bitfield: get rid of __MAKE_OP machinery

The __MAKE_OP machinery hides the fixed-width bitfield helper definitions
from source searches and makes the end result highly obscured and largely
uncontrolled.

This follows earlier discussions about making these helpers easier to
find: [1], [2].

Move the explicit helpers and their shared checks into
linux/bitfield-fix-width.h in both the kernel and tools headers, and
include it from bitfield.h to preserve existing users.

The repeated overflow check is factored into __assert_field(), preserving
the original condition and compile-time diagnostics.

With GCC 15.2.0 and x86-64 defconfig plus the bitfield KUnit tests, the
before/after kernel builds are binary identical.

The __MAKE_OP generates the following 40 functions (including the direct
____MAKE_OP(u8,u8,,) invocation):

  u8_encode_bits()
  u8_replace_bits()
  u8p_replace_bits()
  u8_get_bits()
  le16_encode_bits()
  le16_replace_bits() [dead code]
  le16p_replace_bits()
  le16_get_bits()
  be16_encode_bits()
  be16_replace_bits() [dead code]
  be16p_replace_bits() [dead code]
  be16_get_bits()
  u16_encode_bits()
  u16_replace_bits()
  u16p_replace_bits()
  u16_get_bits()
  le32_encode_bits()
  le32_replace_bits() [dead code]
  le32p_replace_bits()
  le32_get_bits()
  be32_encode_bits()
  be32_replace_bits() [dead code]
  be32p_replace_bits()
  be32_get_bits()
  u32_encode_bits()
  u32_replace_bits()
  u32p_replace_bits()
  u32_get_bits()
  le64_encode_bits()
  le64_replace_bits() [dead code]
  le64p_replace_bits() [dead code]
  le64_get_bits()
  be64_encode_bits()
  be64_replace_bits() [dead code]
  be64p_replace_bits() [dead code]
  be64_get_bits()
  u64_encode_bits()
  u64_replace_bits()
  u64p_replace_bits()
  u64_get_bits()

Functions marked with [dead code] have no in-tree callers and are
removed by this change.

Link: https://lore.kernel.org/all/20250214073402.0129e259@kernel.org/ [1]
Link: https://lore.kernel.org/all/aeub59FBHbCy-KKP@yury/ [2]
Assisted-by: OpenAI Codex
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Yury Norov <ynorov@nvidia.com>

