i2c: Improve dev-interface documentation (fceb2d06) · Commits · EulixOS / Software / Kernel

Documentation/i2c/dev-interface

+40 −25

Original line number	Diff line number	Diff line
		@@ -4,6 +4,10 @@ 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, ...,
		@@ -17,29 +21,33 @@ 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 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);
		@@ -48,9 +56,11 @@ address you want to communicate:
		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) {
		@@ -58,6 +68,7 @@ the device supports them. Both are illustrated below.
		} else {
		/* res contains the read word */
		}

		/* Using I2C Write, equivalent of
		i2c_smbus_write_word_data(file, register, 0x6543) */
		buf[0] = register;
		@@ -66,6 +77,7 @@ the device supports them. Both are illustrated below.
		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 */
		@@ -73,6 +85,12 @@ the device supports them. Both are illustrated below.
		/* 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!

		@@ -80,8 +98,7 @@ IMPORTANT: because of the use of inline functions, you have to use
		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
		@@ -104,7 +121,6 @@ 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
		@@ -120,9 +136,8 @@ ioctl(file,I2C_RDWR,struct i2c_rdwr_ioctl_data *msgset)
		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.
		@@ -148,7 +163,7 @@ what happened. The 'write' transactions return 0 on success; the
		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.