linux-kernel - Re: [PATCH v5 0/4] Introduce the for_each_set

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Date:   Tue, 5 May 2020 16:51:56 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     William Breathitt Gray <vilhelm.gray@...il.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Syed Nayyar Waris <syednwaris@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Simek <michal.simek@...inx.com>,
        Arnd Bergmann <arnd@...db.de>, rrichter@...vell.com,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        "Zhang, Rui" <rui.zhang@...el.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Amit Kucheria <amit.kucheria@...durent.com>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v5 0/4] Introduce the for_each_set_clump macro

On Mon, May 4, 2020 at 5:41 PM William Breathitt Gray
<vilhelm.gray@...il.com> wrote:
> On Mon, May 04, 2020 at 02:41:09PM +0300, Andy Shevchenko wrote:
> > On Sun, May 03, 2020 at 04:38:36AM +0530, Syed Nayyar Waris wrote:

...

> > Looking into the last patches where we have examples I still do not see a
> > benefit of variadic clump sizes. power of 2 sizes would make sense (and be
> > optimized accordingly (64-bit, 32-bit).
> >
> > --
> > With Best Regards,
> > Andy Shevchenko
>
> There is of course benefit in defining for_each_set_clump with clump
> sizes of powers of 2 (we can optimize for 32 and 64 bit sizes and avoid
> boundary checks that we know will not occur), but at the very least the
> variable size bitmap_set_value and bitmap_get_value provide significant
> benefit for the readability of the gpio-xilinx code:
>
>         bitmap_set_value(old, state[0], 0, width[0]);
>         bitmap_set_value(old, state[1], width[0], width[1]);
>         ...
>         state[0] = bitmap_get_value(new, 0, width[0]);
>         state[1] = bitmap_get_value(new, width[0], width[1]);
>
> These lines are simple and clear to read: we know immediately what they
> do. But if we did not have bitmap_set_value/bitmap_get_value, we'd have
> to use several bitwise operations for each line; the obfuscation of the
> code would be an obvious hinderance here.

Do I understand correctly that width[0] and width[1] may not be power
of two and it's actually the case?

-- 
With Best Regards,
Andy Shevchenko