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

From: David Laight <[email-masked]>
Date: Tue, 9 Dec 2025 18:54:52 +0000

On Tue, 9 Dec 2025 17:46:21 +0200
Andy Shevchenko <[email-masked]> wrote:

> On Tue, Dec 09, 2025 at 10:03:07AM +0000, [email-masked] wrote:
> 
> > 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.  
> 
> ...
> 
> > +#define FIELD_PREP_WM16(mask, val)				\
> > +({								\
> > +	__auto_type _mask = mask;				\
> > +	u32 _val = FIELD_PREP(_mask, val);			\  
> 
> > +	BUILD_BUG_ON_MSG(_mask > 0xffffu,			\
> > +			 "FIELD_PREP_WM16: mask too large");	\  
> 
> Can it be static_assert() instead?

No, they have to be 'integer constant expressions' not just
'compile time constants'.
Pretty useless for anything non-trivial.

	David

> 
> > +	_val | (_mask << 16);					\
> > +})  
>