On Thu, Jun 4, 2020 at 1:03 AM Rikard Falkeborn
[off-list ref] 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, cast the inputs to int before doing the
comparisons. The only valid inputs to GENMASK() and GENMASK_ULL() are
are 0 to 31 or 63. Anything outside this is undefined due to the shifts
in GENMASK()/GENMASK_ULL(). Therefore, casting the inputs to int do not
change the values for valid known inputs. For unknown values, the check
does not change anything since it's a compile-time check only.

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))
|   ^~~~~~~~~~~~~~~~~~~

quoted

quoted

drivers/mfd/atmel-smc.c:49:25: note: in expansion of macro 'GENMASK'

49 |  unsigned int lsbmask = GENMASK(msbpos - 1, 0);
|                         ^~~~~~~

Thank you for the patch!

I think there is still a possibility to improve (as I mentioned there
are test cases that are absent right now).
What if we will have unsigned long value 0x100000001? Would it be 1
after casting?

Maybe cast to (long) or (long long) more appropriate?

Please, add test cases.

-- 
With Best Regards,
Andy Shevchenko