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References: <20220510154750.212913-1-yury.norov@gmail.com> <20220510154750.212913-9-yury.norov@gmail.com>
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From:   Max Filippov <jcmvbkbc@gmail.com>
Date:   Tue, 10 May 2022 10:57:25 -0700
Message-ID: <CAMo8Bf+JiyKD=g00whNOYGynx_0KjC_wJtpEBCB2ynFZ3U+e_Q@mail.gmail.com>
Subject: Re: [PATCH 08/22] bitops: introduce MANY_BITS() macro
To:     Yury Norov <yury.norov@gmail.com>
Cc:     Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        David Laight <David.Laight@aculab.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Joe Perches <joe@perches.com>,
        Julia Lawall <Julia.Lawall@inria.fr>,
        =?UTF-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= <mirq-linux@rere.qmqm.pl>,
        Nicholas Piggin <npiggin@gmail.com>,
        Nicolas Palix <nicolas.palix@imag.fr>,
        Peter Zijlstra <peterz@infradead.org>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Matti Vaittinen <Matti.Vaittinen@fi.rohmeurope.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Chris Zankel <chris@zankel.net>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        "Eric W . Biederman" <ebiederm@xmission.com>,
        Kumar Kartikeya Dwivedi <memxor@gmail.com>,
        =?UTF-8?B?VG9rZSBIw7hpbGFuZC1Kw7hyZ2Vuc2Vu?= <toke@redhat.com>,
        "open list:TENSILICA XTENSA PORT (xtensa)" 
        <linux-xtensa@linux-xtensa.org>
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Hi Yury,

On Tue, May 10, 2022 at 8:48 AM Yury Norov <yury.norov@gmail.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, but I can't use is_power_of_2 in a preprocessor condition,
can I?

-- 
Thanks.
-- Max