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

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Date:   Tue, 6 Dec 2016 11:42:40 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Sebastian Frias <sf84@...oste.net>
Cc:     zijun_hu <zijun_hu@....com>, Sasha Levin <sasha.levin@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mason <slash.tmp@...e.fr>,
        Maxime Coquelin <maxime.coquelin@...com>,
        Harvey Harrison <harvey.harrison@...il.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] bitops: add equivalent of BIT(x) for bitfields

On Tue, Dec 6, 2016 at 11:31 AM, Sebastian Frias <sf84@...oste.net> wrote:
> On 05/12/16 18:48, Geert Uytterhoeven wrote:
>> On Mon, Dec 5, 2016 at 2:36 PM, Sebastian Frias <sf84@...oste.net> wrote:
>>> Introduce SETBITFIELD(msb, lsb, value) macro to ease dealing with
>>> continuous bitfields, just as BIT(x) does for single bits.
>>
>> If it's a bitfield, why not calling it that way?
>
> I don't know if you saw v2 (or v3 for that matter), but the name was changed
> to GENVALUE.

... which means "generate a value"??

> Also a small use case was added to the commit message:
>
> "Introduce GENVALUE(msb, lsb, value) macro..."
> "...This is useful mostly for creating values to be packed together
> via OR operations, ex:
>
>    u32 val = 0x11110000;
>    val |= GENVALUE(19, 12, 0x5a);

"val |= 0x5a << 12;" looks much more readable to me...

> now 'val = 0x1115a000'"
>
>> So what about BITFIELD(start ,size), like arch/tile/kernel/tile-desc_32.c has?
>>
>>> SETBITFIELD_ULL(msb, lsb, value) macro is also added.
>>
>> Confused by the need for a "value" parameter...
>
> "value" is the value to be massaged (shifted, masked) into a [msb:lsb] bitfield.

OK. So it inserts a value into a bitfield.

Yes, that can be useful. Now let's find a sensible name for this.
Perhaps inspired by a PowerPC mnemonic? At least that would be more
obvious than "GENVALUE", IMHO...

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds