mirror of
https://github.com/torvalds/linux.git
synced 2024-11-26 22:21:42 +00:00
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>
This commit is contained in:
parent
9def255631
commit
fceb2d0680
@ -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, ...,
|
||||
@ -17,30 +21,34 @@ 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) {
|
||||
|
||||
if (ioctl(file, I2C_SLAVE, addr) < 0) {
|
||||
/* ERROR HANDLING; you can check errno to see what went wrong */
|
||||
exit(1);
|
||||
}
|
||||
@ -48,31 +56,41 @@ 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);
|
||||
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) */
|
||||
i2c_smbus_write_word_data(file, register, 0x6543) */
|
||||
buf[0] = register;
|
||||
buf[1] = 0x43;
|
||||
buf[2] = 0x65;
|
||||
if ( write(file,buf,3) != 3) {
|
||||
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) {
|
||||
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!
|
||||
|
||||
@ -80,31 +98,29 @@ 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)
|
||||
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)
|
||||
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)
|
||||
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)
|
||||
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)
|
||||
|
||||
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
|
||||
@ -120,10 +136,9 @@ 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
|
||||
below.
|
||||
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
|
||||
@ -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.
|
||||
|
Loading…
Reference in New Issue
Block a user