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

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Message-ID: <CAMuHMdXost7vL5uTocMGtrqhEk5AY3QUWvyP5w7_hBtf3MkMfA@mail.gmail.com>
Date: Fri, 17 Oct 2025 12:55:02 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: David Laight <david.laight.linux@...il.com>
Cc: Jakub Kicinski <kuba@...nel.org>, Michael Turquette <mturquette@...libre.com>, 
	Stephen Boyd <sboyd@...nel.org>, Nicolas Ferre <nicolas.ferre@...rochip.com>, 
	Alexandre Belloni <alexandre.belloni@...tlin.com>, Claudiu Beznea <claudiu.beznea@...on.dev>, 
	Giovanni Cabiddu <giovanni.cabiddu@...el.com>, Herbert Xu <herbert@...dor.apana.org.au>, 
	David Miller <davem@...emloft.net>, Linus 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 <ychuang3@...oton.com>, 
	Shan-Chun Hung <schung@...oton.com>, Yury Norov <yury.norov@...il.com>, 
	Rasmus Villemoes <linux@...musvillemoes.dk>, Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>, 
	Johannes Berg <johannes@...solutions.net>, Alex Elder <elder@...e.org>, 
	Vincent Mailhol <mailhol.vincent@...adoo.fr>, linux-clk@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-renesas-soc@...r.kernel.org, 
	linux-crypto@...r.kernel.org, qat-linux@...el.com, linux-gpio@...r.kernel.org, 
	linux-aspeed@...ts.ozlabs.org, 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

Hi David,

On Fri, 14 Feb 2025 at 17:46, David Laight <david.laight.linux@...il.com> wrote:
> 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.

It is not that bad, as most temporary architectures have an instruction
for that.

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

While that would allow some space saving (only 10 or 12 bits needed to
store low + width), gcc would generate quite some code to create the
mask (even on PowerPC, where I expected a single instruction would
do ;-).


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