| Message ID | 1465824594-29662-2-git-send-email-jakub.kicinski@netronome.com |
|---|---|
| State | Superseded, archived |
| Delegated to: | David Miller |
| Headers | show |
On 2016-06-13 15:29, Jakub Kicinski wrote: > 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 <jakub.kicinski@netronome.com> > --- > 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..ae2224464523 > --- /dev/null > +++ b/include/linux/bitfield.h > @@ -0,0 +1,58 @@ > +/* > + * Copyright (C) 2014 Felix Fietkau <nbd@openwrt.org> Please change my email address to nbd@nbd.name here - Felix
diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h new file mode 100644 index 000000000000..ae2224464523 --- /dev/null +++ b/include/linux/bitfield.h @@ -0,0 +1,58 @@ +/* + * Copyright (C) 2014 Felix Fietkau <nbd@openwrt.org> + * 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 */
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 <jakub.kicinski@netronome.com> --- include/linux/bitfield.h | 58 ++++++++++++++++++++++++++++++++++++++++++++++++ include/linux/log2.h | 6 +++++ 2 files changed, 64 insertions(+) create mode 100644 include/linux/bitfield.h