[PATCH v3 2/3] bits: Introduce fixed-type BIT

[Lucas De Marchi]

Implement fixed-type BIT() to help drivers add stricter checks, like was
done for GENMASK.

Signed-off-by: Lucas De Marchi <lucas.demarchi at intel.com>
Acked-by: Jani Nikula <jani.nikula at intel.com>
---
 include/linux/bits.h | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/include/linux/bits.h b/include/linux/bits.h
index bd56f32de44e..811846ce110e 100644
--- a/include/linux/bits.h
+++ b/include/linux/bits.h
@@ -24,12 +24,16 @@
 #define GENMASK_INPUT_CHECK(h, l) \
 	(BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
 		__is_constexpr((l) > (h)), (l) > (h), 0)))
+#define BIT_INPUT_CHECK(type, b) \
+	((BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
+		__is_constexpr(b), (b) >= BITS_PER_TYPE(type), 0))))
 #else
 /*
  * BUILD_BUG_ON_ZERO is not available in h files included from asm files,
  * disable the input check if that is the case.
  */
 #define GENMASK_INPUT_CHECK(h, l) 0
+#define BIT_INPUT_CHECK(type, b) 0
 #endif
 
 /*
@@ -54,4 +58,17 @@
 #define GENMASK_U32(h, l)	__GENMASK(u32, h, l)
 #define GENMASK_U64(h, l)	__GENMASK(u64, h, l)
 
+/*
+ * Fixed-type variants of BIT(), with additional checks like __GENMASK().  The
+ * following examples generate compiler warnings due to shift-count-overflow:
+ *
+ * - BIT_U8(8)
+ * - BIT_U32(-1)
+ * - BIT_U32(40)
+ */
+#define BIT_U8(b)		((u8)(BIT_INPUT_CHECK(u8, b) + BIT(b)))
+#define BIT_U16(b)		((u16)(BIT_INPUT_CHECK(u16, b) + BIT(b)))
+#define BIT_U32(b)		((u32)(BIT_INPUT_CHECK(u32, b) + BIT(b)))
+#define BIT_U64(b)		((u64)(BIT_INPUT_CHECK(u64, b) + BIT(b)))
+
 #endif	/* __LINUX_BITS_H */
-- 
2.43.0