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From:   Andy Shevchenko <andy.shevchenko@gmail.com>
Date:   Fri, 2 Sep 2022 10:20:08 +0300
Message-ID: <CAHp75VchGZV-cPzZnDRBmd_+jEiRap7oA2oFz6w8dBw=7LNtbQ@mail.gmail.com>
Subject: Re: [PATCH v2 1/2] gpio: Add gpio latch driver
To:     Sascha Hauer <s.hauer@pengutronix.de>
Cc:     "open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        Bartosz Golaszewski <brgl@bgdev.pl>,
        Sascha Hauer <kernel@pengutronix.de>
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On Fri, Sep 2, 2022 at 10:16 AM Sascha Hauer <s.hauer@pengutronix.de> wrote:
> 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@pengutronix.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.

But this reminds me of some kind of gpio-aggregator with a specific
layer on top. To me it really feels that we are (semi-)reinventing a
wheel between the lines...

> 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.

Got it, thanks for elaboration.

-- 
With Best Regards,
Andy Shevchenko