linux-kernel - Re: [PATCH v9 1/9] bitops: Introduce the for_each_set

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Date:   Tue, 12 Mar 2019 14:36:21 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     William Breathitt Gray <vilhelm.gray@...il.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v9 1/9] bitops: Introduce the for_each_set_clump8 macro

On Fri, Mar 8, 2019 at 5:57 PM William Breathitt Gray
<vilhelm.gray@...il.com> wrote:
>
> On Fri, Mar 08, 2019 at 09:31:00AM +0100, Linus Walleij wrote:
> > On Sun, Mar 3, 2019 at 8:47 AM William Breathitt Gray
> > <vilhelm.gray@...il.com> wrote:
> >
> > > This macro iterates for each 8-bit group of bits (clump) with set bits,
> > > within a bitmap memory region. For each iteration, "start" is set to the
> > > bit offset of the found clump, while the respective clump value is
> > > stored to the location pointed by "clump". Additionally, the
> > > bitmap_get_value8 and bitmap_set_value8 functions are introduced to
> > > respectively get and set an 8-bit value in a bitmap memory region.
> > >
> > > Suggested-by: Andy Shevchenko <andy.shevchenko@...il.com>
> > > Suggested-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> > > Cc: Arnd Bergmann <arnd@...db.de>
> > > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > > Reviewed-by: Andy Shevchenko <andy.shevchenko@...il.com>
> > > Reviewed-by: Linus Walleij <linus.walleij@...aro.org>
> > > Signed-off-by: William Breathitt Gray <vilhelm.gray@...il.com>
> >
> > Andrew: would you be OK with this being merged in v5.1?
> >
> > If we need to move the code to drivers/gpio that's OK (though
> > I think it's generally useful) but I need to know to proceed with
> > the William's nice optimization of these drivers.
> >
> > Yours,
> > Linus Walleij
>
> I was waiting on Andy to suggest some examples out of the GPIO realm,
> but he may be under a heavy workload right so I decided to do a quick
> search for one.
>
> In drivers/of/unittest.c, there is loop across a bitmap in the
> of_unittest_destroy_tracked_overlays function:
>
>         for (id = MAX_UNITTEST_OVERLAYS - 1; id >= 0; id--) {
>                 if (!(overlay_id_bits[BIT_WORD(id)] & BIT_MASK(id)))
>                         continue;
>
> This section of code is checking each bit individually, and skipping if
> that bit is not set. This looping can be optimized by using the
> for_each_set_clump8 macro


Probably no.


I see this comment before the loop.
/* remove in reverse order */


Also, the unittest code handles per-bit
whereas your helper does per-byte.





> to skip clumps of nonset bits (not to mention
> make the logic of the code much simpler and easier to follow by reducing
> the code to a single line):
>
>         for_each_set_clump8(id, clump, overlay_id_bits, MAX_UNITTEST_OVERLAYS-1)
>
> The for_each_set_clump8 macro is not specific to the GPIO subsystem; I
> just happen to use it in these GPIO drivers simply because I am most
> familar with this section of the kernel (it's where most of my
> contributions occur afterall).
>
> Consider this, if I am able to find a use for this macro outside of the
> GPIO subsystem within a matter minutes, then there must be some benefit
> in allowing the rest of the kernel to use the for_each_set_clump8 macro.
> So let's put it in bitops.h rather than restrict it to just the GPIO
> subsystem.


If we do not find useful cases in other subsystem,
this patch set looks over-engineering to me.






> William Breathitt Gray


--
Best Regards
Masahiro Yamada