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

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Date: Mon, 12 Feb 2024 21:47:49 +0100
From: Linus Walleij <linus.walleij@...aro.org>
To: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc: Srinivas Kandagatla <srinivas.kandagatla@...aro.org>, Banajit Goswami <bgoswami@...cinc.com>, 
	Bjorn Andersson <andersson@...nel.org>, Konrad Dybcio <konrad.dybcio@...aro.org>, 
	Liam Girdwood <lgirdwood@...il.com>, Mark Brown <broonie@...nel.org>, 
	Rob Herring <robh+dt@...nel.org>, 
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>, Conor Dooley <conor+dt@...nel.org>, 
	Philipp Zabel <p.zabel@...gutronix.de>, "Rafael J. Wysocki" <rafael@...nel.org>, 
	Viresh Kumar <viresh.kumar@...aro.org>, Frank Rowand <frowand.list@...il.com>, 
	Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>, alsa-devel@...a-project.org, 
	linux-arm-msm@...r.kernel.org, linux-sound@...r.kernel.org, 
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-pm@...r.kernel.org, Bartosz Golaszewski <brgl@...ev.pl>, 
	Chris Packham <chris.packham@...iedtelesis.co.nz>, Sean Anderson <sean.anderson@...o.com>
Subject: Re: [PATCH v6 4/6] reset: Instantiate reset GPIO controller for
 shared reset-gpios

On Mon, Jan 29, 2024 at 12:53 PM Krzysztof Kozlowski
<krzysztof.kozlowski@...aro.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@...ev.pl>
> Cc: Chris Packham <chris.packham@...iedtelesis.co.nz>
> Cc: Sean Anderson <sean.anderson@...o.com>
> Reviewed-by: Philipp Zabel <p.zabel@...gutronix.de>
> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>

Acked-by: Linus Walleij <linus.walleij@...aro.org>

I can't think of anything better, that is reasonable to ask for.

I feel slightly icky about the way the code reaches into gpiolib, and I think
regulators should be able to reuse the code, but unfortunately only the day
they have no board files left :/

I do feel the core code handling "reset-gpios" could as well have been
used to handle "enable-gpios" in regulators, just that the regulator code
has more requirements, and would be really hard to rewrite, and deals
with descriptors passed in from drivers instead of centralizing it.

Like regulators, reset grows core support for handling GPIO for resets
which is *long due*, given how common it must be. We really need
something like this, and this is certainly elegant enough to do the job.

Yours,
Linus Walleij