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+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.
+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.h>" and "#include <linux/i2c-dev.h>.
+Yes, I know, you should never include kernel header files, but until glibc
+knows about i2c, there is not much choice.
+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.
+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) {
+ /* 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) {
+ /* ERROR HANDLING: i2c transaction failed */
+ } else {
+ /* res contains the read word */
+ }
+ /* Using I2C Write, equivalent of
+ i2c_smbus_write_word_data(file,register,0x6543) */
+ buf[0] = register;
+ 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 */
+ }
+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 and i2c.h):
+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.
+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.
+ioctl(file,I2C_FUNCS,unsigned long *funcs)
+ Gets the adapter functionality and puts it in *funcs.
+ioctl(file,I2C_RDWR,struct i2c_ioctl_rdwr_data *msgset)
+ Do combined read/write transaction without stop in between.
+ The argument is a pointer to a struct i2c_ioctl_rdwr_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.
+Other values are NOT supported at this moment, except for I2C_SMBUS,
+which you should never directly call; instead, use the access functions
+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 macros, 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.