i2c: Improve dev-interface documentation
* Clarify some points. * Point developers to i2c-tools instead of lm_sensors. * Fix coding style in code examples. Signed-off-by: Jean Delvare <khali@linux-fr.org>
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Jean Delvare
Jean Delvare
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9def255631
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fceb2d0680
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@ -4,6 +4,10 @@ the /dev interface. You need to load module i2c-dev for this.
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Each registered i2c adapter gets a number, counting from 0. You can
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examine /sys/class/i2c-dev/ to see what number corresponds to which adapter.
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Alternatively, you can run "i2cdetect -l" to obtain a formated list of all
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i2c adapters present on your system at a given time. i2cdetect is part of
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the i2c-tools package.
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I2C device files are character device files with major device number 89
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and a minor device number corresponding to the number assigned as
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explained above. They should be called "i2c-%d" (i2c-0, i2c-1, ...,
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@ -17,30 +21,34 @@ So let's say you want to access an i2c adapter from a C program. The
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first thing to do is "#include <linux/i2c-dev.h>". Please note that
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there are two files named "i2c-dev.h" out there, one is distributed
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with the Linux kernel and is meant to be included from kernel
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driver code, the other one is distributed with lm_sensors and is
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driver code, the other one is distributed with i2c-tools and is
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meant to be included from user-space programs. You obviously want
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the second one here.
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Now, you have to decide which adapter you want to access. You should
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inspect /sys/class/i2c-dev/ to decide this. Adapter numbers are assigned
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somewhat dynamically, so you can not even assume /dev/i2c-0 is the
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first adapter.
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inspect /sys/class/i2c-dev/ or run "i2cdetect -l" to decide this.
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Adapter numbers are assigned somewhat dynamically, so you can not
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assume much about them. They can even change from one boot to the next.
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Next thing, open the device file, as follows:
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int file;
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int adapter_nr = 2; /* probably dynamically determined */
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char filename[20];
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sprintf(filename,"/dev/i2c-%d",adapter_nr);
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if ((file = open(filename,O_RDWR)) < 0) {
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snprintf(filename, 19, "/dev/i2c-%d", adapter_nr);
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file = open(filename, O_RDWR);
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if (file < 0) {
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/* ERROR HANDLING; you can check errno to see what went wrong */
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exit(1);
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}
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When you have opened the device, you must specify with what device
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address you want to communicate:
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int addr = 0x40; /* The I2C address */
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if (ioctl(file,I2C_SLAVE,addr) < 0) {
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if (ioctl(file, I2C_SLAVE, addr) < 0) {
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/* ERROR HANDLING; you can check errno to see what went wrong */
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exit(1);
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}
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@ -48,31 +56,41 @@ address you want to communicate:
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Well, you are all set up now. You can now use SMBus commands or plain
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I2C to communicate with your device. SMBus commands are preferred if
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the device supports them. Both are illustrated below.
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__u8 register = 0x10; /* Device register to access */
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__s32 res;
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char buf[10];
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/* Using SMBus commands */
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res = i2c_smbus_read_word_data(file,register);
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res = i2c_smbus_read_word_data(file, register);
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if (res < 0) {
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/* ERROR HANDLING: i2c transaction failed */
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} else {
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/* res contains the read word */
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}
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/* Using I2C Write, equivalent of
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i2c_smbus_write_word_data(file,register,0x6543) */
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i2c_smbus_write_word_data(file, register, 0x6543) */
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buf[0] = register;
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buf[1] = 0x43;
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buf[2] = 0x65;
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if ( write(file,buf,3) != 3) {
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if (write(file, buf, 3) ! =3) {
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/* ERROR HANDLING: i2c transaction failed */
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}
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/* Using I2C Read, equivalent of i2c_smbus_read_byte(file) */
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if (read(file,buf,1) != 1) {
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if (read(file, buf, 1) != 1) {
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/* ERROR HANDLING: i2c transaction failed */
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} else {
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/* buf[0] contains the read byte */
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}
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Note that only a subset of the I2C and SMBus protocols can be achieved by
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the means of read() and write() calls. In particular, so-called combined
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transactions (mixing read and write messages in the same transaction)
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aren't supported. For this reason, this interface is almost never used by
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user-space programs.
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IMPORTANT: because of the use of inline functions, you *have* to use
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'-O' or some variation when you compile your program!
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@ -80,31 +98,29 @@ IMPORTANT: because of the use of inline functions, you *have* to use
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Full interface description
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==========================
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The following IOCTLs are defined and fully supported
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(see also i2c-dev.h):
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The following IOCTLs are defined:
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ioctl(file,I2C_SLAVE,long addr)
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ioctl(file, I2C_SLAVE, long addr)
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Change slave address. The address is passed in the 7 lower bits of the
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argument (except for 10 bit addresses, passed in the 10 lower bits in this
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case).
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ioctl(file,I2C_TENBIT,long select)
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ioctl(file, I2C_TENBIT, long select)
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Selects ten bit addresses if select not equals 0, selects normal 7 bit
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addresses if select equals 0. Default 0. This request is only valid
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if the adapter has I2C_FUNC_10BIT_ADDR.
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ioctl(file,I2C_PEC,long select)
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ioctl(file, I2C_PEC, long select)
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Selects SMBus PEC (packet error checking) generation and verification
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if select not equals 0, disables if select equals 0. Default 0.
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Used only for SMBus transactions. This request only has an effect if the
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the adapter has I2C_FUNC_SMBUS_PEC; it is still safe if not, it just
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doesn't have any effect.
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ioctl(file,I2C_FUNCS,unsigned long *funcs)
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ioctl(file, I2C_FUNCS, unsigned long *funcs)
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Gets the adapter functionality and puts it in *funcs.
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ioctl(file,I2C_RDWR,struct i2c_rdwr_ioctl_data *msgset)
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ioctl(file, I2C_RDWR, struct i2c_rdwr_ioctl_data *msgset)
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Do combined read/write transaction without stop in between.
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Only valid if the adapter has I2C_FUNC_I2C. The argument is
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a pointer to a
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@ -120,10 +136,9 @@ ioctl(file,I2C_RDWR,struct i2c_rdwr_ioctl_data *msgset)
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The slave address and whether to use ten bit address mode has to be
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set in each message, overriding the values set with the above ioctl's.
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Other values are NOT supported at this moment, except for I2C_SMBUS,
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which you should never directly call; instead, use the access functions
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below.
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ioctl(file, I2C_SMBUS, struct i2c_smbus_ioctl_data *args)
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Not meant to be called directly; instead, use the access functions
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below.
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You can do plain i2c transactions by using read(2) and write(2) calls.
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You do not need to pass the address byte; instead, set it through
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@ -148,7 +163,7 @@ what happened. The 'write' transactions return 0 on success; the
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returns the number of values read. The block buffers need not be longer
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than 32 bytes.
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The above functions are all macros, that resolve to calls to the
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i2c_smbus_access function, that on its turn calls a specific ioctl
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The above functions are all inline functions, that resolve to calls to
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the i2c_smbus_access function, that on its turn calls a specific ioctl
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with the data in a specific format. Read the source code if you
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want to know what happens behind the screens.
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