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Message-ID: <3f906732-74b7-4219-88ee-59509bff8459@linaro.org>
Date: Wed, 31 Jan 2024 10:52:41 +0100
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Subject: Re: [PATCH v6 4/6] reset: Instantiate reset GPIO controller for
 shared reset-gpios
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From: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
To: Linus Walleij <linus.walleij@linaro.org>,
 Bartosz Golaszewski <brgl@bgdev.pl>,
 Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>,
 Banajit Goswami <bgoswami@quicinc.com>,
 Bjorn Andersson <andersson@kernel.org>,
 Konrad Dybcio <konrad.dybcio@linaro.org>, Liam Girdwood
 <lgirdwood@gmail.com>, Mark Brown <broonie@kernel.org>,
 Rob Herring <robh+dt@kernel.org>,
 Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
 Conor Dooley <conor+dt@kernel.org>, Philipp Zabel <p.zabel@pengutronix.de>,
 "Rafael J. Wysocki" <rafael@kernel.org>,
 Viresh Kumar <viresh.kumar@linaro.org>, Frank Rowand
 <frowand.list@gmail.com>, Jaroslav Kysela <perex@perex.cz>,
 Takashi Iwai <tiwai@suse.com>, alsa-devel@alsa-project.org,
 linux-arm-msm@vger.kernel.org, linux-sound@vger.kernel.org,
 devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
 linux-pm@vger.kernel.org, Chris Packham <chris.packham@alliedtelesis.co.nz>,
 Sean Anderson <sean.anderson@seco.com>
References: <20240129115216.96479-1-krzysztof.kozlowski@linaro.org>
 <20240129115216.96479-5-krzysztof.kozlowski@linaro.org>
 <CACRpkdYf4HUaV-Pjr81WjLbzy9zdAnyFWs9gPayPC6-3OjHQwA@mail.gmail.com>
 <6473952d-893d-4591-9bfd-dd983313bee9@linaro.org>
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In-Reply-To: <6473952d-893d-4591-9bfd-dd983313bee9@linaro.org>
Content-Type: text/plain; charset=UTF-8
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On 31/01/2024 10:50, Krzysztof Kozlowski wrote:
> 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@linaro.org/
>>
>> On Mon, Jan 29, 2024 at 12:53 PM Krzysztof Kozlowski
>> <krzysztof.kozlowski@linaro.org> 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@robh.at.kernel.org/ [1]
>>> Cc: Bartosz Golaszewski <brgl@bgdev.pl>
>>> Cc: Chris Packham <chris.packham@alliedtelesis.co.nz>
>>> Cc: Sean Anderson <sean.anderson@seco.com>
>>> Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
>>> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
>> (...)
>>
>> 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@ti.com/
> https://lore.kernel.org/all/9eebec9b-e6fd-4a22-89ea-b434f446e061@linaro.org/
> https://lore.kernel.org/all/20231018100055.140847-1-krzysztof.kozlowski@linaro.org/
> https://social.treehouse.systems/@marcan/111268780311634160
> 

And here:

https://lore.kernel.org/all/CAL_JsqL3oZXJJ5_i4BRGpvWu1X8QFB7OGG=D+zLvvWb=UR1mWg@mail.gmail.com/
which the place where this idea of using resets appeared. I agree that
you were not CCed there, but that only means you miss lei filters or
pattern in MAINTAINERS.

Best regards,
Krzysztof