Re: [PATCH v2 03/16] bitmap: Use FIELD_PREP() in expansion of FIELD_PREP_WM16()

Yury Norov
2025-12-18T00:16:12Z

On Wed, Dec 17, 2025 at 02:22:32PM +0100, Nicolas Frattaroli wrote:
> On Friday, 12 December 2025 20:37:08 Central European Standard Time david.laight.linux@gmail.com wrote:
> > From: David Laight <david.laight.linux@gmail.com>
> >
> > Instead of directly expanding __BF_FIELD_CHECK() (which really ought
> > not be used outside bitfield) and open-coding the generation of the
> > masked value, just call FIELD_PREP() and add an extra check for
> > the mask being at most 16 bits.
> > The extra check is added after calling FIELD_PREP() to get a sane
> > error message if 'mask' isn't constant.
> >
> > Remove the leading _ from the formal parameter names.
> > Prefix the local variables with _wm16_ to hopefully make them
> > unique.
> >
> > Signed-off-by: David Laight <david.laight.linux@gmail.com>
> > ---
> >
> > Changes for v2:
> > - Update kerneldoc to match changed formal parameter names.
> > - Change local variables to not collide with those in FIELD_PREP().
> >
> > Most of the examples are constants and get optimised away.
> >
> > include/linux/hw_bitfield.h | 21 ++++++++++-----------
> > 1 file changed, 10 insertions(+), 11 deletions(-)
> >
> > diff --git a/include/linux/hw_bitfield.h b/include/linux/hw_bitfield.h
> > index df202e167ce4..0bd1040a5f93 100644
> > --- a/include/linux/hw_bitfield.h
> > +++ b/include/linux/hw_bitfield.h
> > @@ -12,8 +12,8 @@
> >
> > /**
> > * FIELD_PREP_WM16() - prepare a bitfield element with a mask in the upper half
> > - * @_mask: shifted mask defining the field's length and position
> > - * @_val: value to put in the field
> > + * @mask: shifted mask defining the field's length and position
> > + * @val: value to put in the field
> > *
> > * FIELD_PREP_WM16() masks and shifts up the value, as well as bitwise ORs the
> > * result with the mask shifted up by 16.
> > @@ -23,15 +23,14 @@
> > * register, a bit in the lower half is only updated if the corresponding bit
> > * in the upper half is high.
> > */
> > -#define FIELD_PREP_WM16(_mask, _val) \
> > - ({ \
> > - typeof(_val) __val = _val; \
> > - typeof(_mask) __mask = _mask; \
> > - __BF_FIELD_CHECK(__mask, ((u16)0U), __val, \
> > - "HWORD_UPDATE: "); \
> > - (((typeof(__mask))(__val) << __bf_shf(__mask)) & (__mask)) | \
> > - ((__mask) << 16); \
> > - })
> > +#define FIELD_PREP_WM16(mask, val) \
> > +({ \
> > + __auto_type _wm16_mask = mask; \
> > + u32 _wm16_val = FIELD_PREP(_wm16_mask, val); \
> > + BUILD_BUG_ON_MSG(_wm16_mask > 0xffffu, \
> > + "FIELD_PREP_WM16: mask too large"); \
> > + _wm16_val | (_wm16_mask << 16); \
> > +})
> >
> > /**
> > * FIELD_PREP_WM16_CONST() - prepare a constant bitfield element with a mask in
> >
>
> Tested-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
>
> Compiled it with my usual config and booted it on a board that uses
> drivers that make use of these macros, and checked that things are
> working.

Nicolas, thanks for testing! Would you also want to add an explicit
ack or review tag?

David, I'm OK with this change. Please add bloat-o-meter and code
generation examples, and minimize the diff as I asked in v1, before I
can merge it.

Thanks,
Yury