i2c: Improve dev-interface documentation

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, ..., 

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 lm_sensors and is

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/ to decide this. Adapter numbers are assigned

somewhat dynamically, so you can not even assume /dev/i2c-0 is the

first adapter.

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];

  sprintf(filename,"/dev/i2c-%d",adapter_nr);

  if ((file = open(filename,O_RDWR)) < 0) {

  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 register = 0x10; /* Device register to access */

  __s32 res;

  char buf[10];

  /* Using SMBus commands */

  res = i2c_smbus_read_word_data(file, register);

  if (res < 0) {

  } else {

    /* res contains the read word */

  }

  /* Using I2C Write, equivalent of 

     i2c_smbus_write_word_data(file, register, 0x6543) */

  buf[0] = register;

  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 */

    /* 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 and fully supported 

(see also i2c-dev.h):

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

  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

  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.

Other values are NOT supported at this moment, except for I2C_SMBUS,

which you should never directly call; instead, use the access functions

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.

returns the number of values read. The block buffers need not be longer

than 32 bytes.

The above functions are all macros, that resolve to calls to the

i2c_smbus_access function, that on its turn calls a specific ioctl

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.