2 files changed, 237 insertions, 48 deletions

diff --git a/tools/include/linux/bitfield-fix-width.h b/tools/include/linux/bitfield-fix-width.h
new file mode 100644
index 000000000000..8c2a0fa224e9
--- /dev/null
+++ b/tools/include/linux/bitfield-fix-width.h
@@ -0,0 +1,236 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef _LINUX_BITFIELD_FIX_WIDTH_H
+#define _LINUX_BITFIELD_FIX_WIDTH_H
+
+#include <linux/compiler.h>
+#include <linux/kernel.h>
+#include <linux/types.h>
+#include <asm/byteorder.h>
+
+extern void __compiletime_error("value doesn't fit into mask")
+__field_overflow(void);
+
+extern void __compiletime_error("bad bitfield mask")
+__bad_mask(void);
+
+static __always_inline
+u64 field_multiplier(u64 field)
+{
+	if ((field | (field - 1)) & ((field | (field - 1)) + 1))
+		__bad_mask();
+	return field & -field;
+}
+
+static __always_inline
+u64 field_mask(u64 field)
+{
+	return field / field_multiplier(field);
+}
+
+#define field_max(field)	((typeof(field))field_mask(field))
+
+#define __assert_field(v, field)						\
+	do {								\
+		if (__builtin_constant_p(v) &&				\
+		    ((v) & ~field_mask(field)))				\
+			__field_overflow();				\
+	} while (0)
+
+static __always_inline
+__u8 u8_encode_bits(u8 v, u8 field)
+{
+	__assert_field(v, field);
+	return (v & field_mask(field)) * field_multiplier(field);
+}
+
+static __always_inline
+__u8 u8_replace_bits(__u8 old, u8 val, u8 field)
+{
+	return (old & ~field) | u8_encode_bits(val, field);
+}
+
+static __always_inline
+void u8p_replace_bits(__u8 *p, u8 val, u8 field)
+{
+	*p = (*p & ~field) | u8_encode_bits(val, field);
+}
+
+static __always_inline
+u8 u8_get_bits(__u8 v, u8 field)
+{
+	return (v & field) / field_multiplier(field);
+}
+
+static __always_inline
+__le16 le16_encode_bits(u16 v, u16 field)
+{
+	__assert_field(v, field);
+	return cpu_to_le16((v & field_mask(field)) * field_multiplier(field));
+}
+
+static __always_inline
+void le16p_replace_bits(__le16 *p, u16 val, u16 field)
+{
+	*p = (*p & ~cpu_to_le16(field)) | le16_encode_bits(val, field);
+}
+
+static __always_inline
+u16 le16_get_bits(__le16 v, u16 field)
+{
+	return (le16_to_cpu(v) & field) / field_multiplier(field);
+}
+
+static __always_inline
+__be16 be16_encode_bits(u16 v, u16 field)
+{
+	__assert_field(v, field);
+	return cpu_to_be16((v & field_mask(field)) * field_multiplier(field));
+}
+
+static __always_inline
+u16 be16_get_bits(__be16 v, u16 field)
+{
+	return (be16_to_cpu(v) & field) / field_multiplier(field);
+}
+
+static __always_inline
+__u16 u16_encode_bits(u16 v, u16 field)
+{
+	__assert_field(v, field);
+	return (v & field_mask(field)) * field_multiplier(field);
+}
+
+static __always_inline
+__u16 u16_replace_bits(__u16 old, u16 val, u16 field)
+{
+	return (old & ~field) | u16_encode_bits(val, field);
+}
+
+static __always_inline
+void u16p_replace_bits(__u16 *p, u16 val, u16 field)
+{
+	*p = (*p & ~field) | u16_encode_bits(val, field);
+}
+
+static __always_inline
+u16 u16_get_bits(__u16 v, u16 field)
+{
+	return (v & field) / field_multiplier(field);
+}
+
+static __always_inline
+__le32 le32_encode_bits(u32 v, u32 field)
+{
+	__assert_field(v, field);
+	return cpu_to_le32((v & field_mask(field)) * field_multiplier(field));
+}
+
+static __always_inline
+void le32p_replace_bits(__le32 *p, u32 val, u32 field)
+{
+	*p = (*p & ~cpu_to_le32(field)) | le32_encode_bits(val, field);
+}
+
+static __always_inline
+u32 le32_get_bits(__le32 v, u32 field)
+{
+	return (le32_to_cpu(v) & field) / field_multiplier(field);
+}
+
+static __always_inline
+__be32 be32_encode_bits(u32 v, u32 field)
+{
+	__assert_field(v, field);
+	return cpu_to_be32((v & field_mask(field)) * field_multiplier(field));
+}
+
+static __always_inline
+void be32p_replace_bits(__be32 *p, u32 val, u32 field)
+{
+	*p = (*p & ~cpu_to_be32(field)) | be32_encode_bits(val, field);
+}
+
+static __always_inline
+u32 be32_get_bits(__be32 v, u32 field)
+{
+	return (be32_to_cpu(v) & field) / field_multiplier(field);
+}
+
+static __always_inline
+__u32 u32_encode_bits(u32 v, u32 field)
+{
+	__assert_field(v, field);
+	return (v & field_mask(field)) * field_multiplier(field);
+}
+
+static __always_inline
+__u32 u32_replace_bits(__u32 old, u32 val, u32 field)
+{
+	return (old & ~field) | u32_encode_bits(val, field);
+}
+
+static __always_inline
+void u32p_replace_bits(__u32 *p, u32 val, u32 field)
+{
+	*p = (*p & ~field) | u32_encode_bits(val, field);
+}
+
+static __always_inline
+u32 u32_get_bits(__u32 v, u32 field)
+{
+	return (v & field) / field_multiplier(field);
+}
+
+static __always_inline
+__le64 le64_encode_bits(u64 v, u64 field)
+{
+	__assert_field(v, field);
+	return cpu_to_le64((v & field_mask(field)) * field_multiplier(field));
+}
+
+static __always_inline
+u64 le64_get_bits(__le64 v, u64 field)
+{
+	return (le64_to_cpu(v) & field) / field_multiplier(field);
+}
+
+static __always_inline
+__be64 be64_encode_bits(u64 v, u64 field)
+{
+	__assert_field(v, field);
+	return cpu_to_be64((v & field_mask(field)) * field_multiplier(field));
+}
+
+static __always_inline
+u64 be64_get_bits(__be64 v, u64 field)
+{
+	return (be64_to_cpu(v) & field) / field_multiplier(field);
+}
+
+static __always_inline
+__u64 u64_encode_bits(u64 v, u64 field)
+{
+	__assert_field(v, field);
+	return (v & field_mask(field)) * field_multiplier(field);
+}
+
+static __always_inline
+__u64 u64_replace_bits(__u64 old, u64 val, u64 field)
+{
+	return (old & ~field) | u64_encode_bits(val, field);
+}
+
+static __always_inline
+void u64p_replace_bits(__u64 *p, u64 val, u64 field)
+{
+	*p = (*p & ~field) | u64_encode_bits(val, field);
+}
+
+static __always_inline
+u64 u64_get_bits(__u64 v, u64 field)
+{
+	return (v & field) / field_multiplier(field);
+}
+
+#endif /* _LINUX_BITFIELD_FIX_WIDTH_H */
diff --git a/tools/include/linux/bitfield.h b/tools/include/linux/bitfield.h
index ddf81f24956b..4508cefabec9 100644
--- a/tools/include/linux/bitfield.h
+++ b/tools/include/linux/bitfield.h
@@ -9,7 +9,7 @@
 
