power of 2 revisited

This started out as a direct question to Ingo, but I have readdressed it
to misc@

This is around documenting peculiar behaviour around power of 2 math in
the kernel.

I noticed that in sys/dev/pci/drm/radeon/radeon_device.c:1137 there is a
call to radeon_check_pot_argument, this function isn't correct in that
it returns true for a value of zero (as you know, 0 is not a power of two).

I sent a diff to Jonathan to correct the behaviour which he rejected
because he doesn't want to add local changes to change behaviour which
isn't hitting a bug here. Fair Enough.

Then reading through old mail archives I came across this discussion
from a few years back:
https://marc.info/?t=144321527600004&r=1&w=2

I'm wondering if it's worth documenting the peculiarities here, and
possibly putting an actually mathematically correct check somewhere in
the kernel or maybe userland? Perhaps hypothetical PEER REVIEWED
functions with prototypes like isnpotu(uint64_t n) or isnpot(int64_t n).

Is this at all worthwhile? Maybe this would help stop people from
incorrectly reinventing the wheel?

For instance in the kernel right now there is :
radeon_check_pot_argument
IS_POWER_OF_2
is_power_of_2
is_power_of_2_u64
powerof2
probably others too.

And manual checks like
sys/arch/amd64/amd64/identcpu.c:804
powerof2 = ((x - 1) & x) == 0;

--
Ted Bullock <tbullock@comlore.com>