linux-kernel - Re: [PATCH v6 12/26] bitfield: Add less-checking __FIELD

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Message-ID: <CAMuHMdXVUJq36GvNUQE8FnHsX+=1jG4GOJ_034r=fgr_Rw4Djg@mail.gmail.com>
Date: Thu, 6 Nov 2025 15:49:03 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Andy Shevchenko <andriy.shevchenko@...el.com>
Cc: Yury Norov <yury.norov@...il.com>, Michael Turquette <mturquette@...libre.com>, 
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Subject: Re: [PATCH v6 12/26] bitfield: Add less-checking __FIELD_{GET,PREP}()

Hi Andy,

On Thu, 6 Nov 2025 at 15:44, Andy Shevchenko
<andriy.shevchenko@...el.com> wrote:
> On Thu, Nov 06, 2025 at 02:34:00PM +0100, Geert Uytterhoeven wrote:
> > The BUILD_BUG_ON_MSG() check against "~0ull" works only with "unsigned
> > (long) long" _mask types.  For constant masks, that condition is usually
> > met, as GENMASK() yields an UL value.  The few places where the
> > constant mask is stored in an intermediate variable were fixed by
> > changing the variable type to u64 (see e.g. [1] and [2]).
> >
> > However, for non-constant masks, smaller unsigned types should be valid,
> > too, but currently lead to "result of comparison of constant
> > 18446744073709551615 with expression of type ... is always
> > false"-warnings with clang and W=1.
> >
> > Hence refactor the __BF_FIELD_CHECK() helper, and factor out
> > __FIELD_{GET,PREP}().  The later lack the single problematic check, but
> > are otherwise identical to FIELD_{GET,PREP}(), and are intended to be
> > used in the fully non-const variants later.
> >
> > [1] commit 5c667d5a5a3ec166 ("clk: sp7021: Adjust width of _m in
> >     HWM_FIELD_PREP()")
> > [2] commit cfd6fb45cfaf46fa ("crypto: ccree - avoid out-of-range
> >     warnings from clang")
>
> Also can be made as
>
> Link: https://git.kernel.org/torvalds/c/5c667d5a5a3ec166 [1]

Nooooh... torvalds might click on it, and complain ;-)

> > +     BUILD_BUG_ON_MSG(__bf_cast_unsigned(mask, mask) >               \
> > +                      __bf_cast_unsigned(reg, ~0ull),                \
> > +                      pfx "type of reg too small for mask")
>
> Perhaps we may convert this (and others?) to static_assert():s at some point?

Nick tried that before, without success:
https://lore.kernel.org/all/CAKwvOdm_prtk1UQNJQGidZm44Lk582S3p=of0y46+rVjnSgXJg@mail.gmail.com

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