C bitfields are problematic and best avoided. Developers
interacting with hardware registers find themselves searching
for easy-to-use alternatives. Common approach is to define
structures or sets of macros containing mask and shift pair.
Operations on the register are then performed as follows:
field = (reg >> shift) & mask;
reg &= ~(mask << shift);
reg |= (field & mask) << shift;
Defining shift and mask separately is tedious. Ivo van Doorn
came up with an idea of computing them at compilation time
based on a single shifted mask (later refined by Felix) which
can be used like this:
#define REG_FIELD 0x000ff000
field = FIELD_GET(REG_FIELD, reg);
reg &= ~REG_FIELD;
reg |= FIELD_PUT(REG_FIELD, field);
FIELD_{GET,PUT} macros take care of finding out what the
appropriate shift is based on compilation time ffs operation.
GENMASK can be used to define registers (which is usually
less error-prone and easier to match with datasheets).
This approach is the most convenient I've seen so to limit code
multiplication let's move the macros to a global header file.
Signed-off-by: Jakub Kicinski <jakub.kicinski@xxxxxxxxxxxxx>
---
include/linux/bitfield.h | 58 ++++++++++++++++++++++++++++++++++++++++++++++++
include/linux/bug.h | 3 +++
2 files changed, 61 insertions(+)
create mode 100644 include/linux/bitfield.h
diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
new file mode 100644
index 000000000000..d6a36c3c1775
--- /dev/null
+++ b/include/linux/bitfield.h
@@ -0,0 +1,58 @@
+/*
+ * Copyright (C) 2014 Felix Fietkau <nbd@xxxxxxxx>
+ * Copyright (C) 2004 - 2009 Ivo van Doorn <IvDoorn@xxxxxxxxx>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2
+ * as published by the Free Software Foundation
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ */
+
+#ifndef _LINUX_BITFIELD_H
+#define _LINUX_BITFIELD_H
+
+#include <asm/types.h>
+#include <linux/bug.h>
+#include <linux/log2.h>
+
+#define _bf_shf(x) (__builtin_ffsll(x) - 1)
+
+#define _BF_FIELD_CHECK(_mask, _val) \
+ ({ \
+ BUILD_BUG_ON(!(_mask)); \
+ BUILD_BUG_ON(__builtin_constant_p(_val) ? \
+ ~((_mask) >> _bf_shf(_mask)) & (_val) : \
+ 0); \
+ __BUILD_BUG_ON_NOT_POWER_OF_2((_mask) + \
+ (1ULL << _bf_shf(_mask))); \
+ })
+
+#define FIELD_PUT(_mask, _val) \
+ ({ \
+ _BF_FIELD_CHECK(_mask, _val); \
+ ((u32)(_val) << _bf_shf(_mask)) & (_mask); \
+ })
+
+#define FIELD_GET(_mask, _val) \
+ ({ \
+ _BF_FIELD_CHECK(_mask, 0); \
+ (u32)(((_val) & (_mask)) >> _bf_shf(_mask)); \
+ })
+
+#define FIELD_PUT64(_mask, _val) \
+ ({ \
+ _BF_FIELD_CHECK(_mask, _val); \
+ ((u64)(_val) << _bf_shf(_mask)) & (_mask); \
+ })
+
+#define FIELD_GET64(_mask, _val) \
+ ({ \
+ _BF_FIELD_CHECK(_mask, 0); \
+ (u64)(((_val) & (_mask)) >> _bf_shf(_mask)); \
+ })
+
+#endif
diff --git a/include/linux/bug.h b/include/linux/bug.h
index e51b0709e78d..bba5bdae1681 100644
--- a/include/linux/bug.h
+++ b/include/linux/bug.h
@@ -13,6 +13,7 @@ enum bug_trap_type {
struct pt_regs;
#ifdef __CHECKER__
+#define __BUILD_BUG_ON_NOT_POWER_OF_2(n) (0)
#define BUILD_BUG_ON_NOT_POWER_OF_2(n) (0)
#define BUILD_BUG_ON_ZERO(e) (0)
#define BUILD_BUG_ON_NULL(e) ((void*)0)
@@ -24,6 +25,8 @@ struct pt_regs;
#else /* __CHECKER__ */
/* Force a compilation error if a constant expression is not a power of 2 */
+#define __BUILD_BUG_ON_NOT_POWER_OF_2(n) \
+ BUILD_BUG_ON(((n) & ((n) - 1)) != 0)
#define BUILD_BUG_ON_NOT_POWER_OF_2(n) \
BUILD_BUG_ON((n) == 0 || (((n) & ((n) - 1)) != 0))
--
1.9.1
--
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