On Sun, Jun 7, 2020 at 11:34 PM Rikard Falkeborn
<rikard.falkeb...@gmail.com> wrote:
>
> When calling the GENMASK and GENMASK_ULL macros with zero lower bit and
> an unsigned unknown high bit, some gcc versions warn due to the
> comparisons of the high and low bit in GENMASK_INPUT_CHECK.
>
> To silence the warnings, only perform the check if both inputs are
> known. This does not trigger any warnings, from the Wtype-limits help:
>
> Warn if a comparison is always true or always false due to the
> limited range of the data type, but do not warn for constant
> expressions.
>
> As an example of the warning, kindly reported by the kbuild test robot:
>
> from drivers/mfd/atmel-smc.c:11:
> drivers/mfd/atmel-smc.c: In function 'atmel_smc_cs_encode_ncycles':
> include/linux/bits.h:26:28: warning: comparison of unsigned expression < 0 is
> always false [-Wtype-limits]
> 26 | __builtin_constant_p((l) > (h)), (l) > (h), 0)))
> | ^
> include/linux/build_bug.h:16:62: note: in definition of macro
> 'BUILD_BUG_ON_ZERO'
> 16 | #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
> | ^
> include/linux/bits.h:39:3: note: in expansion of macro 'GENMASK_INPUT_CHECK'
> 39 | (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
> | ^~~~~~~~~~~~~~~~~~~
> >> drivers/mfd/atmel-smc.c:49:25: note: in expansion of macro 'GENMASK'
> 49 | unsigned int lsbmask = GENMASK(msbpos - 1, 0);
> | ^~~~~~~
It's much better, than previous variant, thanks!
FWIW,
Reviewed-by: Andy Shevchenko <andy.shevche...@gmail.com>
> Fixes: 295bcca84916 ("linux/bits.h: add compile time sanity check of GENMASK
> inputs")
> Reported-by: kbuild test robot <l...@intel.com>
> Reported-by: Emil Velikov <emil.l.veli...@gmail.com>
> Reported-by: Syed Nayyar Waris <syednwa...@gmail.com>
> Signed-off-by: Rikard Falkeborn <rikard.falkeb...@gmail.com>
> ---
> v1->v2
> * Change to require both high and low bit to be constant expressions
> instead of introducing somewhat arbitrary casts
>
> include/linux/bits.h | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/bits.h b/include/linux/bits.h
> index 4671fbf28842..35ca3f5d11a0 100644
> --- a/include/linux/bits.h
> +++ b/include/linux/bits.h
> @@ -23,7 +23,8 @@
> #include <linux/build_bug.h>
> #define GENMASK_INPUT_CHECK(h, l) \
> (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> - __builtin_constant_p((l) > (h)), (l) > (h), 0)))
> + __builtin_constant_p(l) && __builtin_constant_p(h), \
> + (l) > (h), 0)))
> #else
> /*
> * BUILD_BUG_ON_ZERO is not available in h files included from asm files,
> --
> 2.27.0
>
--
With Best Regards,
Andy Shevchenko
