linux-kernel - Re: [PATCH 08/22] bitops: introduce MANY

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Date:   Tue, 10 May 2022 12:16:55 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Max Filippov <jcmvbkbc@...il.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        David Laight <David.Laight@...lab.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joe Perches <joe@...ches.com>,
        Julia Lawall <Julia.Lawall@...ia.fr>,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        Nicholas Piggin <npiggin@...il.com>,
        Nicolas Palix <nicolas.palix@...g.fr>,
        Peter Zijlstra <peterz@...radead.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Matti Vaittinen <Matti.Vaittinen@...rohmeurope.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Chris Zankel <chris@...kel.net>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        "open list:TENSILICA XTENSA PORT (xtensa)" 
        <linux-xtensa@...ux-xtensa.org>
Subject: Re: [PATCH 08/22] bitops: introduce MANY_BITS() macro

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 <yury.norov@...il.com> 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