Subject: Re: [PATCH 4/9] bitfield: Copy #define parameters to locals

From: Yury Norov <[email-masked]>
Date: Wed, 10 Dec 2025 13:45:41 -0500

On Tue, Dec 09, 2025 at 05:51:48PM +0200, Andy Shevchenko wrote:
> On Tue, Dec 09, 2025 at 10:03:08AM +0000, [email-masked] wrote:
> 
> > Use __auto_type to take copies of parameters to both ensure they are
> > evaluated only once and to avoid bloating the pre-processor output.
> > In particular 'mask' is likely to be GENMASK() and the expension
> > of FIELD_GET() is then about 18KB.
> > 
> > Remove any extra (), update kerneldoc.
> 
> > Consistently use xxx for #define formal parameters and _xxx for
> > local variables.
> 
> Okay, I commented below, and I think this is too huge to be in this commit.
> Can we make it separate?

I'm next to Andy. The commit message covers 6 or 7 independent
changes, and patch body itself seems to be above my abilities to
review. This should look like a series if nice cleanups, now it looks
like a patch bomb.
 
> > Rather than use (typeof(mask))(val) to ensure bits aren't lost when
> > val is shifted left, use '__auto_type _val = 1 ? (val) : _mask;'
> > relying on the ?: operator to generate a type that is large enough.
> > 
> > Remove the (typeof(mask)) cast from __FIELD_GET(), it can only make
> > a difference if 'reg' is larger than 'mask' and the caller cares about
> > the actual type.
> > Note that mask usually comes from GENMASK() and is then 'unsigned long'.
> > 
> > Rename the internal defines __FIELD_PREP to __BF_FIELD_PREP and
> > __FIELD_GET to __BF_FIELD_GET.
> > 
> > Now that field_prep() and field_get() copy their parameters there is
> > no need for the __field_prep() and __field_get() defines.
> > But add a define to generate the required 'shift' to use in both defines.
> 
> ...
> 
> > -#define __BF_FIELD_CHECK_MASK(_mask, _val, _pfx)			\
> > +#define __BF_FIELD_CHECK_MASK(mask, val, pfx)				\
> >  	({								\
> > -		BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask),		\
> > -				 _pfx "mask is not constant");		\
> > -		BUILD_BUG_ON_MSG((_mask) == 0, _pfx "mask is zero");	\
> > -		BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ?		\
> > -				 ~((_mask) >> __bf_shf(_mask)) &	\
> > -					(0 + (_val)) : 0,		\
> > -				 _pfx "value too large for the field"); \
> > -		__BUILD_BUG_ON_NOT_POWER_OF_2((_mask) +			\
> > -					      (1ULL << __bf_shf(_mask))); \
> > +		BUILD_BUG_ON_MSG(!__builtin_constant_p(mask),		\
> > +				 pfx "mask is not constant");		\
> > +		BUILD_BUG_ON_MSG((mask) == 0, _pfx "mask is zero");	\
> > +		BUILD_BUG_ON_MSG(__builtin_constant_p(val) ?		\
> > +				 ~((mask) >> __bf_shf(mask)) &		\
> > +					(0 + (val)) : 0,		\
> > +				 pfx "value too large for the field");	\
> > +		__BUILD_BUG_ON_NOT_POWER_OF_2((mask) +			\
> > +					      (1ULL << __bf_shf(mask))); \
> >  	})
> 
> I looks like renaming parameters without any benefit, actually the opposite
> it's very hard to see if there is any interesting change here. Please, drop
> this or make it clear to focus only on the things that needs to be changed.
> 
> -- 
> With Best Regards,
> Andy Shevchenko
>