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

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Message-ID: <20251211185319.4daf6e4f@pumpkin>
Date: Thu, 11 Dec 2025 18:53:19 +0000
From: David Laight <david.laight.linux@...il.com>
To: Yury Norov <yury.norov@...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()

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

> + 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()?

There is likely to be pressure for a global change soon.
6.19 contains '#define auto __auto_type' to match C23.

> 
> > +	u32 _val = FIELD_PREP(_mask, val);			\
> > +	BUILD_BUG_ON_MSG(_mask > 0xffffu,			\
> > +			 "FIELD_PREP_WM16: mask too large");	\  
> 
> Not necessary to split this line.

Only because I removed a level on indentation.

> 
> > +	_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.

Even the minimal changes (not moving the continuation markers) change pretty
much all the lines.
So It really isn't worth trying to keep it.

	David