On Wed, Dec 29, 2021 at 09:27:34PM -0700, Ted Bullock wrote: > 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.
IS_POWER_OF_2 and is_power_of_2_u64 are inteldrm specific is_power_of_2 is a linux interface. radeon_check_pot_argument should be replaced in linux by is_power_of_2. https://lists.freedesktop.org/archives/amd-gfx/2021-December/073108.html No code outside of drm should be using the linux interfaces and I don't think we should be documenting them. > > And manual checks like > sys/arch/amd64/amd64/identcpu.c:804 > powerof2 = ((x - 1) & x) == 0; > > -- > Ted Bullock <tbull...@comlore.com> > >