 #include <linux/build_bug.h>
 #include <linux/kernel.h>
-#include <asm/byteorder.h>
+#include <linux/bitfield-fix-width.h>
 
 /*
  * Bitfield access macros
@@ -127,51 +127,4 @@
 		(typeof(_mask))(((_reg) & (_mask)) >> __bf_shf(_mask));	\
 	})
 
-extern void __compiletime_error("value doesn't fit into mask")
-__field_overflow(void);
-extern void __compiletime_error("bad bitfield mask")
-__bad_mask(void);
-static __always_inline u64 field_multiplier(u64 field)
-{
-	if ((field | (field - 1)) & ((field | (field - 1)) + 1))
-		__bad_mask();
-	return field & -field;
-}
-static __always_inline u64 field_mask(u64 field)
-{
-	return field / field_multiplier(field);
-}
-#define field_max(field)	((typeof(field))field_mask(field))
-#define ____MAKE_OP(type,base,to,from)					\
-static __always_inline __##type type##_encode_bits(base v, base field)	\
-{									\
-	if (__builtin_constant_p(v) && (v & ~field_mask(field)))	\
-		__field_overflow();					\
-	return to((v & field_mask(field)) * field_multiplier(field));	\
-}									\
-static __always_inline __##type type##_replace_bits(__##type old,	\
-					base val, base field)		\
-{									\
-	return (old & ~to(field)) | type##_encode_bits(val, field);	\
-}									\
-static __always_inline void type##p_replace_bits(__##type *p,		\
-					base val, base field)		\
-{									\
-	*p = (*p & ~to(field)) | type##_encode_bits(val, field);	\
-}									\
-static __always_inline base type##_get_bits(__##type v, base field)	\
-{									\
-	return (from(v) & field)/field_multiplier(field);		\
-}
-#define __MAKE_OP(size)							\
-	____MAKE_OP(le##size,u##size,cpu_to_le##size,le##size##_to_cpu)	\
-	____MAKE_OP(be##size,u##size,cpu_to_be##size,be##size##_to_cpu)	\
-	____MAKE_OP(u##size,u##size,,)
-____MAKE_OP(u8,u8,,)
-__MAKE_OP(16)
-__MAKE_OP(32)
-__MAKE_OP(64)
-#undef __MAKE_OP
-#undef ____MAKE_OP
-
 #endif