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

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Message-ID: <CAMuHMdXZKNtAmiMP8uuSngZMsDLGcYwrLS0xNWzN4UfLaccdyA@mail.gmail.com>
Date: Mon, 3 Feb 2025 14:59:36 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Vincent Mailhol <mailhol.vincent@...adoo.fr>
Cc: Johannes Berg <johannes@...solutions.net>, Yury Norov <yury.norov@...il.com>, 
	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>, Rasmus Villemoes <linux@...musvillemoes.dk>, 
	Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>, Jakub Kicinski <kuba@...nel.org>, 
	Alex Elder <elder@...e.org>
Subject: Re: [PATCH treewide v2 1/3] bitfield: Add non-constant
 field_{prep,get}() helpers

Hi Vincent,

On Mon, 3 Feb 2025 at 14:37, Vincent Mailhol <mailhol.vincent@...adoo.fr> wrote:
> On 03/02/2025 at 16:44, Johannes Berg wrote:
> > On Sun, 2025-02-02 at 12:53 -0500, Yury Norov wrote:
> >>> Instead of creating another variant for
> >>> non-constant bitfields, wouldn't it be better to make the existing macro
> >>> accept both?
> >>
> >> Yes, it would definitely be better IMO.
> >
> > On the flip side, there have been discussions in the past (though I
> > think not all, if any, on the list(s)) about the argument order. Since
> > the value is typically not a constant, requiring the mask to be a
> > constant has ensured that the argument order isn't as easily mixed up as
> > otherwise.
>
> If this is a concern, then it can be checked with:
>
>   BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask) &&
>                    __builtin_constant_p(_val),
>                    _pfx "mask is not constant");
>
> It means that we forbid FIELD_PREP(non_const_mask, const_val) but allow
> any other combination.

Even that case looks valid to me. Actually there is already such a user
in drivers/iio/temperature/mlx90614.c:

    ret |= field_prep(chip_info->fir_config_mask, MLX90614_CONST_FIR);

So if you want enhanced safety, having both the safer/const upper-case
variants and the less-safe/non-const lower-case variants makes sense.

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