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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.h>" and "#include <linux/i2c-dev.h>.
18Yes, I know, you should never include kernel header files, but until glibc
19knows about i2c, there is not much choice.
20
21Now, you have to decide which adapter you want to access. You should
22inspect /sys/class/i2c-dev/ to decide this. Adapter numbers are assigned
23somewhat dynamically, so you can not even assume /dev/i2c-0 is the
24first adapter.
25
26Next thing, open the device file, as follows:
27 int file;
28 int adapter_nr = 2; /* probably dynamically determined */
29 char filename[20];
30
31 sprintf(filename,"/dev/i2c-%d",adapter_nr);
32 if ((file = open(filename,O_RDWR)) < 0) {
33 /* ERROR HANDLING; you can check errno to see what went wrong */
34 exit(1);
35 }
36
37When you have opened the device, you must specify with what device
38address you want to communicate:
39 int addr = 0x40; /* The I2C address */
40 if (ioctl(file,I2C_SLAVE,addr) < 0) {
41 /* ERROR HANDLING; you can check errno to see what went wrong */
42 exit(1);
43 }
44
45Well, you are all set up now. You can now use SMBus commands or plain
46I2C to communicate with your device. SMBus commands are preferred if
47the device supports them. Both are illustrated below.
48 __u8 register = 0x10; /* Device register to access */
49 __s32 res;
50 char buf[10];
51 /* Using SMBus commands */
52 res = i2c_smbus_read_word_data(file,register);
53 if (res < 0) {
54 /* ERROR HANDLING: i2c transaction failed */
55 } else {
56 /* res contains the read word */
57 }
58 /* Using I2C Write, equivalent of
59 i2c_smbus_write_word_data(file,register,0x6543) */
60 buf[0] = register;
61 buf[1] = 0x43;
62 buf[2] = 0x65;
63 if ( write(file,buf,3) != 3) {
64 /* ERROR HANDLING: i2c transaction failed */
65 }
66 /* Using I2C Read, equivalent of i2c_smbus_read_byte(file) */
67 if (read(file,buf,1) != 1) {
68 /* ERROR HANDLING: i2c transaction failed */
69 } else {
70 /* buf[0] contains the read byte */
71 }
72
73IMPORTANT: because of the use of inline functions, you *have* to use
74'-O' or some variation when you compile your program!
75
76
77Full interface description
78==========================
79
80The following IOCTLs are defined and fully supported
81(see also i2c-dev.h and i2c.h):
82
83ioctl(file,I2C_SLAVE,long addr)
84 Change slave address. The address is passed in the 7 lower bits of the
85 argument (except for 10 bit addresses, passed in the 10 lower bits in this
86 case).
87
88ioctl(file,I2C_TENBIT,long select)
89 Selects ten bit addresses if select not equals 0, selects normal 7 bit
90 addresses if select equals 0. Default 0.
91
92ioctl(file,I2C_PEC,long select)
93 Selects SMBus PEC (packet error checking) generation and verification
94 if select not equals 0, disables if select equals 0. Default 0.
95 Used only for SMBus transactions.
96
97ioctl(file,I2C_FUNCS,unsigned long *funcs)
98 Gets the adapter functionality and puts it in *funcs.
99
100ioctl(file,I2C_RDWR,struct i2c_ioctl_rdwr_data *msgset)
101
102 Do combined read/write transaction without stop in between.
103 The argument is a pointer to a struct i2c_ioctl_rdwr_data {
104
105 struct i2c_msg *msgs; /* ptr to array of simple messages */
106 int nmsgs; /* number of messages to exchange */
107 }
108
109 The msgs[] themselves contain further pointers into data buffers.
110 The function will write or read data to or from that buffers depending
111 on whether the I2C_M_RD flag is set in a particular message or not.
112 The slave address and whether to use ten bit address mode has to be
113 set in each message, overriding the values set with the above ioctl's.
114
115
116Other values are NOT supported at this moment, except for I2C_SMBUS,
117which you should never directly call; instead, use the access functions
118below.
119
120You can do plain i2c transactions by using read(2) and write(2) calls.
121You do not need to pass the address byte; instead, set it through
122ioctl I2C_SLAVE before you try to access the device.
123
124You can do SMBus level transactions (see documentation file smbus-protocol
125for details) through the following functions:
126 __s32 i2c_smbus_write_quick(int file, __u8 value);
127 __s32 i2c_smbus_read_byte(int file);
128 __s32 i2c_smbus_write_byte(int file, __u8 value);
129 __s32 i2c_smbus_read_byte_data(int file, __u8 command);
130 __s32 i2c_smbus_write_byte_data(int file, __u8 command, __u8 value);
131 __s32 i2c_smbus_read_word_data(int file, __u8 command);
132 __s32 i2c_smbus_write_word_data(int file, __u8 command, __u16 value);
133 __s32 i2c_smbus_process_call(int file, __u8 command, __u16 value);
134 __s32 i2c_smbus_read_block_data(int file, __u8 command, __u8 *values);
135 __s32 i2c_smbus_write_block_data(int file, __u8 command, __u8 length,
136 __u8 *values);
137All these transactions return -1 on failure; you can read errno to see
138what happened. The 'write' transactions return 0 on success; the
139'read' transactions return the read value, except for read_block, which
140returns the number of values read. The block buffers need not be longer
141than 32 bytes.
142
143The above functions are all macros, that resolve to calls to the
144i2c_smbus_access function, that on its turn calls a specific ioctl
145with the data in a specific format. Read the source code if you
146want to know what happens behind the screens.