Hi Yury,
On Tue, May 10, 2022 at 8:48 AM Yury Norov <[email protected]> wrote:
> arch/xtensa/kernel/traps.c and include/linux/log2.h define very similar
> functions with different behaviour. XTENSA defines IS_POW2(), and
> log2.h defines is_power_of_2(). The difference is that IS_POW2()
> considers 0 as power of 2, while is_power_of_2() - does not.
IS_POW2 is constructed this way because we know that there is at least
one non-zero bit in the value that it tests.
> This discrepancy may confuse reader. From mathematical point of view,
> 0 is not a power of 2.
If it would reduce the confusion we can add a check that the value is
non-zero in the IS_POW2 macro.
I'd really like to not introduce the local macro and just use something
standard, but I can't use is_power_of_2 in a preprocessor condition,
can I?
--
Thanks.
-- Max
Hi Max,
On Tue, May 10, 2022 at 10:57:25AM -0700, Max Filippov wrote:
> Hi Yury,
>
> On Tue, May 10, 2022 at 8:48 AM Yury Norov <[email protected]> wrote:
> > arch/xtensa/kernel/traps.c and include/linux/log2.h define very similar
> > functions with different behaviour. XTENSA defines IS_POW2(), and
> > log2.h defines is_power_of_2(). The difference is that IS_POW2()
> > considers 0 as power of 2, while is_power_of_2() - does not.
>
> IS_POW2 is constructed this way because we know that there is at least
> one non-zero bit in the value that it tests.
>
> > This discrepancy may confuse reader. From mathematical point of view,
> > 0 is not a power of 2.
>
> If it would reduce the confusion we can add a check that the value is
> non-zero in the IS_POW2 macro.
>
> I'd really like to not introduce the local macro and just use something
> standard,
This patch introduces a macro MANY_BITS() in include/linux/bitops.h, which
is a full analogue of IS_POW2(). Would it work for you to switch to
MANY_BITS()?
> but I can't use is_power_of_2 in a preprocessor condition, can I?
I believe you can't.
Thanks,
Yury
On Tue, May 10, 2022 at 12:16 PM Yury Norov <[email protected]> wrote:
>
> Hi Max,
>
> On Tue, May 10, 2022 at 10:57:25AM -0700, Max Filippov wrote:
> > Hi Yury,
> >
> > On Tue, May 10, 2022 at 8:48 AM Yury Norov <[email protected]> wrote:
> > > arch/xtensa/kernel/traps.c and include/linux/log2.h define very similar
> > > functions with different behaviour. XTENSA defines IS_POW2(), and
> > > log2.h defines is_power_of_2(). The difference is that IS_POW2()
> > > considers 0 as power of 2, while is_power_of_2() - does not.
> >
> > IS_POW2 is constructed this way because we know that there is at least
> > one non-zero bit in the value that it tests.
> >
> > > This discrepancy may confuse reader. From mathematical point of view,
> > > 0 is not a power of 2.
> >
> > If it would reduce the confusion we can add a check that the value is
> > non-zero in the IS_POW2 macro.
> >
> > I'd really like to not introduce the local macro and just use something
> > standard,
>
> This patch introduces a macro MANY_BITS() in include/linux/bitops.h, which
> is a full analogue of IS_POW2(). Would it work for you to switch to
> MANY_BITS()?
It would, I guess. It would also work if is_power_of_2 was a macro.
--
Thanks.
-- Max