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bitfield: Add non-constant field_{prep,get}() helpers

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The existing FIELD_{GET,PREP}() macros are limited to compile-time
constants.  However, it is very common to prepare or extract bitfield
elements where the bitfield mask is not a compile-time constant.

To avoid this limitation, the AT91 clock driver and several other
drivers already have their own non-const field_{prep,get}() macros.
Make them available for general use by adding them to
<linux/bitfield.h>, and improve them slightly:
  1. Avoid evaluating macro parameters more than once,
  2. Replace "ffs() - 1" by "__ffs()",
  3. Support 64-bit use on 32-bit architectures,
  4. Wire field_{get,prep}() to FIELD_{GET,PREP}() when mask is
     actually constant.

This is deliberately not merged into the existing FIELD_{GET,PREP}()
macros, as people expressed the desire to keep stricter variants for
increased safety, or for performance critical paths.

Yury: use __mask withing new macros.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Crt Mori <cmo@melexis.com>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Acked-by: Richard Genoud <richard.genoud@bootlin.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Reviewed-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
This commit is contained in:
Geert Uytterhoeven
2025-11-06 14:34:01 +01:00
committed by Yury Norov (NVIDIA)
parent 2a6c045640
commit c1c6ab80b2
1 changed files with 59 additions and 0 deletions
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59
include/linux/bitfield.h
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@@ -17,6 +17,7 @@
* FIELD_{GET,PREP} macros take as first parameter shifted mask
* from which they extract the base mask and shift amount.
* Mask must be a compilation time constant.
* field_{get,prep} are variants that take a non-const mask.
*
* Example:
*
@@ -240,4 +241,62 @@ __MAKE_OP(64)
#undef __MAKE_OP
#undef ____MAKE_OP
#define __field_prep(mask, val) \
({ \
__auto_type __mask = (mask); \
typeof(__mask) __val = (val); \
unsigned int __shift = BITS_PER_TYPE(__mask) <= 32 ? \
__ffs(__mask) : __ffs64(__mask); \
(__val << __shift) & __mask; \
})
#define __field_get(mask, reg) \
({ \
__auto_type __mask = (mask); \
typeof(__mask) __reg = (reg); \
unsigned int __shift = BITS_PER_TYPE(__mask) <= 32 ? \
__ffs(__mask) : __ffs64(__mask); \
(__reg & __mask) >> __shift; \
})
/**
* field_prep() - prepare a bitfield element
* @mask: shifted mask defining the field's length and position, must be
* non-zero
* @val: value to put in the field
*
* Return: field value masked and shifted to its final destination
*
* field_prep() masks and shifts up the value. The result should be
* combined with other fields of the bitfield using logical OR.
* Unlike FIELD_PREP(), @mask is not limited to a compile-time constant.
* Typical usage patterns are a value stored in a table, or calculated by
* shifting a constant by a variable number of bits.
* If you want to ensure that @mask is a compile-time constant, please use
* FIELD_PREP() directly instead.
*/
#define field_prep(mask, val) \
(__builtin_constant_p(mask) ? __FIELD_PREP(mask, val, "field_prep: ") \
: __field_prep(mask, val))
/**
* field_get() - extract a bitfield element
* @mask: shifted mask defining the field's length and position, must be
* non-zero
* @reg: value of entire bitfield
*
* Return: extracted field value
*
* field_get() extracts the field specified by @mask from the
* bitfield passed in as @reg by masking and shifting it down.
* Unlike FIELD_GET(), @mask is not limited to a compile-time constant.
* Typical usage patterns are a value stored in a table, or calculated by
* shifting a constant by a variable number of bits.
* If you want to ensure that @mask is a compile-time constant, please use
* FIELD_GET() directly instead.
*/
#define field_get(mask, reg) \
(__builtin_constant_p(mask) ? __FIELD_GET(mask, reg, "field_get: ") \
: __field_get(mask, reg))
#endif