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

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Date:   Tue, 5 May 2020 10:53:49 -0400
From:   William Breathitt Gray <vilhelm.gray@...il.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Michal Simek <michal.simek@...inx.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Syed Nayyar Waris <syednwaris@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>, rrichter@...vell.com,
        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 Tue, May 05, 2020 at 04:51:56PM +0300, Andy Shevchenko wrote:
> 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

I'm under the impression that width[0] and width[1] are arbitrarily
chosen by the user and could be any integer. I have never used this
hardware so I'm hoping one of the gpio-xilinx or GPIO subsystem
maintainers in this thread will respond with some guidance.

If the values of width[0] and width[1] are restricted to powers of 2,
then I agree that there is no need for generic bitmap_set_value and
bitmap_get_value functions and we can instead use more optimized power
of 2 versions.

William Breathitt Gray

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