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.
Operation 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:
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.
Compared to Felix Fietkau's version I:
- edited the comments a bit;
- gave auxiliary macros _bf_ prefix;
- dropped the UL specifier from 1 in _bf_low_bits,
- renamed the FIELD_SET to FIELD_PUT;
- added 64bit versions.
CC: Ivo van Doorn <ivdo...@gmail.com>
CC: Felix Fietkau <n...@openwrt.org>
Signed-off-by: Jakub Kicinski <jakub.kicin...@netronome.com>
Reviewed-by: Dinan Gunawardena <dgunaward...@netronome.com>
Reviewed-by: Simon Horman <simon.hor...@netronome.com>
---
include/linux/bitfield.h | 89 ++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 89 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..3815c81f5b10
--- /dev/null
+++ b/include/linux/bitfield.h
@@ -0,0 +1,89 @@
+/*
+ * Copyright (C) 2014 Felix Fietkau <n...@openwrt.org>
+ * Copyright (C) 2004 - 2009 Ivo van Doorn <ivdo...@gmail.com>
+ *
+ * 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>
+
+/*
+ * Macros to find first set bit in a variable.
+ * These macros behave the same as the __ffs() functions but the most important
+ * difference that this is done during compile-time rather than run-time.
+ */
+#define compile_ffs2(__x) \
+ __builtin_choose_expr(((__x) & 0x1), 0, 1)
+
+#define compile_ffs4(__x) \
+ __builtin_choose_expr(((__x) & 0x3), \
+ (compile_ffs2((__x))), \
+ (compile_ffs2((__x) >> 2) + 2))
+
+#define compile_ffs8(__x) \
+ __builtin_choose_expr(((__x) & 0xf), \
+ (compile_ffs4((__x))), \
+ (compile_ffs4((__x) >> 4) + 4))
+
+#define compile_ffs16(__x) \
+ __builtin_choose_expr(((__x) & 0xff), \
+ (compile_ffs8((__x))), \
+ (compile_ffs8((__x) >> 8) + 8))
+
+#define compile_ffs32(__x) \
+ __builtin_choose_expr(((__x) & 0xffff), \
+ (compile_ffs16((__x))), \
+ (compile_ffs16((__x) >> 16) + 16))
+
+#define compile_ffs64(__x) \
+ __builtin_choose_expr(((__x) & 0xffffffff), \
+ (compile_ffs32((__x))), \
+ (compile_ffs32(((__x) >> 16) >> 16) + 32))
+
+/*
+ * Macros to validate that the mask given contains a contiguous set of bits.
+ * Note that we cannot use the is_power_of_2() function since this check
+ * must be done at compile-time.
+ */
+#define _bf_is_power_of_two(x) ( !((x) & ((x) - 1)) )
+#define _bf_low_bits(x) ( ((x) - 1) & ~(x) )
+#define _bf_is_valid_mask(x) _bf_is_power_of_two(1U + (x) + _bf_low_bits(x))
+
+#define _BF_FIELD_CHECK(_mask) \
+ BUILD_BUG_ON(!(_mask) || !_bf_is_valid_mask(_mask))
+
+#define FIELD_PUT(_mask, _val) \
+ ({ \
+ _BF_FIELD_CHECK(_mask); \
+ (((u32)(_val)) << compile_ffs32(_mask)) & _mask; \
+ })
+
+#define FIELD_GET(_mask, _val) \
+ ({ \
+ _BF_FIELD_CHECK(_mask); \
+ (u32)(((_val) & _mask) >> compile_ffs32(_mask)); \
+ })
+
+#define FIELD_PUT64(_mask, _val) \
+ ({ \
+ _BF_FIELD_CHECK(_mask); \
+ (((u64)(_val)) << compile_ffs64(_mask)) & _mask; \
+ })
+
+#define FIELD_GET64(_mask, _val) \
+ ({ \
+ _BF_FIELD_CHECK(_mask); \
+ (u64)(((_val) & _mask) >> compile_ffs64(_mask)); \
+ })
+
+#endif
--
1.9.1
