[svlug] find_first_bit()

[Tim Flagg]

> in the kernel bitops, like find_first_bit(), is size in bits, bytes or
> words?
>
> static inline int
> find_first_bit(const unsigned long * addr, unsigned int size)

size is in bits.  See include/asm-i386/bitops.h (with some
comments I added):

     ...
     321 /**
     322  * find_first_bit - find the first set bit in a memory region
     323  * @addr: The address to start the search at
     324  * @size: The maximum size to search
     325  *
     326  * Returns the bit-number of the first set bit, not the number of the by        te
     327  * containing a bit.
     328  */
     329 static inline unsigned find_first_bit(const unsigned long *addr, unsigne        d size)
     330 {
     331     unsigned x = 0;
     332
     333     while (x < size) {  // While we have more bits to check
     334         unsigned long val = *addr++;    // Check next 4 bytes.
     335         if (val)          // Any of these 4 bytes have a 1?
     336             return __ffs(val) + x;  // Return bit # of the 1
--> 337         x += (sizeof(*addr)<<3);  // Increment by 4 words * 8 bits each
     338     }
     339     return x;
     340 }
     ...

Additional notes:
     Line 337:  At this point, we have checked the next 32 bits
     (for the i386 architecture an "unsigned long" is 4 bytes).  and they
     didn't have any 1's

     sizeof(*addr) is the number of BYTES we look at in each loop.
     The "<< 3" multiplies by 8 (since there are 8 bits in a byte).

     So another way to look at line 337:
         x += 4 * 8;  // We've checked 4 more bytes * 8 bits each

     So line 333 is comparing "x" (the number of bits we've already
     checked) with "size" (the number of bits we are supposed to check).

Tim