Re: [PATCH v6 4/6] reset: Instantiate reset GPIO controller for shared reset-gpios

[Bartosz Golaszewski]

On Wed, Jan 31, 2024 at 9:57 AM Linus Walleij <linus.walleij@linaroorg> wrote:
>
> Hi Krzysztof,
>
> something is odd with the addresses on this patch, because neither GPIO
> maintainer is on CC nor linux-gpio@vger, and it's such a GPIO-related
> patch. We only saw it through side effects making <linux/gpio/driver.h>
> optional, as required by this patch.
>
> Please also CC Geert Uytterhoeven, the author of the GPIO aggregator.
>
> i.e. this:
> > 2. !GPIOLIB stub:
> >    https://lore.kernel.org/all/20240125081601.118051-3-krzysztof.kozlowski@xxxxxxxxxx/
>
> On Mon, Jan 29, 2024 at 12:53 PM Krzysztof Kozlowski
> <krzysztof.kozlowski@xxxxxxxxxx> wrote:
>
> > Devices sharing a reset GPIO could use the reset framework for
> > coordinated handling of that shared GPIO line.  We have several cases of
> > such needs, at least for Devicetree-based platforms.
> >
> > If Devicetree-based device requests a reset line, while "resets"
> > Devicetree property is missing but there is a "reset-gpios" one,
> > instantiate a new "reset-gpio" platform device which will handle such
> > reset line.  This allows seamless handling of such shared reset-gpios
> > without need of changing Devicetree binding [1].
> >
> > To avoid creating multiple "reset-gpio" platform devices, store the
> > Devicetree "reset-gpios" GPIO specifiers used for new devices on a
> > linked list.  Later such Devicetree GPIO specifier (phandle to GPIO
> > controller, GPIO number and GPIO flags) is used to check if reset
> > controller for given GPIO was already registered.
> >
> > If two devices have conflicting "reset-gpios" property, e.g. with
> > different ACTIVE_xxx flags, this would allow to spawn two separate
> > "reset-gpio" devices, where the second would fail probing on busy GPIO
> > request.
> >
> > Link: https://lore.kernel.org/all/YXi5CUCEi7YmNxXM@xxxxxxxxxxxxxxxxxx/ [1]
> > Cc: Bartosz Golaszewski <brgl@xxxxxxxx>
> > Cc: Chris Packham <chris.packham@xxxxxxxxxxxxxxxxxxx>
> > Cc: Sean Anderson <sean.anderson@xxxxxxxx>
> > Reviewed-by: Philipp Zabel <p.zabel@xxxxxxxxxxxxxx>
> > Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@xxxxxxxxxx>
> (...)
>
> In my naive view, this implements the following:
>
> reset -> virtual "gpio" -> many physical gpios[0..n]

This is a different problem: it supports many users enabling the same
GPIO (in Krzysztof's patch it's one but could be more if needed) but -
unlike the broken NONEXCLUSIVE GPIOs in GPIOLIB - it counts the number
of users and doesn't disable the GPIO for as long as there's at least
one.

Bart

>
> So if there was already a way in the kernel to map one GPIO to
> many GPIOs, the framework could just use that with a simple
> single GPIO?
>
> See the bindings in:
> Documentation/devicetree/bindings/gpio/gpio-delay.yaml
>
> This is handled by drivers/gpio/gpio-aggregator.c.
>
> This supports a 1-to-1 map: one GPIO in, one GPIO out, same offset.
> So if that is extended to support 1-to-many, this problem is solved.
>
> Proposed solution: add a single boolean property such as
> aggregate-all-gpios; to the gpio-delay node, making it provide
> one single gpio at offset 0 on the consumer side, and refuse any
> more consumers.
>
> This will also solve the problem with induced delays on
> some GPIO lines as I can see was discussed in the bindings,
> the gpio aggregator already supports that, but it would work
> fine with a delay being zero as well.
>
> This avoids all the hackery with driver stubs etc as well.
>
> Yours,
> Linus Walleij