From: Geert Uytterhoeven <geert@linux-m68k.org>
To: Frank Rowand <frowand.list@gmail.com>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>,
Linus Walleij <linus.walleij@linaro.org>,
Bartosz Golaszewski <bgolaszewski@baylibre.com>,
Pantelis Antoniou <pantelis.antoniou@konsulko.com>,
Rob Herring <robh+dt@kernel.org>,
Peter Ujfalusi <peter.ujfalusi@ti.com>,
Chris Brandt <chris.brandt@renesas.com>,
"open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS"
<devicetree@vger.kernel.org>,
Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH/RFC 0/2] gpio: of: Add DT overlay support for GPIO hogs
Date: Tue, 7 Jan 2020 08:51:13 +0100 [thread overview]
Message-ID: <CAMuHMdXuGf71whvtRsaDfoaCnCj+CrTWhhsQ0H3sGsQu7hMtfg@mail.gmail.com> (raw)
In-Reply-To: <db13664e-f01a-5492-942b-f021f1bbe4e2@gmail.com>
Hi Frank,
On Tue, Jan 7, 2020 at 12:34 AM Frank Rowand <frowand.list@gmail.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@linux-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
prev parent reply other threads:[~2020-01-07 7:51 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-30 13:38 [PATCH/RFC 0/2] gpio: of: Add DT overlay support for GPIO hogs Geert Uytterhoeven
2019-12-30 13:38 ` [PATCH/RFC 1/2] gpio: of: Extract of_gpiochip_add_hog() Geert Uytterhoeven
2019-12-30 13:38 ` [PATCH/RFC 2/2] gpio: of: Add DT overlay support for GPIO hogs Geert Uytterhoeven
2020-01-06 23:34 ` Frank Rowand
2020-01-07 7:10 ` Frank Rowand
2020-01-07 7:25 ` Frank Rowand
2020-01-07 8:02 ` Geert Uytterhoeven
2020-01-07 8:11 ` Geert Uytterhoeven
2020-01-24 21:57 ` Frank Rowand
2020-01-24 22:02 ` Frank Rowand
2020-01-07 7:59 ` Geert Uytterhoeven
2020-01-03 9:51 ` [PATCH/RFC 0/2] " Bartosz Golaszewski
2020-01-07 7:46 ` Geert Uytterhoeven
2020-01-07 9:03 ` Bartosz Golaszewski
2020-01-07 9:49 ` Geert Uytterhoeven
2020-01-06 23:34 ` Frank Rowand
2020-01-07 7:51 ` Geert Uytterhoeven [this message]
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