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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.
+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 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 */
+ }
+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
+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.