Why I2C_SMBUS_BLOCK_MAX is limited to 32 bytes?

Question

I'm trying to configure a SAA6752HS chip (a MPEG-2 encoder) through I2C bus using a Raspberry Pi as a development kit. It was a piece of cake until I had to write at the address 0xC2 of the chip. For this task, I have to use an I2C command that expects a payload of size 189 bytes. So then I stumbled upon a 32 bytes limitation inside the I2C driver, defined by I2C_SMBUS_BLOCK_MAX, in /usr/include/linux/i2c.h. It is not possible to force different values of max limit. Everything around I2C lib end up into the function i2c_smbus_access and any request with more then 32 bytes makes ioctl returns -1. I have no idea how to debug it so far.

static inline __s32 i2c_smbus_access(int file, char read_write, __u8 command,
                                     int size, union i2c_smbus_data *data)
{
        struct i2c_smbus_ioctl_data args;

        args.read_write = read_write;
        args.command = command;
        args.size = size;
        args.data = data;
        return ioctl(file,I2C_SMBUS,&args);
}

I can't understand why there is such limitation, considering that there are devices that require more than 32 bytes of payload data to work (SAA6752HS is such an example).

Are there a way to overcome such limitation without rewrite a new driver?

Thank you in advance.

Answer 1

Here's the documentation for the Linux i2c interface: https://www.kernel.org/doc/Documentation/i2c/dev-interface

At the simplest level you can use ioctl(I2C_SLAVE) to set the slave address and the write system call to write the command. Something like:

i2c_write(int file, int address, int subaddress, int size, char *data) {
    char buf[size + 1];               // note: variable length array
    ioctl(file, I2C_SLAVE, address);  // real code would need to check for an error
    buf[0] = subaddress;              // need to send everything in one call to write
    memcpy(buf + 1, data, size);      // so copy subaddress and data to a buffer 
    write(file, buf, size + 1); 
}