[PATCH v3] add equivalent of BIT(x) for bitfields

[Sebastian Frias]

Introduce GENVALUE(msb, lsb, value) macro to ease dealing with
continuous bitfields, just as BIT(x) does for single bits.

GENVALUE_ULL(msb, lsb, value) macro is also added.

This is useful mostly for creating values to be packed together
via OR operations, ex:

   u32 val = 0x11110000;
   val |= GENVALUE(19, 12, 0x5a);

now 'val = 0x1115a000'


Signed-off-by: Sebastian Frias <sf84@xxxxxxxxxxx>
Link: https://marc.info/?l=linux-kernel&m=148094498711000&w=2
---

Change in v2:
- rename the macro to GENVALUE as proposed by Linus
- longer comment attempts to show use case for the macro as
proposed by Borislav

Change in v3:
- use BUILD_BUG_ON_ZERO() to break if some input parameters
(essentially 'lsb' but also 'msb') are not constants as
proposed by Linus.
Indeed, 'lsb' is used twice so it cannot have side-effects;
'msb' is subjected to same constraints for consistency.

---
 include/linux/bitops.h | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index a83c822..419529a0 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -24,6 +24,31 @@
 #define GENMASK_ULL(h, l) \
 	(((~0ULL) << (l)) & (~0ULL >> (BITS_PER_LONG_LONG - 1 - (h))))
 
+#ifdef	__KERNEL__
+/*
+ * Equivalent of BIT(x) but for contiguous bitfields
+ * GENVALUE(1, 0,0xff) = 0x00000003
+ * GENVALUE(3, 0,0xff) = 0x0000000f
+ * GENVALUE(15,8,0xff) = 0x0000ff00
+ * GENVALUE(6, 6,   1) = 0x00000040 == BIT(6)
+ */
+#define GENVALUE(msb, lsb, val)					\
+	(								\
+		/* BUILD_BUG_ON_ZERO() evaluates to 0 */		\
+		BUILD_BUG_ON_ZERO(!__builtin_constant_p(msb)) |		\
+		BUILD_BUG_ON_ZERO(!__builtin_constant_p(lsb)) |		\
+		(((val) << (lsb)) & (GENMASK((msb), (lsb))))		\
+	)
+
+#define GENVALUE_ULL(msb, lsb, val)					\
+	(								\
+		/* BUILD_BUG_ON_ZERO() evaluates to 0 */		\
+		BUILD_BUG_ON_ZERO(!__builtin_constant_p(msb)) |		\
+		BUILD_BUG_ON_ZERO(!__builtin_constant_p(lsb)) |		\
+		(((val) << (lsb)) & (GENMASK_ULL((msb), (lsb))))	\
+	)
+#endif
+
 extern unsigned int __sw_hweight8(unsigned int w);
 extern unsigned int __sw_hweight16(unsigned int w);
 extern unsigned int __sw_hweight32(unsigned int w);
-- 
1.8.3.1