![Jan Veldeman](/assets/img/avatar_default.png)
Fix documentation to match code in include/linux/i2c-dev.h Signed-off-by: Jan Veldeman <jan@mind.be> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
147 lines
5.8 KiB
Plaintext
147 lines
5.8 KiB
Plaintext
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_rdwr_ioctl_data *msgset)
|
|
|
|
Do combined read/write transaction without stop in between.
|
|
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.
|
|
|
|
|
|
Other values are NOT supported at this moment, except for I2C_SMBUS,
|
|
which you should never directly call; 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 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.
|