[PATCH v6 12/26] bitfield: Add less-checking __FIELD_{GET,PREP}()

Geert Uytterhoeven
2025-11-06T13:36:36Z

The BUILD_BUG_ON_MSG() check against "~0ull" works only with "unsigned
(long) long" _mask types. For constant masks, that condition is usually
met, as GENMASK() yields an UL value. The few places where the
constant mask is stored in an intermediate variable were fixed by
changing the variable type to u64 (see e.g. [1] and [2]).

However, for non-constant masks, smaller unsigned types should be valid,
too, but currently lead to "result of comparison of constant
18446744073709551615 with expression of type ... is always
false"-warnings with clang and W=1.

Hence refactor the __BF_FIELD_CHECK() helper, and factor out
__FIELD_{GET,PREP}(). The later lack the single problematic check, but
are otherwise identical to FIELD_{GET,PREP}(), and are intended to be
used in the fully non-const variants later.

[1] commit 5c667d5a5a3ec166 ("clk: sp7021: Adjust width of _m in
HWM_FIELD_PREP()")
[2] commit cfd6fb45cfaf46fa ("crypto: ccree - avoid out-of-range
warnings from clang")

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
---
v6:
- New.

Alternatively, FIELD_{GET,PREP}() could be duplicated, with the second
parameter of __BF_FIELD_CHECK() changed from "0ULL" resp. "_reg" to
"_mask":

#define __FIELD_PREP(_mask, _val) \
({ \
__BF_FIELD_CHECK(_mask, _mask, _val, "__FIELD_PREP: "); \
((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask); \
})

#define __FIELD_GET(_mask, _reg) \
({ \
__BF_FIELD_CHECK(_mask, _mask, 0U, "__FIELD_GET: "); \
(typeof(_mask))(((_reg) & (_mask)) >> __bf_shf(_mask)); \
})

But I think that is less intuitive, and prevents defining
FIELD_{GET,PREP}() using __FIELD_{GET,PREP}().
---
include/linux/bitfield.h | 36 ++++++++++++++++++++++++++++--------
1 file changed, 28 insertions(+), 8 deletions(-)

diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
index 5355f8f806a97974..bf8e0ae4b5b41038 100644
--- a/include/linux/bitfield.h
+++ b/include/linux/bitfield.h
@@ -60,7 +60,7 @@

#define __bf_cast_unsigned(type, x) ((__unsigned_scalar_typeof(type))(x))

-#define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx) \
+#define __BF_FIELD_CHECK_MASK(_mask, _val, _pfx) \
({ \
BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask), \
_pfx "mask is not constant"); \
@@ -69,13 +69,33 @@
~((_mask) >> __bf_shf(_mask)) & \
(0 + (_val)) : 0, \
_pfx "value too large for the field"); \
- BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) > \
- __bf_cast_unsigned(_reg, ~0ull), \
- _pfx "type of reg too small for mask"); \
__BUILD_BUG_ON_NOT_POWER_OF_2((_mask) + \
(1ULL << __bf_shf(_mask))); \
})

+#define __BF_FIELD_CHECK_REG(mask, reg, pfx) \
+ BUILD_BUG_ON_MSG(__bf_cast_unsigned(mask, mask) > \
+ __bf_cast_unsigned(reg, ~0ull), \
+ pfx "type of reg too small for mask")
+
+#define __BF_FIELD_CHECK(mask, reg, val, pfx) \
+ ({ \
+ __BF_FIELD_CHECK_MASK(mask, val, pfx); \
+ __BF_FIELD_CHECK_REG(mask, reg, pfx); \
+ })
+
+#define __FIELD_PREP(mask, val, pfx) \
+ ({ \
+ __BF_FIELD_CHECK_MASK(mask, val, pfx); \
+ ((typeof(mask))(val) << __bf_shf(mask)) & (mask); \
+ })
+
+#define __FIELD_GET(mask, reg, pfx) \
+ ({ \
+ __BF_FIELD_CHECK_MASK(mask, 0U, pfx); \
+ (typeof(mask))(((reg) & (mask)) >> __bf_shf(mask)); \
+ })
+
/**
* FIELD_MAX() - produce the maximum value representable by a field
* @_mask: shifted mask defining the field's length and position
@@ -112,8 +132,8 @@
*/
#define FIELD_PREP(_mask, _val) \
({ \
- __BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: "); \
- ((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask); \
+ __BF_FIELD_CHECK_REG(_mask, 0ULL, "FIELD_PREP: "); \
+ __FIELD_PREP(_mask, _val, "FIELD_PREP: "); \
})

#define __BF_CHECK_POW2(n) BUILD_BUG_ON_ZERO(((n) & ((n) - 1)) != 0)
@@ -152,8 +172,8 @@
*/
#define FIELD_GET(_mask, _reg) \
({ \
- __BF_FIELD_CHECK(_mask, _reg, 0U, "FIELD_GET: "); \
- (typeof(_mask))(((_reg) & (_mask)) >> __bf_shf(_mask)); \
+ __BF_FIELD_CHECK_REG(_mask, _reg, "FIELD_GET: "); \
+ __FIELD_GET(_mask, _reg, "FIELD_GET: "); \
})

/**
--
2.43.0