From: Johannes Berg <johannes@xxxxxxxxxxxxxxxx> Date: Wed, 18 Jan 2023 14:26:53 +0100 > From: Johannes Berg <johannes.berg@xxxxxxxxx> > > Neither FIELD_PREP() nor *_encode_bits() can be used > in constant contexts (such as initializers), but we > don't want to define shift constants for all masks > just for use in initializers, and having checks that > the values fit is also useful. > > Therefore, add FIELD_PREP_CONST() which is a smaller > version of FIELD_PREP() that can only take constant > arguments and has less friendly (but not less strict) > error checks, and expands to a constant value. > > Signed-off-by: Johannes Berg <johannes.berg@xxxxxxxxx> > --- > include/linux/bitfield.h | 26 ++++++++++++++++++++++++++ > 1 file changed, 26 insertions(+) > > diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h > index c9be1657f03d..ebfa12f69501 100644 > --- a/include/linux/bitfield.h > +++ b/include/linux/bitfield.h > @@ -115,6 +115,32 @@ > ((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask); \ > }) > > +#define __BF_CHECK_POW2(n) BUILD_BUG_ON_ZERO(((n) & ((n) - 1)) != 0) > + > +/** > + * FIELD_PREP_CONST() - prepare a constant bitfield element > + * @_mask: shifted mask defining the field's length and position > + * @_val: value to put in the field > + * > + * FIELD_PREP_CONST() masks and shifts up the value. The result should > + * be combined with other fields of the bitfield using logical OR. > + * > + * Unlike FIELD_PREP() this is a constant expression and can therefore > + * be used in initializers. Error checking is less comfortable for this > + * version, and non-constant masks cannot be used. > + */ > +#define FIELD_PREP_CONST(_mask, _val) \ Have you tried combining it with FIELD_PREP() using __builtin_choose_expr() + __builtin_is_constexpr() (or __builtin_constant_p() depending on which will satisfy the compiler)? I'm not saying it's 100% possible, but worth trying. > + ( \ > + /* mask must be non-zero */ \ > + BUILD_BUG_ON_ZERO((_mask) == 0) + \ > + /* check if value fits */ \ > + BUILD_BUG_ON_ZERO(~((_mask) >> __bf_shf(_mask)) & (_val)) + \ > + /* check if mask is contiguous */ \ > + __BF_CHECK_POW2((_mask) + (1ULL << __bf_shf(_mask))) + \ > + /* and create the value */ \ > + (((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask)) \ > + ) > + > /** > * FIELD_GET() - extract a bitfield element > * @_mask: shifted mask defining the field's length and position Thanks, Olek
