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

From: David Laight <[email-masked]>
Date: Thu, 18 Dec 2025 08:44:46 +0000

On Wed, 17 Dec 2025 19:16:10 -0500
Yury Norov <[email-masked]> wrote:

> On Wed, Dec 17, 2025 at 02:22:32PM +0100, Nicolas Frattaroli wrote:
> > On Friday, 12 December 2025 20:37:08 Central European Standard Time [email-masked] wrote:  
> > > From: David Laight <[email-masked]>
> > > 
> > > 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 <[email-masked]>
> > > ---
> > > 
> > > 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 <[email-masked]>
> > 
> > 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.  
> 
> Nicolas, thanks for testing! Would you also want to add an explicit
> ack or review tag?
> 
> David, I'm OK with this change. Please add bloat-o-meter and code
> generation examples, and minimize the diff as I asked in v1, before I
> can merge it.

That is pretty much the minimal diff.
By the time you've changed the three lines that do anything 'real'
and realigned the continuation markers all the lines have changed.

I did look at the generated code, but since 'mask' is a constant
the whole thing is just ((val << const_a) | const_b) pretty much
regardless of the actual source - most of which is compile-time checks.
There isn't anything for the bloat-o-meter to find.
	
	David

> Thanks,
> Yury