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

[Linus Walleij]

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]

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