Re: [PATCH 3/9] bitmap: Use FIELD_PREP() in expansion of FIELD_PREP_WM16()

David Laight
2025-12-09T09:56:29Z

On Mon, 8 Dec 2025 19:54:37 -0500
Yury Norov <yury.norov@gmail.com> wrote:

> + Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
>
> It's always good to CC an author of the original implementation,
> isn't?
>
> On Mon, Dec 08, 2025 at 10:42:44PM +0000, 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()
>
> That's fair.
>
> > and add an extra check for
> > the mask being at most 16 bits.
>
> Maybe it's time to introduce FIELD_PREP16()?
>
> > Signed-off-by: David Laight <david.laight.linux@gmail.com>
> > ---
> > include/linux/hw_bitfield.h | 17 ++++++++---------
> > 1 file changed, 8 insertions(+), 9 deletions(-)
> >
> > diff --git a/include/linux/hw_bitfield.h b/include/linux/hw_bitfield.h
> > index df202e167ce4..d7f21b60449b 100644
> > --- a/include/linux/hw_bitfield.h
> > +++ b/include/linux/hw_bitfield.h
> > @@ -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 _mask = mask; \
>
> Is __auto_type any better than typeof()?

Generally yes.
Mainly because you only get one expansion of the #define parameter.

>
> > + u32 _val = FIELD_PREP(_mask, val); \
> > + BUILD_BUG_ON_MSG(_mask > 0xffffu, \
> > + "FIELD_PREP_WM16: mask too large"); \
>
> Not necessary to split this line.
>
> > + _val | (_mask << 16); \
> > +})
>
> Can you share bloat-o-meter and code generation examples?

The generated code is going to be pretty much unchanged.
The changes only really affect the compile-time checks.

>
> For the next version please try to keep as much history
> untouched as possible.

I'm going to resend the same patches with the full cc list for now.

David

>
> Thanks,
> Yury
>
> >
> > /**
> > * FIELD_PREP_WM16_CONST() - prepare a constant bitfield element with a mask in
> > --
> > 2.39.5