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

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Date:   Tue, 6 Dec 2016 11:31:36 +0100
From:   Sebastian Frias <sf84@...oste.net>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
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 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.
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);

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.

> 
> 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
>