[PATCH v4 3/8] bits: introduce fixed-type genmasks

Vincent Mailhol via B4 Relay
2025-03-05T13:01:03Z

From: Yury Norov <yury.norov@gmail.com>

Add __GENMASK_t() which generalizes __GENMASK() to support different
types, and implement fixed-types versions of GENMASK() based on it.
The fixed-type version allows more strict checks to the min/max values
accepted, which is useful for defining registers like implemented by
i915 and xe drivers with their REG_GENMASK*() macros.

The strict checks rely on shift-count-overflow compiler check to fail
the build if a number outside of the range allowed is passed.
Example:

#define FOO_MASK GENMASK_U32(33, 4)

will generate a warning like:

../include/linux/bits.h:41:31: error: left shift count >= width of type [-Werror=shift-count-overflow]
41 | (((t)~0ULL - ((t)(1) << (l)) + 1) & \
| ^~

Signed-off-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
---
Changelog:

v3 -> v4:

- The v3 is one year old. Meanwhile people started using
__GENMASK() directly. So instead of generalizing __GENMASK() to
support different types, add a new GENMASK_t().

- replace ~0ULL by ~_ULL(0). Otherwise, __GENMASK_t() would fail
in asm code.

- Make GENMASK_U8() and GENMASK_U16() return an unsigned int. In
v3, due to the integer promotion rules, these were returning a
signed integer. By casting these to unsigned int, at least the
signedness is kept.
---
include/linux/bitops.h | 1 -
include/linux/bits.h | 33 +++++++++++++++++++++++++++++----
2 files changed, 29 insertions(+), 5 deletions(-)

diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index c1cb53cf2f0f8662ed3e324578f74330e63f935d..9be2d50da09a417966b3d11c84092bb2f4cd0bef 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -8,7 +8,6 @@

#include <uapi/linux/kernel.h>

-#define BITS_PER_TYPE(type) (sizeof(type) * BITS_PER_BYTE)
#define BITS_TO_LONGS(nr) __KERNEL_DIV_ROUND_UP(nr, BITS_PER_TYPE(long))
#define BITS_TO_U64(nr) __KERNEL_DIV_ROUND_UP(nr, BITS_PER_TYPE(u64))
#define BITS_TO_U32(nr) __KERNEL_DIV_ROUND_UP(nr, BITS_PER_TYPE(u32))
diff --git a/include/linux/bits.h b/include/linux/bits.h
index 5f68980a1b98d771426872c74d7b5c0f79e5e802..f202e46d2f4b7899c16d975120f3fa3ae41556ae 100644
--- a/include/linux/bits.h
+++ b/include/linux/bits.h
@@ -12,6 +12,7 @@
#define BIT_ULL_MASK(nr) (ULL(1) << ((nr) % BITS_PER_LONG_LONG))
#define BIT_ULL_WORD(nr) ((nr) / BITS_PER_LONG_LONG)
#define BITS_PER_BYTE 8
+#define BITS_PER_TYPE(type) (sizeof(type) * BITS_PER_BYTE)

/*
* Create a contiguous bitmask starting at bit position @l and ending at
@@ -25,14 +26,38 @@

#define GENMASK_INPUT_CHECK(h, l) BUILD_BUG_ON_ZERO(const_true((l) > (h)))

-#define GENMASK(h, l) \
- (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
-#define GENMASK_ULL(h, l) \
- (GENMASK_INPUT_CHECK(h, l) + __GENMASK_ULL(h, l))
+/*
+ * Generate a mask for the specified type @t. Additional checks are made to
+ * guarantee the value returned fits in that type, relying on
+ * shift-count-overflow compiler check to detect incompatible arguments.
+ * For example, all these create build errors or warnings:
+ *
+ * - GENMASK(15, 20): wrong argument order
+ * - GENMASK(72, 15): doesn't fit unsigned long
+ * - GENMASK_U32(33, 15): doesn't fit in a u32
+ */
+#define GENMASK_t(t, h, l) \
+ (GENMASK_INPUT_CHECK(h, l) + \
+ (((t)~ULL(0) - ((t)1 << (l)) + 1) & \
+ ((t)~ULL(0) >> (BITS_PER_TYPE(t) - 1 - (h)))))
+
+#define GENMASK(h, l) GENMASK_t(unsigned long, h, l)
+#define GENMASK_ULL(h, l) GENMASK_t(unsigned long long, h, l)

/*
* Missing asm support
*
+ * __GENMASK_U*() depends on BITS_PER_TYPE() which would not work in the asm
+ * code as BITS_PER_TYPE() relies on sizeof(), something not available in
+ * asm. Nethertheless, the concept of fixed width integers is a C thing which
+ * does not apply to assembly code.
+ */
+#define GENMASK_U8(h, l) ((unsigned int)GENMASK_t(u8, h, l))
+#define GENMASK_U16(h, l) ((unsigned int)GENMASK_t(u16, h, l))
+#define GENMASK_U32(h, l) GENMASK_t(u32, h, l)
+#define GENMASK_U64(h, l) GENMASK_t(u64, h, l)
+
+/*
* __GENMASK_U128() depends on _BIT128() which would not work
* in the asm code, as it shifts an 'unsigned __int128' data
* type instead of direct representation of 128 bit constants

--
2.45.3