I2C_SLAVE ioctl purpose - linux

Question

I am writing code for implementing a simple i2c read/write function using the general linux i2c driver linux/i2c-dev.h
I am confused about the ioctl : I2C_SLAVE
The kernel documentation states as follows :
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
However I am using the ioctl I2C_RDWR where I again set the slave address using i2c_msg.addr.
The kernel documentation also mentions the following :
Some ioctl() calls are for administrative tasks and are handled by
i2c-dev directly. Examples include I2C_SLAVE
So is it must to use the ioctl I2C_SLAVE? If so do I need to set it just once or every time I perform a read and write?
If I had an i2c device I could have just tested the code on the device and would not have bothered you guys but unfortunately I don't have one right now.
Thanks for the help.

Answer 1

There are three major methods of communicating with i2c devices from userspace.
1. IOCTL I2C_RDWR
This method allows for simultaneous read/write and sending an uninterrupted sequence of message. Not all i2c devices support this method.
Before performing i/o with this method, you should check whether the device supports this method using an ioctl I2C_FUNCS operation.
Using this method, you do not need to perform an ioctl I2C_SLAVE operation -- it is done behind the scenes using the information embedded in the messages.
2. IOCTL SMBUS
This method of i/o is more powerful but the resulting code is more verbose. This method can be used if the device does not support the I2C_RDWR method.
Using this method, you do need to perform an ioctl I2C_SLAVE operation (or, if the device is busy, an I2C_SLAVE_FORCE operation).
3. SYSFS I/O
This method uses the basic file i/o system calls read() and write(). Uninterrupted sequential operations are not possible using this method. This method can be used if the device does not support the I2C_RDWR method.
Using this method, you do need to perform an ioctl I2C_SLAVE operation (or, if the device is busy, an I2C_SLAVE_FORCE operation).
I can't think of any situation when this method would be preferable to others, unless you need the chip to be treated like a file.
Full IOCTL Example
I haven't tested this example, but it shows the conceptual flow of writing to an i2c device.-- automatically detecting whether to use the ioctl I2C_RDWR or smbus technique.
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <errno.h>
#include <string.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <linux/i2c.h>
#include <linux/i2c-dev.h>
#include <sys/ioctl.h>
#define I2C_ADAPTER "/dev/i2c-0"
#define I2C_DEVICE 0x00
int i2c_ioctl_write (int fd, uint8_t dev, uint8_t regaddr, uint16_t *data, size_t size)
{
int i, j = 0;
int ret;
uint8_t *buf;
// the extra byte is for the regaddr
size_t buff_size = 1 + size;
buf = malloc(buff_size);
if (buf == NULL) {
return -ENOMEM;
}
buf[j ++] = regaddr;
for (i = 0; i < size / sizeof(uint16_t); i ++) {
buf[j ++] = (data[i] & 0xff00) >> 8;
buf[j ++] = data[i] & 0xff;
}
struct i2c_msg messages[] = {
{
.addr = dev,
.buf = buf,
.len = buff_size,
},
};
struct i2c_rdwr_ioctl_data payload = {
.msgs = messages,
.nmsgs = sizeof(messages) / sizeof(messages[0]),
};
ret = ioctl(fd, I2C_RDWR, &payload);
if (ret < 0) {
ret = -errno;
}
free (buf);
return ret;
}
int i2c_ioctl_smbus_write (int fd, uint8_t dev, uint8_t regaddr, uint16_t *data, size_t size)
{
int i, j = 0;
int ret;
uint8_t *buf;
buf = malloc(size);
if (buf == NULL) {
return -ENOMEM;
}
for (i = 0; i < size / sizeof(uint16_t); i ++) {
buf[j ++] = (data[i] & 0xff00) >> 8;
buf[j ++] = data[i] & 0xff;
}
struct i2c_smbus_ioctl_data payload = {
.read_write = I2C_SMBUS_WRITE,
.size = I2C_SMBUS_WORD_DATA,
.command = regaddr,
.data = (void *) buf,
};
ret = ioctl (fd, I2C_SLAVE_FORCE, dev);
if (ret < 0)
{
ret = -errno;
goto exit;
}
ret = ioctl (fd, I2C_SMBUS, &payload);
if (ret < 0)
{
ret = -errno;
goto exit;
}
exit:
free(buf);
return ret;
}
int i2c_write (int fd, uint8_t dev, uint8_t regaddr, uint16_t *data, size_t size)
{
unsigned long funcs;
if (ioctl(fd, I2C_FUNCS, &funcs) < 0) {
return -errno;
}
if (funcs & I2C_FUNC_I2C) {
return i2c_ioctl_write (fd, dev, regaddr, data, size);
} else if (funcs & I2C_FUNC_SMBUS_WORD_DATA) {
return i2c_ioctl_smbus_write (fd, dev, regaddr, data, size);
} else {
return -ENOSYS;
}
}
int parse_args (uint8_t *regaddr, uint16_t *data, size_t size, char *argv[])
{
char *endptr;
int i;
*regaddr = (uint8_t) strtol(argv[1], &endptr, 0);
if (errno || endptr == argv[1]) {
return -1;
}
for (i = 0; i < size / sizeof(uint16_t); i ++) {
data[i] = (uint16_t) strtol(argv[i + 2], &endptr, 0);
if (errno || endptr == argv[i + 2]) {
return -1;
}
}
return 0;
}
void usage (int argc, char *argv[])
{
fprintf(stderr, "Usage: %s regaddr data [data]*\n", argv[0]);
fprintf(stderr, " regaddr The 8-bit register address to write to.\n");
fprintf(stderr, " data The 16-bit data to be written.\n");
exit(-1);
}
int main (int argc, char *argv[])
{
uint8_t regaddr;
uint16_t *data;
size_t size;
int fd;
int ret = 0;
if (argc < 3) {
usage(argc, argv);
}
size = (argc - 2) * sizeof(uint16_t);
data = malloc(size);
if (data == NULL) {
fprintf (stderr, "%s.\n", strerror(ENOMEM));
return -ENOMEM;
}
if (parse_args(&regaddr, data, size, argv) != 0) {
free(data);
usage(argc, argv);
}
fd = open(I2C_ADAPTER, O_RDWR | O_NONBLOCK);
ret = i2c_write(fd, I2C_DEVICE, regaddr, data);
close(fd);
if (ret) {
fprintf (stderr, "%s.\n", strerror(-ret));
}
free(data);
return ret;
}

Answer 2

If you use the read() and write() methods, calling ioctl with I2C_SLAVE once is enough. You can also use I2C_SLAVE_FORCE if the device is already in use.
However I haven't yet found a consistent way to read specific registers for every device using the read()/write() methods.

Answer 3

I'm not too sure if this helps because I don't use ioctl I2C_RDWR but I've been using the following code with success:
int fd;
fd = open("/dev/i2c-5", O_RDWR);
ioctl(fd, I2C_SLAVE_FORCE, 0x20);
i2c_smbus_write_word_data(fd, ___, ___);
i2c_smbus_read_word_data(fd, ___);
All I do is set I2C_SLAVE_FORCE once at the beginning and I can read and write as much as I want to after that.
PS - This is just a code sample and obviously you should check the returns of all of these functions. I'm using this code to communicate with a digital I/O chip. The two i2c_* functions are just wrappers that call ioctl(fd, I2C_SMBUS, &args); where args is a struct i2c_smbus_ioctl_data type.

Answer 4

For the interested, SLAVE_FORCE is used when the device in question is already being managed by a kernel driver. (i2cdetect will show UU for that address)