[PATCHv2 1/2] add basic register-field manipulation macros
From: Jakub Kicinski <hidden>
Date: 2016-06-14 11:49:13
Also in:
lkml, netdev
Subsystem:
bitmap api, the rest · Maintainers:
Yury Norov, Linus Torvalds
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 X_REG_FIELD 0x000ff000
field = FIELD_GET(X_REG_FIELD, reg);
reg &= ~X_REG_FIELD;
reg |= FIELD_PUT(X_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 implementation from mt76 this one
uses standard Linux and GCC functions such as is_power_of_2()
and __builtin_ffsll().
Signed-off-by: Jakub Kicinski <redacted>
---
v2:
- change Felix's email address.
include/linux/bitfield.h | 58 ++++++++++++++++++++++++++++++++++++++++++++++++
include/linux/log2.h | 6 +++++
2 files changed, 64 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..9560d1877cbc
--- /dev/null
+++ b/include/linux/bitfield.h@@ -0,0 +1,58 @@ +/* + * Copyright (C) 2014 Felix Fietkau <nbd@nbd.name> + * Copyright (C) 2004 - 2009 Ivo van Doorn <IvDoorn@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> +#include <linux/bug.h> +#include <linux/log2.h> + +#define _bf_shf(x) (__builtin_ffsll(x) - 1) + +#define _BF_FIELD_CHECK(_mask, _val) \ + ({ \ + const u64 hi = (_mask) + (1ULL << _bf_shf(_mask)); \ + \ + BUILD_BUG_ON(!(_mask) || (hi && !is_power_of_2_u64(hi))); \ + BUILD_BUG_ON(__builtin_constant_p(_val) ? \ + ~((_mask) >> _bf_shf(_mask)) & (_val) : \ + 0); \ + }) + +#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/log2.h b/include/linux/log2.h
index fd7ff3d91e6a..1b5e1f4da043 100644
--- a/include/linux/log2.h
+++ b/include/linux/log2.h@@ -54,6 +54,12 @@ bool is_power_of_2(unsigned long n) return (n != 0 && ((n & (n - 1)) == 0)); } +static inline __attribute__((const)) +bool is_power_of_2_u64(u64 n) +{ + return (n != 0 && ((n & (n - 1)) == 0)); +} + /* * round up to nearest power of two */
--
1.9.1