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 01:29:43 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Janusz Krzysztofik <jmkrzyszt@...il.com>
Cc:     Boris Brezillon <boris.brezillon@...tlin.com>,
        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

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?

Yours,
Linus Walleij