linux-kernel - Re: [PATCH 1/6] bitops: Introduce the the for_each_set

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Message-ID: <20200424163410.GD185537@smile.fi.intel.com>
Date:   Fri, 24 Apr 2020 19:34:10 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     William Breathitt Gray <vilhelm.gray@...il.com>
Cc:     Lukas Wunner <lukas@...ner.de>,
        Syed Nayyar Waris <syednwaris@...il.com>,
        akpm@...ux-foundation.org, arnd@...db.de,
        Linus Walleij <linus.walleij@...aro.org>,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] bitops: Introduce the the for_each_set_clump macro

On Fri, Apr 24, 2020 at 11:09:26AM -0400, William Breathitt Gray wrote:
> On Fri, Apr 24, 2020 at 05:00:58PM +0200, Lukas Wunner wrote:
> > On Fri, Apr 24, 2020 at 08:22:38PM +0530, Syed Nayyar Waris wrote:
> > > On Fri, Apr 24, 2020 at 7:40 PM Lukas Wunner <lukas@...ner.de> wrote:
> > > > On Fri, Apr 24, 2020 at 05:55:21PM +0530, Syed Nayyar Waris wrote:

...

> > > So, this function preserves the behaviour of earlier
> > > bitmap_set_value8() function and also adds extra functionality to
> > > that.
> > 
> > Please leave drivers as is which use exclusively 8-bit accesses,
> > e.g. gpio-max3191x.c and gpio-74x164.c.  I'm fearing a performance
> > regression if your new generic variant is used.  They work perfectly
> > fine the way they are and I don't see any benefit this series may have
> > for them.
> > 
> > If there are other drivers which benefit from the flexibility of your
> > generic variant then I'm not opposed to changing those.

> We can leave of course bitmap_set_value8 alone, but for 8-bit values the
> difference in latency I suspect is primarily due to the conditional test
> for the word boundaries. This latency is surely overshadowed by the I/O
> latency of the GPIO drivers, so I don't think there's much harm in
> changing those to use the generic function when the bottleneck will not
> be due to the bitmap_set_value/bitmap_get_value operations.

Okay, how many new (non-8-bit) users this will target?

-- 
With Best Regards,
Andy Shevchenko