Re: [PATCH 04/13] lib: introduce BITS_{FIRST,LAST} macro

[Andy Shevchenko]

On Tue, Mar 16, 2021 at 09:35:35AM +0100, Rasmus Villemoes wrote:
> On 16/03/2021 02.54, Yury Norov wrote:
> > BITMAP_{LAST,FIRST}_WORD_MASK() in linux/bitmap.h duplicates the
> > functionality of GENMASK(). The scope of BITMAP* macros is wider
> > than just bitmaps. This patch defines 4 new macros: BITS_FIRST(),
> > BITS_LAST(), BITS_FIRST_MASK() and BITS_LAST_MASK() in linux/bits.h
> > on top of GENMASK() and replaces BITMAP_{LAST,FIRST}_WORD_MASK()
> > to avoid duplication and increase the scope of the macros.
> > 
> > This change doesn't affect code generation. On ARM64:
> > scripts/bloat-o-meter vmlinux.before vmlinux
> > add/remove: 1/2 grow/shrink: 2/0 up/down: 17/-16 (1)
> > Function                                     old     new   delta
> > ethtool_get_drvinfo                          900     908      +8
> > e843419@0cf2_0001309d_7f0                      -       8      +8
> > vermagic                                      48      49      +1
> > e843419@0d45_000138bb_f68                      8       -      -8
> > e843419@0cc9_00012bce_198c                     8       -      -8
> 
> [what on earth are those weird symbols?]
> 
> 
> > diff --git a/include/linux/bits.h b/include/linux/bits.h
> > index 7f475d59a097..8c191c29506e 100644
> > --- a/include/linux/bits.h
> > +++ b/include/linux/bits.h
> > @@ -37,6 +37,12 @@
> >  #define GENMASK(h, l) \
> >  	(GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
> >  
> > +#define BITS_FIRST(nr)		GENMASK((nr), 0)
> > +#define BITS_LAST(nr)		GENMASK(BITS_PER_LONG - 1, (nr))
> > +
> > +#define BITS_FIRST_MASK(nr)	BITS_FIRST((nr) % BITS_PER_LONG)
> > +#define BITS_LAST_MASK(nr)	BITS_LAST((nr) % BITS_PER_LONG)
> 
> I don't think it's a good idea to propagate the unusual closed-range
> semantics of GENMASK to those wrappers. Almost all C and kernel code
> uses the 'inclusive lower bound, exclusive upper bound', and I'd expect
> BITS_FIRST(5) to result in a word with five bits set, not six. So I
> think these changes as-is make the code much harder to read and understand.
> 
> Regardless, please add some comments on the valid input ranges to the
> macros, whether that ends up being 0 <= nr < BITS_PER_LONG or 0 < nr <=
> BITS_PER_LONG or whatnot.
> 
> It would also be much easier to review if you just redefined the
> BITMAP_LAST_WORD_MASK macros etc. in terms of these new things, so you
> wouldn't have to do a lot of mechanical changes at the same time as
> introducing the new ones - especially when those mechanical changes
> involve adding a "minus 1" everywhere.

I tend to agree with Rasmus here.

-- 
With Best Regards,
Andy Shevchenko