linux-kernel - Re: [PATCH RFC 0/9] gpio: improve support for shared GPIOs

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Message-ID: <fydmplp5z4hjic2wlmvcy6yr3s5t5u4qsgo7yzbqq3xu2g6hdk@v4tzjj3ww4s6>
Date: Tue, 21 Oct 2025 17:49:44 +0530
From: Manivannan Sadhasivam <mani@...nel.org>
To: Bartosz Golaszewski <brgl@...ev.pl>
Cc: Dmitry Torokhov <dmitry.torokhov@...il.com>, 
	Kees Cook <kees@...nel.org>, Mika Westerberg <westeri@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Linus Walleij <linus.walleij@...aro.org>, 
	Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, 
	Conor Dooley <conor+dt@...nel.org>, Saravana Kannan <saravanak@...gle.com>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Andy Shevchenko <andy@...nel.org>, 
	Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, 
	Srinivas Kandagatla <srini@...nel.org>, Liam Girdwood <lgirdwood@...il.com>, 
	Mark Brown <broonie@...nel.org>, Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>, 
	linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org, linux-gpio@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-sound@...r.kernel.org, linux-arm-msm@...r.kernel.org, 
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH RFC 0/9] gpio: improve support for shared GPIOs

On Tue, Oct 21, 2025 at 02:06:30PM +0200, Bartosz Golaszewski wrote:
> On Tue, Oct 21, 2025 at 3:53 AM Manivannan Sadhasivam <mani@...nel.org> wrote:
> > >
> > > That only happens if the driver uses the reset API. If you go with the
> > > GPIOLIB then none of this matters. I definitely don't want to change
> > > the existing DT sources either but I want to find out if the code in
> > > this series is suitable (with some modifications) for supporting the
> > > PERST# line or if the logic behind it is more complex and possibly
> > > requires separate, more fine-grained handling.
> > >
> >
> > All PCI controllers relied on '{reset/perst}-gpios' property for handling the
> > PERST# signal. Now if we change it to a reset line, then the drivers have to
> > first detect it as a reset line and use the reset APIs, if not fallback to gpiod
> > APIs (for DT backwards compatibility), which will add unncessary churn IMO.
> >
> 
> Ok so some platforms define perst-gpios while others use reset-gpios,
> I see now. Yeah, it's better to go with explicit GPIOs then.
> 
> > But if there is no way the GPIO subsystem is going to support shared GPIOs, then
> > we have to live with it.
> >
> 
> Well, there is going to be. We already de-facto have it but it doesn't
> work very well and is fragile (I'm talking about the non-exclusive
> flag). I very much intend to bring this upstream.
> 
> My question wrt PCI PERST# was whether this is useful for it because
> IIRC all endpoints sharing the signal will assert it (or rather their
> pwrctl drivers will) and then only deassert it once all endpoints are
> powered up. This would translate to the pwrctl driver doing the
> following for each endpoint:
> 
> perst = gpiod_get(dev, "perst");
> gpiod_set_value_cansleep(perst, 1);
> 
> Do the power up.
> 
> gpiod_set_value_cansleep(perst, 0);
> 
> And with the implementation this series proposes it would mean that
> the perst signal will go high after the first endpoint pwrctl driver
> sets it to high and only go down once the last driver sets it to low.
> The only thing I'm not sure about is the synchronization between the
> endpoints - how do we wait for all of them to be powered-up before
> calling the last gpiod_set_value()?
> 

That will be handled by the pwrctrl core. Not today, but in the coming days.

- Mani

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