Re: [PATCH v6 12/26] bitfield: Add less-checking __FIELD_{GET,PREP}()

Andy Shevchenko
2025-11-06T14:44:48Z

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]

The positive effect that one may click that on Git Web.
Ideally, of course, would be an additional parses on Git Web kernel.org uses to
parse that standard "commit ...()" notation to add the respective HREF link.

> Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>

...

> + 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?

--
With Best Regards,
Andy Shevchenko