linux-kernel - Re: [PATCH v3] add equivalent of BIT(x) for bitfields

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Message-ID: <87bmwom8on.fsf@purkki.adurom.net>
Date:   Wed, 07 Dec 2016 10:42:48 +0200
From:   Kalle Valo <kvalo@...eaurora.org>
To:     Sebastian Frias <sf84@...oste.net>
Cc:     zijun_hu <zijun_hu@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mason <slash.tmp@...e.fr>,
        Harvey Harrison <harvey.harrison@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>
Subject: Re: [PATCH v3] add equivalent of BIT(x) for bitfields

Sebastian Frias <sf84@...oste.net> writes:

> Introduce GENVALUE(msb, lsb, value) macro to ease dealing with
> continuous bitfields, just as BIT(x) does for single bits.
>
> GENVALUE_ULL(msb, lsb, value) macro is also added.
>
> This is useful mostly for creating values to be packed together
> via OR operations, ex:
>
>    u32 val = 0x11110000;
>    val |= GENVALUE(19, 12, 0x5a);
>
> now 'val = 0x1115a000'
>
>
> Signed-off-by: Sebastian Frias <sf84@...oste.net>
> Link: https://marc.info/?l=linux-kernel&m=148094498711000&w=2
> ---
>
> Change in v2:
> - rename the macro to GENVALUE as proposed by Linus
> - longer comment attempts to show use case for the macro as
> proposed by Borislav
>
> Change in v3:
> - use BUILD_BUG_ON_ZERO() to break if some input parameters
> (essentially 'lsb' but also 'msb') are not constants as
> proposed by Linus.
> Indeed, 'lsb' is used twice so it cannot have side-effects;
> 'msb' is subjected to same constraints for consistency.

(I missed there was v3 already, but I'll repeat what I said in v1.)

Please check FIELD_PREP() from include/linux/bitfield.h, doesn't it
already do the same?

Adding Jakub to comment more.

-- 
Kalle Valo