linux-kernel - Re: [PATCH v2 1/2] gpio: Add gpio latch driver

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Message-ID: <20220902071601.GU24324@pengutronix.de>
Date:   Fri, 2 Sep 2022 09:16:01 +0200
From:   Sascha Hauer <s.hauer@...gutronix.de>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <brgl@...ev.pl>,
        Sascha Hauer <kernel@...gutronix.de>
Subject: Re: [PATCH v2 1/2] gpio: Add gpio latch driver

On Fri, Sep 02, 2022 at 09:42:21AM +0300, Andy Shevchenko wrote:
> On Wed, Aug 31, 2022 at 9:02 AM Sascha Hauer <s.hauer@...gutronix.de> wrote:
> >
> > This driver implements a GPIO multiplexer based on latches connected to
> > other GPIOs. A set of data GPIOs is connected to the data input of
> > multiple latches. The clock input of each latch is driven by another
> > set of GPIOs. With two 8-bit latches 10 GPIOs can be multiplexed into
> > 16 GPIOs. GPOs might be a better term as in fact the multiplexed pins
> > are output only.
> 
> So, this is for only one type of latches, now I'm wondering why
> gpio-74xx-mmio can't cover this case (with probably small
> modifications to the code)?

gpio-74xx-mmio is about latches connected to a parallel bus. You can
access the GPIOs by doing readl/writel operations. The latches are
driven by the bus logic and likely an additional address decoder.

What I have here instead is a latch fully driven by GPIOs.

Yes, with enough force you could implement it in the gpio-74xx-mmio
driver, but that wouldn't be mmio at all and likely completely different
code pathes.

Sascha

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