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

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Message-ID: <aTdzTdoPLJqvgqZ_@yury>
Date: Mon, 8 Dec 2025 19:54:37 -0500
From: Yury Norov <yury.norov@...il.com>
To: david.laight.linux@...il.com
Cc: Rasmus Villemoes <linux@...musvillemoes.dk>,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	Nicolas Frattaroli <nicolas.frattaroli@...labora.com>
Subject: Re: [PATCH 3/9] bitmap: Use FIELD_PREP() in expansion of
 FIELD_PREP_WM16()

+ Nicolas Frattaroli <nicolas.frattaroli@...labora.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@...il.com wrote:
> From: David Laight <david.laight.linux@...il.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@...il.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