linux-kernel - Re: [RFC PATCH v2 09/12] gpiolib: Identify GPIO descriptor arrays with direct mapping

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Date:   Tue, 7 Aug 2018 19:10:42 +0200
From:   Boris Brezillon <boris.brezillon@...tlin.com>
To:     Janusz Krzysztofik <jmkrzyszt@...il.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Jonathan Corbet <corbet@....net>,
        Miquèl Raynal <miquel.raynal@...tlin.com>,
        Richard Weinberger <richard@....at>,
        David Woodhouse <dwmw2@...radead.org>,
        Brian Norris <computersforpeace@...il.com>,
        Mark Vasut <marek.vasut@...il.com>,
        ext Tony Lindgren <tony@...mide.com>,
        Aaro Koskinen <aaro.koskinen@....fi>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux-OMAP <linux-omap@...r.kernel.org>,
        linux-mtd@...ts.infradead.org, linux-doc@...r.kernel.org,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v2 09/12] gpiolib: Identify GPIO descriptor arrays
 with direct mapping

On Tue, 07 Aug 2018 18:50:22 +0200
Janusz Krzysztofik <jmkrzyszt@...il.com> wrote:

> Hi Linus,
> 
> On Tuesday, August 7, 2018 1:29:43 AM CEST Linus Walleij wrote:
> > Hi Janusz!
> > 
> > On Tue, Aug 7, 2018 at 12:29 AM Janusz Krzysztofik <jmkrzyszt@...il.com>   
> wrote:
> >   
> > > Certain GPIO array lookup results may map directly to GPIO pins of a
> > > single GPIO chip in hardware order.  If that condition is recognized
> > > and handled efficiently, significant performance gain of get/set array
> > > functions may be possible.
> > >
> > > While processing a request for an array of GPIO descriptors, verify if
> > > the descriptors just collected represent consecutive pins of a single
> > > GPIO chip.  Pass that information with the array to the caller so it
> > > can benefit from enhanced performance as soon as bitmap based get/set
> > > array functions which can make efficient use of that are available.
> > >
> > > Signed-off-by: Janusz Krzysztofik <jmkrzyszt@...il.com>  
> > (...)  
> > >  This function returns a struct gpio_descs which contains an array of
> > > -descriptors::
> > > +descriptors.  It may also contain a valid descriptor of a single GPIO   
> chip in
> > > +case the array strictly matches pin hardware layout of the chip::
> > >
> > >         struct gpio_descs {
> > >                 unsigned int ndescs;
> > >                 struct gpio_desc *desc[];
> > > +               struct gpio_chip *chip;  
> > 
> > This must be motivated: if the only purpose is to indicate to the consumer   
> that
> > all GPIOs are on the same chip, why not just have a
> > 
> > bool all_on_same_chip;
> > 
> > That you set to true if these are all on the same chip?  
> 
> My approach would probably save one or two instructions per get/set call, but 
> I'm not stuck to it and will be happy to find a better solution.
> 
> How about folding the chip descriptor inside an additional structure, private 
> to drivers, with internals not revealed to consumers?

Or just get the chip from gpio_descs->desc[0]->gdev->chip when
->all_on_same_chip is true...

That adds 2 dereferencing though.