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.
4
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.
7I2C device files are character device files with major device number 89
8and a minor device number corresponding to the number assigned as 
9explained above. They should be called "i2c-%d" (i2c-0, i2c-1, ..., 
10i2c-10, ...). All 256 minor device numbers are reserved for i2c.
11
12
13C example
14=========
15
16So let's say you want to access an i2c adapter from a C program. The
17first thing to do is "#include <linux/i2c-dev.h>". Please note that
18there are two files named "i2c-dev.h" out there, one is distributed
19with the Linux kernel and is meant to be included from kernel
20driver code, the other one is distributed with lm_sensors and is
21meant to be included from user-space programs. You obviously want
22the second one here.
23
24Now, you have to decide which adapter you want to access. You should
25inspect /sys/class/i2c-dev/ to decide this. Adapter numbers are assigned
26somewhat dynamically, so you can not even assume /dev/i2c-0 is the
27first adapter.
28
29Next thing, open the device file, as follows:
30  int file;
31  int adapter_nr = 2; /* probably dynamically determined */
32  char filename[20];
33  
34  sprintf(filename,"/dev/i2c-%d",adapter_nr);
35  if ((file = open(filename,O_RDWR)) < 0) {
36    /* ERROR HANDLING; you can check errno to see what went wrong */
37    exit(1);
38  }
39
40When you have opened the device, you must specify with what device
41address you want to communicate:
42  int addr = 0x40; /* The I2C address */
43  if (ioctl(file,I2C_SLAVE,addr) < 0) {
44    /* ERROR HANDLING; you can check errno to see what went wrong */
45    exit(1);
46  }
47
48Well, you are all set up now. You can now use SMBus commands or plain
49I2C to communicate with your device. SMBus commands are preferred if
50the device supports them. Both are illustrated below.
51  __u8 register = 0x10; /* Device register to access */
52  __s32 res;
53  char buf[10];
54  /* Using SMBus commands */
55  res = i2c_smbus_read_word_data(file,register);
56  if (res < 0) {
57    /* ERROR HANDLING: i2c transaction failed */
58  } else {
59    /* res contains the read word */
60  }
61  /* Using I2C Write, equivalent of 
62           i2c_smbus_write_word_data(file,register,0x6543) */
63  buf[0] = register;
64  buf[1] = 0x43;
65  buf[2] = 0x65;
66  if ( write(file,buf,3) != 3) {
67    /* ERROR HANDLING: i2c transaction failed */
68  }
69  /* Using I2C Read, equivalent of i2c_smbus_read_byte(file) */
70  if (read(file,buf,1) != 1) {
71    /* ERROR HANDLING: i2c transaction failed */
72  } else {
73    /* buf[0] contains the read byte */
74  }
75
76IMPORTANT: because of the use of inline functions, you *have* to use
77'-O' or some variation when you compile your program!
78
79
80Full interface description
81==========================
82
83The following IOCTLs are defined and fully supported 
84(see also i2c-dev.h):
85
86ioctl(file,I2C_SLAVE,long addr)
87  Change slave address. The address is passed in the 7 lower bits of the
88  argument (except for 10 bit addresses, passed in the 10 lower bits in this
89  case).
90
91ioctl(file,I2C_TENBIT,long select)
92  Selects ten bit addresses if select not equals 0, selects normal 7 bit
93  addresses if select equals 0. Default 0.
94
95ioctl(file,I2C_PEC,long select)
96  Selects SMBus PEC (packet error checking) generation and verification
97  if select not equals 0, disables if select equals 0. Default 0.
98  Used only for SMBus transactions.
99
100ioctl(file,I2C_FUNCS,unsigned long *funcs)
101  Gets the adapter functionality and puts it in *funcs.
102
103ioctl(file,I2C_RDWR,struct i2c_rdwr_ioctl_data *msgset)
104
105  Do combined read/write transaction without stop in between.
106  The argument is a pointer to a struct i2c_rdwr_ioctl_data {
107
108      struct i2c_msg *msgs;  /* ptr to array of simple messages */
109      int nmsgs;             /* number of messages to exchange */
110  }
111
112  The msgs[] themselves contain further pointers into data buffers.
113  The function will write or read data to or from that buffers depending
114  on whether the I2C_M_RD flag is set in a particular message or not.
115  The slave address and whether to use ten bit address mode has to be
116  set in each message, overriding the values set with the above ioctl's.
117
118
119Other values are NOT supported at this moment, except for I2C_SMBUS,
120which you should never directly call; instead, use the access functions
121below.
122
123You can do plain i2c transactions by using read(2) and write(2) calls.
124You do not need to pass the address byte; instead, set it through
125ioctl I2C_SLAVE before you try to access the device.
126
127You can do SMBus level transactions (see documentation file smbus-protocol 
128for details) through the following functions:
129  __s32 i2c_smbus_write_quick(int file, __u8 value);
130  __s32 i2c_smbus_read_byte(int file);
131  __s32 i2c_smbus_write_byte(int file, __u8 value);
132  __s32 i2c_smbus_read_byte_data(int file, __u8 command);
133  __s32 i2c_smbus_write_byte_data(int file, __u8 command, __u8 value);
134  __s32 i2c_smbus_read_word_data(int file, __u8 command);
135  __s32 i2c_smbus_write_word_data(int file, __u8 command, __u16 value);
136  __s32 i2c_smbus_process_call(int file, __u8 command, __u16 value);
137  __s32 i2c_smbus_read_block_data(int file, __u8 command, __u8 *values);
138  __s32 i2c_smbus_write_block_data(int file, __u8 command, __u8 length, 
139                                   __u8 *values);
140All these transactions return -1 on failure; you can read errno to see
141what happened. The 'write' transactions return 0 on success; the
142'read' transactions return the read value, except for read_block, which
143returns the number of values read. The block buffers need not be longer
144than 32 bytes.
145
146The above functions are all macros, that resolve to calls to the
147i2c_smbus_access function, that on its turn calls a specific ioctl
148with the data in a specific format. Read the source code if you
149want to know what happens behind the screens.
150