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

[Krzysztof Kozlowski]

On 31/01/2024 09:57, Linus Walleij wrote:
> Hi Krzysztof,
> 
> something is odd with the addresses on this patch, because neither GPIO

Nothing is odd - I use get_maintainers.pl which just don't print your
names. I can add your addresses manually, no problem, but don't blame
the contributor that get_maintainers.pl has a missing content-regex. If
you want to be Cced on usage of GPIOs, you need to be sure that
MAINTAINERS file has appropriate pattern.


> 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]

It does not, there is no single GPIO here. There is a single reset
controller, though, but still multiple GPIOs in DTS.

> 
> 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.

It does not match the hardware thus I don't know how to implement it in
DTS while keeping the requirement that we are describing hardware, not
OS abstractions.

> 
> 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.

And how do you solve the reset requirements? The problem is not just to
share GPIO. The problem is to share in a way that devices operate
properly when they assert/deassert reset.

> 
> 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.


So none of these ideas were posted in previous threads, just because you
were not CCed (except one thread)?

https://lore.kernel.org/lkml/20191030120440.3699-1-peter.ujfalusi@xxxxxx/
https://lore.kernel.org/all/9eebec9b-e6fd-4a22-89ea-b434f446e061@xxxxxxxxxx/
https://lore.kernel.org/all/20231018100055.140847-1-krzysztof.kozlowski@xxxxxxxxxx/
https://social.treehouse.systems/@marcan/111268780311634160

Please implement some custom lei filter, so you will get such
notifications earlier. We keep discussing this for many months on
various attempts and this specific attempt already reached v6.

Best regards,
Krzysztof