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Date: Tue, 16 Mar 2021 13:42:45 +0200 From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com> To: Rasmus Villemoes <linux@...musvillemoes.dk> Cc: Yury Norov <yury.norov@...il.com>, linux-kernel@...r.kernel.org, linux-m68k@...ts.linux-m68k.org, linux-arch@...r.kernel.org, linux-sh@...r.kernel.org, Alexey Klimov <aklimov@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>, Arnd Bergmann <arnd@...db.de>, David Sterba <dsterba@...e.com>, Dennis Zhou <dennis@...nel.org>, Geert Uytterhoeven <geert@...ux-m68k.org>, Jianpeng Ma <jianpeng.ma@...el.com>, Joe Perches <joe@...ches.com>, John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>, Josh Poimboeuf <jpoimboe@...hat.com>, Rich Felker <dalias@...c.org>, Stefano Brivio <sbrivio@...hat.com>, Wei Yang <richard.weiyang@...ux.alibaba.com>, Wolfram Sang <wsa+renesas@...g-engineering.com>, Yoshinori Sato <ysato@...rs.sourceforge.jp> Subject: Re: [PATCH 04/13] lib: introduce BITS_{FIRST,LAST} macro 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@...2_0001309d_7f0 - 8 +8 > > vermagic 48 49 +1 > > e843419@...5_000138bb_f68 8 - -8 > > e843419@...9_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
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