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

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Message-ID: <aQzGY4AKiMQpuL0R@yury>
Date: Thu, 6 Nov 2025 11:01:39 -0500
From: Yury Norov <yury.norov@...il.com>
To: Andy Shevchenko <andriy.shevchenko@...el.com>
Cc: Geert Uytterhoeven <geert+renesas@...der.be>,
	Michael Turquette <mturquette@...libre.com>,
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Subject: Re: [PATCH v6 12/26] bitfield: Add less-checking __FIELD_{GET,PREP}()

On Thu, Nov 06, 2025 at 04:44:31PM +0200, Andy Shevchenko 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]
> 
> 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.

Those flying over Atlantic or cruising cross Caribbean would disagree. :)
 
> > Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.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
>