linux-kernel - Re: [PATCH treewide v3 2/4] bitfield: Add non-constant field

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Message-ID: <20250214164614.29bbc620@pumpkin>
Date: Fri, 14 Feb 2025 16:46:14 +0000
From: David Laight <david.laight.linux@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Geert Uytterhoeven <geert+renesas@...der.be>, Michael Turquette
 <mturquette@...libre.com>, Stephen Boyd <sboyd@...nel.org>, Nicolas Ferre
 <nicolas.ferre@...rochip.com>, Alexandre Belloni
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 Giovanni Cabiddu <giovanni.cabiddu@...el.com>, Herbert Xu
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 Walleij <linus.walleij@...aro.org>, Bartosz Golaszewski <brgl@...ev.pl>,
 Joel Stanley <joel@....id.au>, Andrew Jeffery
 <andrew@...econstruct.com.au>, Crt Mori <cmo@...exis.com>, Jonathan Cameron
 <jic23@...nel.org>, Lars-Peter Clausen <lars@...afoo.de>, Jacky Huang
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 <yury.norov@...il.com>, Rasmus Villemoes <linux@...musvillemoes.dk>,
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 Berg <johannes@...solutions.net>, Alex Elder <elder@...e.org>, Vincent
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 linux-arm-kernel@...ts.infradead.org, linux-renesas-soc@...r.kernel.org,
 linux-crypto@...r.kernel.org, qat-linux@...el.com,
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 linux-iio@...r.kernel.org, linux-sound@...r.kernel.org,
 linux-kernel@...r.kernel.org, Jonathan Cameron
 <Jonathan.Cameron@...wei.com>
Subject: Re: [PATCH treewide v3 2/4] bitfield: Add non-constant
 field_{prep,get}() helpers

On Fri, 14 Feb 2025 07:34:02 -0800
Jakub Kicinski <kuba@...nel.org> wrote:

> On Fri, 14 Feb 2025 14:55:51 +0100 Geert Uytterhoeven wrote:
> > The existing FIELD_{GET,PREP}() macros are limited to compile-time
> > constants.  However, it is very common to prepare or extract bitfield
> > elements where the bitfield mask is not a compile-time constant.
> > 
> > To avoid this limitation, the AT91 clock driver and several other
> > drivers already have their own non-const field_{prep,get}() macros.
> > Make them available for general use by consolidating them in
> > <linux/bitfield.h>, and improve them slightly:
> >   1. Avoid evaluating macro parameters more than once,
> >   2. Replace "ffs() - 1" by "__ffs()",
> >   3. Support 64-bit use on 32-bit architectures.
> > 
> > This is deliberately not merged into the existing FIELD_{GET,PREP}()
> > macros, as people expressed the desire to keep stricter variants for
> > increased safety, or for performance critical paths.  
> 
> I really really think that people should just use the static inline
> helpers if the field is not constant. And we should do something like
> below so that people can actually find them.

Especially since you really don't want to be calling ffs() on variables.

Much better to have saved the low bit and field width/mask.

	David