linux-kernel - Re: [PATCH treewide v2 1/3] bitfield: Add non-constant field

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Message-ID: <CAMuHMdWe1Fe4ZvWHB1tf_NQNg6W9PmJzpEK_28g+ix9an3jw_Q@mail.gmail.com>
Date: Fri, 14 Feb 2025 12:00:44 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Jakub Kicinski <kuba@...nel.org>
Cc: 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, 
	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 S . 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>
Subject: Re: [PATCH treewide v2 1/3] bitfield: Add non-constant
 field_{prep,get}() helpers

Hi Jakub,

On Tue, 4 Feb 2025 at 16:30, Jakub Kicinski <kuba@...nel.org> wrote:
> On Sun, 2 Feb 2025 17:26:04 +0900 Vincent Mailhol wrote:
> > On 31/01/2025 at 22:46, 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.
> >
> > Why is it that the existing FIELD_{GET,PREP}() macros must be limited to
> > compile time constants?
>
> Hard no, some high performance networking drivers use this on
> the fastpath. We want to make sure that the compiler doesn't
> do anything stupid, and decomposes the masks at build time.
>
> The macros work just fine for a *lot* of code:
>
> $ git grep -E 'FIELD_(PREP|GET)\(' | wc -l
> 22407

Indeed.

> BTW aren't u32_get_bits(), u32_replace_bits() etc. not what
> you need in the first place? I think people don't know about
> those, with all due respect the way they are coded up looks
> like an IOCCC submission..

These support only compile-time constants, too.

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