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 <[email protected]>
