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.
Rasmus
