[patch 10/32] linux/bits.h: fix unsigned less than zero warnings

[Andrew Morton <akpm@linux-foundation.org>]

From: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Subject: linux/bits.h: fix unsigned less than zero warnings

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);
|                         ^~~~~~~

Link: http://lkml.kernel.org/r/20200621054210.14804-1-rikard.falkeborn@gmail.com
Link: http://lkml.kernel.org/r/20200608221823.35799-1-rikard.falkeborn@gmail.com
Fixes: 295bcca84916 ("linux/bits.h: add compile time sanity check of GENMASK inputs")
Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Emil Velikov <emil.l.velikov@gmail.com>
Reported-by: Syed Nayyar Waris <syednwaris@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: William Breathitt Gray <vilhelm.gray@gmail.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/bits.h |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/include/linux/bits.h~linux-bitsh-fix-unsigned-less-than-zero-warnings
+++ a/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,
_