[PATCH v5 2/7] bits: introduce fixed-type genmasks

Vincent Mailhol via B4 Relay
2025-03-06T11:31:07Z

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

Add GENMASK_TYPE() 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:51:27: error: right shift count >= width of type [-Werror=shift-count-overflow]
51 | type_max(t) >> (BITS_PER_TYPE(t) - 1 - (h)))))
| ^~

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>
Co-developed-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
---
Changelog:

v4 -> v5:

- Rename GENMASK_t() to GENMASK_TYPE().

- Fix typo in patch description.

- Use tab indentations instead of single space to separate the
macro name from its body.

- s/__GENMASK_U*()/GENMASK_U*()/g in the comment.

- Add a tag to credit myself as Co-developer. Keep Yury as the
main author.

- Modify GENMASK_TYPE() to match the changes made to __GENMASK()
in: https://github.com/norov/linux/commit/1e7933a575ed

- Replace (t)~_ULL(0) with type_max(t). This is OK because
GENMASK_TYPE() is not available in asm.

- linux/const.h and asm/bitsperlong.h are not used anymore. Remove
them.

- Apply GENMASK_TYPE() to GENMASK_U128().

- Remove the unsigned int cast for the U8 and U16 variants. Cast
to the target type instead. Do that cast directly in
GENMASK_TYPE().

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 | 47 +++++++++++++++++++++++++++++++----------------
2 files changed, 31 insertions(+), 17 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 4819cbe7bd48fbae796fc6005c9f92b1c93a034c..74219521a56e2639ccff7fdc899d6805ee355a0c 100644
--- a/include/linux/bits.h
+++ b/include/linux/bits.h
@@ -2,16 +2,15 @@
#ifndef __LINUX_BITS_H
#define __LINUX_BITS_H

-#include <linux/const.h>
#include <vdso/bits.h>
#include <uapi/linux/bits.h>
-#include <asm/bitsperlong.h>

#define BIT_MASK(nr) (UL(1) << ((nr) % BITS_PER_LONG))
#define BIT_WORD(nr) ((nr) / BITS_PER_LONG)
#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
@@ -20,28 +19,44 @@
*/
#if !defined(__ASSEMBLY__)

+/*
+ * Missing asm support
+ *
+ * GENMASK_U*() depends on BITS_PER_TYPE() which relies on sizeof(),
+ * something not available in asm. Nethertheless, fixed width integers
+ * is a C concept. Assembly code can rely on the long and long long
+ * versions instead.
+ */
+
#include <linux/build_bug.h>
#include <linux/compiler.h>
+#include <linux/overflow.h>

#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))
-
/*
- * Missing asm support
+ * 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_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
- * such as long and unsigned long. The fundamental problem is
- * that a 128 bit constant will get silently truncated by the
- * gcc compiler.
+ * - 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_U128(h, l) \
- (GENMASK_INPUT_CHECK(h, l) + __GENMASK_U128(h, l))
+#define GENMASK_TYPE(t, h, l) \
+ ((t)(GENMASK_INPUT_CHECK(h, l) + \
+ (type_max(t) << (l) & \
+ type_max(t) >> (BITS_PER_TYPE(t) - 1 - (h)))))
+
+#define GENMASK(h, l) GENMASK_TYPE(unsigned long, h, l)
+#define GENMASK_ULL(h, l) GENMASK_TYPE(unsigned long long, h, l)
+
+#define GENMASK_U8(h, l) GENMASK_TYPE(u8, h, l)
+#define GENMASK_U16(h, l) GENMASK_TYPE(u16, h, l)
+#define GENMASK_U32(h, l) GENMASK_TYPE(u32, h, l)
+#define GENMASK_U64(h, l) GENMASK_TYPE(u64, h, l)
+#define GENMASK_U128(h, l) GENMASK_TYPE(u128, h, l)

#else /* defined(__ASSEMBLY__) */

--
2.45.3