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

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Message-ID: <CAMuHMdU+0HGG22FbO3wNmXtbUm9RhTopYrGghF6UrkFu-iww2A@mail.gmail.com>
Date: Fri, 17 Oct 2025 12:55:00 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Jakub Kicinski <kuba@...nel.org>
Cc: 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>, 
	David Laight <david.laight.linux@...il.com>, 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, 
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	Jonathan Cameron <Jonathan.Cameron@...wei.com>
Subject: Re: [PATCH treewide v3 2/4] bitfield: Add non-constant
 field_{prep,get}() helpers

Hi Jakub,

On Fri, 14 Feb 2025 at 16:34, 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.
>
> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
> index 63928f173223..e02afcd7aeee 100644
> --- a/include/linux/bitfield.h
> +++ b/include/linux/bitfield.h
> @@ -156,6 +156,80 @@
>                 (typeof(_mask))(((_reg) & (_mask)) >> __bf_shf(_mask)); \
>         })
>
> +/**
> + * u32_encode_bits() - prepare a u32 bitfield element (non-const)
> + * @v: value to put in the field
> + * @field: shifted mask defining the field's length and position
> + *
> + * Equivalent of FIELD_PREP() for u32, field does not have to be constant.
> + *
> + * Note that the helper is available for other field widths (generated below).
> + */
> +static __always_inline __u32 u32_encode_bits(u32 v, u32 field)
> +{
> +       if (__builtin_constant_p(v) && (v & ~field_mask(field)))
> +               __field_overflow();
> +       return ((v & field_mask(field)) * field_multiplier(field));
> +}

Unfortunately gcc emits actual divisions or __*div*() calls, and
multiplications in the non-constant case.

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