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

Yury Norov
2025-12-09T00:54:39Z

+ 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()?

> + 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?

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

Thanks,
Yury

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