why XOR ext2_set_bit?

Hi,

I just had a problem writing to a file on a ext2 filesystem with my port of kernel 2.4 on powerpc (G4) running in little-endian. When the function ext2_set_bit was called by ext2_new_block, it returned something so I got the warning :

"ext2_new_block bit already set for block 161"

and I was stuck in an infinite loop. I found the definition for the function in include/asm-ppc/bitops.h as follow:

#define ext2_set_bit(nr, addr) test_and_set_bit((nr) ^ 0x18, addr)

Now my question: Why is "nr" XORed with 0x18 ??? I tried to do this function on paper and with this XOR, they were always trying to change the wrong bit. Now, since nobody's been reporting this problem before, I guess this XOR is ok but I'd really like to understand it's utility to correct things on my side (for now, I just removed it and everything appears to be fine).

Could someone give me an explanation?

Thanks,

Sébastien Côté

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Ext2 metadata is little endian. The XOR converts from big endian to little endian bit numbering inside a 32-bit word in native format.

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]

In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds

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