how to disable fast frames in ath5k wireless driver

Question

By default, fast frames are enabled in ath5k. (http://wireless.kernel.org/en/users/Drivers/ath5k) I have found the macro which disables it

#define AR5K_EEPROM_FF_DIS(_v)      (((_v) >> 2) & 0x1

The question is what do I do with it? Do I replace the above line with

#define AR5K_EEPROM_FF_DIS(_v)  1

?

Do I compile it passing some parameter?

The bit-shift expression confuses me. Is _v a variable?

The question is more general as to how to deal with such macros in drivers. I've seen them in other codes too and always got confused.

Answer 1

Ok, I try to explain with a simplified example

#include <stdio.h>

/* Just for print in binary mode */
char *chartobin(unsigned char c)
{
    static char a[9];
    int i;

    for (i = 0; i < 8; i++)
          a[7 - i] = (c & (1 << i)) == (1 << i) ? '1' : '0';
    a[8] = '\0';
    return a;
}

int main(void)
{
    unsigned char u = 0xf;

    printf("%s\n", chartobin(u)); 
    u >>= 2; // Shift bits 2 positions (to the right)
    printf("%s\n", chartobin(u));       
    printf("%s\n", chartobin(u & 0x1)); // Check if the last bit is on

    return 0;
}

Output:

00001111
00000011
00000001

Do I replace the above line with #define AR5K_EEPROM_FF_DIS(_v) 1?

Nooooo!!

If you initialize u with 0xb instead of 0xf you get:

00001011
00000010
00000000

As you can see (((_v) >> 2) & 0x1 != 1