUNC_SMBUS_PEC; it is still safe if not, it just
  doesn't have any effect.

ioctl(file, I2C_FUNCS, unsigned long *funcs)
  Gets the adapter functionality and puts it in *funcs.

ioctl(file, I2C_RDWR, struct i2c_rdwr_ioctl_data *msgset)
  Do combined read/write transaction without stop in between.
  Only valid if the adapter has I2C_FUNC_I2C.  The argument is
  a pointer to a

  struct i2c_rdwr_ioctl_data {
      struct i2c_msg *msgs;  /* ptr to array of simple messages */
      int nmsgs;             /* number of messages to exchange */
  }

  The msgs[] themselves contain further pointers into data buffers.
  The function will write or read data to or from that buffers depending
  on whether the I2C_M_RD flag is set in a particular message or not.
  The slave address and whether to use ten bit address mode has to be
  set in each message, overriding the values set with the above ioctl's.

ioctl(file, I2C_SMBUS, struct i2c_smbus_ioctl_data *args)
  Not meant to be called  directly; instead, use the access functions
  below.

You can do plain i2c transactions by using read(2) and write(2) calls.
You do not need to pass the address byte; instead, set it through
ioctl I2C_SLAVE before you try to access the device.

You can do SMBus level transactions (see documentation file smbus-protocol 
for details) through the following functions:
  __s32 i2c_smbus_write_quick(int file, __u8 value);
  __s32 i2c_smbus_read_byte(int file);
  __s32 i2c_smbus_write_byte(int file, __u8 value);
  __s32 i2c_smbus_read_byte_data(int file, __u8 command);
  __s32 i2c_smbus_write_byte_data(int file, __u8 command, __u8 value);
  __s32 i2c_smbus_read_word_data(int file, __u8 command);
  __s32 i2c_smbus_write_word_data(int file, __u8 command, __u16 value);
  __s32 i2c_smbus_process_call(int file, __u8 command, __u16 value);
  __s32 i2c_smbus_read_block_data(int file, __u8 command, __u8 *values);
  __s32 i2c_smbus_write_block_data(int file, __u8 command, __u8 length, 
                                   __u8 *values);
All these transactions return -1 on failure; you can read errno to see
what happened. The 'write' transactions return 0 on success; the
'read' transactions return the read value, except for read_block, which
returns the number of values read. The block buffers need not be longer
than 32 bytes.

The above functions are all inline functions, that resolve to calls to
the i2c_smbus_access function, that on its turn calls a specific ioctl
with the data in a specific format. Read the source code if you
want to know what happens behind the screens.


Implementation details
======================

For the interested, here's the code flow which happens inside the kernel
when you use the /dev interface to I2C:

1* Your program opens /dev/i2c-N and calls ioctl() on it, as described in
section "C example" above.

2* These open() and ioctl() calls are handled by the i2c-dev kernel
driver: see i2c-dev.c:i2cdev_open() and i2c-dev.c:i2cdev_ioctl(),
respectively. You can think of i2c-dev as a generic I2C chip driver
that can be programmed from user-space.

3* Some ioctl() calls are for administrative tasks and are handled by
i2c-dev directly. Examples include I2C_SLAVE (set the address of the
device you want to access) and I2C_PEC (enable or disable SMBus error
checking on future transactions.)

4* Other ioctl() calls are converted to in-kernel function calls by
i2c-dev. Examples include I2C_FUNCS, which queries the I2C adapter
functionality using i2c.h:i2c_get_functionality(), and I2C_SMBUS, which
performs an SMBus transaction using i2c-core.c:i2c_smbus_xfer().

The i2c-dev driver is responsible for checking all the parameters that
come from user-space for validity. After this point, there is no
difference between these calls that came from user-space through i2c-dev
and calls that would have been performed by kernel I2C chip drivers
directly. This means that I2C bus drivers don't need to implement
anything special to support access from user-space.

5* These i2c-core.c/i2c.h functions are wrappers to the actual
implementation of your I2C bus driver. Each adapter must declare
callback functions implementing these standard calls.
i2c.h:i2c_get_functionality() calls i2c_adapter.algo->functionality(),
while i2c-core.c:i2c_smbus_xfer() calls either
adapter.algo->smbus_xfer() if it is implemented, or if not,
i2c-core.c:i2c_smbus_xfer_emulated() which in turn calls
i2c_adapter.algo->master_xfer().

After your I2C bus driver has processed these requests, execution runs
up the call chain, with almost no processing done, except by i2c-dev to
package the returned data, if any, in suitable format for the ioctl.