Subject: [RFC][PATCH] new byteorder primitives - ..._{replace,get}_bits()

From: Al Viro <[email-masked]>
Date: Mon, 11 Dec 2017 15:54:22 +0000

A lot of drivers are open-coding the "replace these bits in __be32 with
the following value" kind of primitives.  Let's add them to byteorder.h.

Primitives:
	{be,le}{16,32,64}_replace_bits(old, v, bit, nbits)
	{be,le}{16,32,64}_get_bits(val, bit, nbits)

Essentially, it gives helpers for work with bitfields in fixed-endian.
Suppose we have e.g. a little-endian 32bit value with fixed layout;
expressing that as a bitfield would go like
	struct foo {
		unsigned foo:4;		/* bits 0..3 */
		unsigned :2;
		unsigned bar:12;	/* bits 6..17 */
		unsigned baz:14;	/* bits 18..31 */
	}
Even for host-endian it doesn't work all that well - you end up with
ifdefs in structure definition and generated code stinks.  For fixed-endian
it gets really painful, and people tend to use explicit shift-and-mask
kind of macros for accessing the fields (and often enough get the
endianness conversions wrong, at that).  With these primitives

struct foo v		<=>	__le32 v
v.foo = i ? 1 : 2	<=>	v = le32_replace_bits(v, i ? 1 : 2, 0, 4)
f(4 + v.baz)		<=>	f(4 + le32_get_bits(v, 18, 14))

Signed-off-by: Al Viro <[email-masked]>
---
diff --git a/include/linux/byteorder/generic.h b/include/linux/byteorder/generic.h
index 451aaa0786ae..d8f169a7104a 100644
--- a/include/linux/byteorder/generic.h
+++ b/include/linux/byteorder/generic.h
@@ -187,4 +187,26 @@ static inline void be32_to_cpu_array(u32 *dst, const __be32 *src, size_t len)
 		dst[i] = be32_to_cpu(src[i]);
 }
 
+#define ____MASK(bit, nbits) ((((1ULL << ((nbits) - 1)) << 1) - 1) << (bit))
+#define ____MAKE_OP(type,base)						\
+static inline __##type type##_replace_bits(__##type old,		\
+					base val, int bit, int nbits)	\
+{									\
+	__##type mask = cpu_to_##type(____MASK(bit, nbits));		\
+	return (old & ~mask) | (cpu_to_##type(val << bit) & mask);	\
+}									\
+static inline base type##_get_bits(__##type val, int bit, int nbits)	\
+{									\
+	return (type##_to_cpu(val) >> bit) & ____MASK(0, nbits);	\
+}
+
+____MAKE_OP(le16,u16)
+____MAKE_OP(le32,u32)
+____MAKE_OP(le64,u64)
+____MAKE_OP(be16,u16)
+____MAKE_OP(be32,u32)
+____MAKE_OP(be64,u64)
+#undef ____MAKE_OP
+#undef ____MASK
+
 #endif /* _LINUX_BYTEORDER_GENERIC_H */