[PATCH] linux/bits.h: adjust GENMASK_INPUT_CHECK() check

[Emil Velikov]

Recently the GENMASK_INPUT_CHECK() was added, aiming to catch cases
where there GENMASK arguments are flipped.

Although it seems to be triggering -Wtype-limits in the following cases:

   unsigned foo = (10 + x);
   unsigned bar = GENMASK(foo, 0);

   const unsigned foo = (10 + x);
   unsigned bar = GENMASK(foo, 0);

Here are the warnings, from my GCC 9.2 box.

warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
   __builtin_constant_p((l) > (h)), (l) > (h), 0)))
                            ^
warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
   __builtin_constant_p((l) > (h)), (l) > (h), 0)))
                                        ^

This results in people disabling the warning all together or promoting
foo to signed. Either of which being a sub par option IMHO.

Add a trivial "+ 1" to each h and l in the constant expression.

Fixes: 295bcca84916 ("linux/bits.h: add compile time sanity check of
GENMASK inputs")
Cc: Rikard Falkeborn <rikard.falkeborn at gmail.com>
Cc: Linus Torvalds <torvalds at linux-foundation.org>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
Cc: dri-devel at lists.freedesktop.org
Signed-off-by: Emil Velikov <emil.l.velikov at gmail.com>
---
>From some quick testing, this works as expected although I could be
wrong.

-Emil
---
 include/linux/bits.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/bits.h b/include/linux/bits.h
index 4671fbf28842..5e3ca57adc20 100644
--- a/include/linux/bits.h
+++ b/include/linux/bits.h
@@ -22,8 +22,8 @@
 	(!defined(CONFIG_CC_IS_GCC) || CONFIG_GCC_VERSION >= 49000)
 #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)))
+	(BUILD_BUG_ON_ZERO(!__builtin_choose_expr( \
+		__builtin_constant_p((l + 1) > (h + 1)), (l + 1) > (h + 1), 0)))
 #else
 /*
  * BUILD_BUG_ON_ZERO is not available in h files included from asm files,
-- 
2.23.0