linux-kernel - Re: [PATCH/RFC 0/2] gpio: of: Add DT overlay support for GPIO hogs

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Date:   Tue, 7 Jan 2020 08:51:13 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Frank Rowand <frowand.list@...il.com>
Cc:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        Rob Herring <robh+dt@...nel.org>,
        Peter Ujfalusi <peter.ujfalusi@...com>,
        Chris Brandt <chris.brandt@...esas.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH/RFC 0/2] gpio: of: Add DT overlay support for GPIO hogs

Hi Frank,

On Tue, Jan 7, 2020 at 12:34 AM Frank Rowand <frowand.list@...il.com> wrote:
> On 12/30/19 7:38 AM, Geert Uytterhoeven wrote:
> > As GPIO hogs are configured at GPIO controller initialization time,
> > adding/removing GPIO hogs in Device Tree overlays currently does not
> > work.  Hence this patch series adds support for that, by registering an
> > of_reconfig notifier, as is already done for platform, i2c, and SPI
> > devices.
> >
> > Perhaps this would be better served through a pinctrl-gpio driver?
> > Pinctrl is already working fine with DT overlays, as the pinctrl-*
> > properties are part of the slave device node, and thus looked up at
> > slave device node attachment time, not at pin controller initialization
> > time.
> >
> > In my particular use case (talking to SPI devices connected to a PMOD
> > connector on the RSK+RZA1 development board), the GPIO performs board
> > level muxing of a.o. the SPI MOSI/MISO/SCK signals.  Hence the hog
> > really needs to be active only while talking to the SPI device, so the
> > muxing could (in theory) be done upon demand.
> > But how to describe that in DT, and implement it (using Runtime PM?)?
>
> I'm trying to understand the use case.  I can easily imagine two cases:
>
>   (1) want to configure the GPIO to be able to use the SPI bus sometimes,
>       but configure the GPIO differently when not using the SPI bus
>
>   (2) want to describe a device on the SPI bus in an overlay, thus
>       also needing to describe the associate gpio hog node in the
>       same overlay
>
> For use case (2), the proposed patch seems to be a good solution.
>
> For use case (1), this is a case of trying to use devicetree as a
> way to control configuration instead of describing the hardware.
> In this case, Bartosz' reply may indicate the way forward.
>
> I'll assume use case (2) for patch comments.

Yes, my main interest is use case (2).
I have no plans to pursue use case (1).

However, I have some more comments and questions for use case (1).
Before you can control configuration, you have to describe the hardware.
Hence isn't that a job for DT?
Furthermore, I'd like you to step back and answer the following question:
what is the difference between a GPIO serving as a chip select for an
SPI slave, and a GPIO controlling board level muxing?  In both cases the
GPIO controls to which hardware other signals are routed, and both may
be changed at runtime.

Thanks!

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds