From c1c6ab80b25c8db1e2ef5ae3ac8075d2c242ae13 Mon Sep 17 00:00:00 2001 From: Geert Uytterhoeven Date: Thu, 6 Nov 2025 14:34:01 +0100 Subject: [PATCH] bitfield: Add non-constant field_{prep,get}() helpers MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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 , 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 Acked-by: Alexandre Belloni Acked-by: Jonathan Cameron Acked-by: Crt Mori Acked-by: Nuno Sá Acked-by: Richard Genoud Reviewed-by: Andy Shevchenko Reviewed-by: Yury Norov (NVIDIA) Signed-off-by: Yury Norov (NVIDIA) --- include/linux/bitfield.h | 59 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 59 insertions(+) diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h index bf8e0ae4b5b41..126dc5b380afa 100644 --- a/include/linux/bitfield.h +++ b/include/linux/bitfield.h @@ -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 -- 2.47.3