linux-kernel - [PATCH v2] add equivalent of BIT(x) for bitfields

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Message-ID: <93523b80-6033-bafa-c1ec-4bc2c3f9c53b@laposte.net>
Date:   Mon, 5 Dec 2016 18:49:30 +0100
From:   Sebastian Frias <sf84@...oste.net>
To:     zijun_hu <zijun_hu@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Mason <slash.tmp@...e.fr>,
        Harvey Harrison <harvey.harrison@...il.com>,
        Borislav Petkov <bp@...en8.de>
Subject: [PATCH v2] add equivalent of BIT(x) for bitfields

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@...oste.net>
---

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

---
 include/linux/bitops.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index a83c822..641675d 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -24,6 +24,20 @@
 #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)     \
+	(((val) << (lsb)) & (GENMASK((msb), (lsb))))
+#define GENVALUE_ULL(msb, lsb, val) \
+	(((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