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

Nicolas Frattaroli
2025-12-17T13:23:16Z

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.

Thank you!

Kind regards,
Nicolas Frattaroli