Re: [PATCH] gpio: Revert regression in sysfs-gpio (gpiolib.c)

From: Marcelo Roberto Jimenez <marcelo.jimenez@gmail.com>
To: Will McVicker <willmcvicker@google.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>,
	Bartosz Golaszewski <brgl@bgdev.pl>,
	 Thorsten Leemhuis <regressions@leemhuis.info>,
	stable <stable@vger.kernel.org>,
	 regressions@lists.linux.dev,
	Linus Walleij <linus.walleij@linaro.org>,
	 "open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
	Thierry Reding <treding@nvidia.com>,
	 Vidya Sagar <vidyas@nvidia.com>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	 Edmond Chung <edmondchung@google.com>,
	Andrew Chant <achant@google.com>,
	 Sergio Tanzilli <tanzilli@acmesystems.it>
Subject: Re: [PATCH] gpio: Revert regression in sysfs-gpio (gpiolib.c)
Date: Mon, 20 Dec 2021 17:41:42 -0300	[thread overview]
Message-ID: <CACjc_5qQv_vWawtbA8O01EaiY6GqSXQYs0jgC8XLAGYuwtnUUg@mail.gmail.com> (raw)
In-Reply-To: <CABYd82b2i4Uuyi5+zLoTgiC-QMS1y=VkwmMznZqxLca0iP9qTQ@mail.gmail.com>

Hi Will,

On Mon, Dec 20, 2021 at 4:25 PM Will McVicker <willmcvicker@google.com> wrote:
>
> On Mon, Dec 20, 2021 at 7:14 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> >
> > On Mon, Dec 20, 2021 at 3:57 PM Bartosz Golaszewski <brgl@bgdev.pl> wrote:
> > > On Sat, Dec 18, 2021 at 7:28 AM Thorsten Leemhuis
> > > <regressions@leemhuis.info> wrote:
> > > > [TLDR: I'm adding this regression to regzbot, the Linux kernel
> > > > regression tracking bot; most text you find below is compiled from a few
> > > > templates paragraphs some of you might have seen already.]
> > > >
> > > > On 17.12.21 16:35, Marcelo Roberto Jimenez wrote:
> > > > > Some GPIO lines have stopped working after the patch
> > > > > commit 2ab73c6d8323f ("gpio: Support GPIO controllers without pin-ranges")
> > > > >
> > > > > And this has supposedly been fixed in the following patches
> > > > > commit 89ad556b7f96a ("gpio: Avoid using pin ranges with !PINCTRL")
> > > > > commit 6dbbf84603961 ("gpiolib: Don't free if pin ranges are not defined")
> > > >
> > > > There seems to be a backstory here. Are there any entries and bug
> > > > trackers or earlier discussions everyone that looks into this should be
> > > > aware of?
> > > >
> > >
> > > Agreed with Thorsten. I'd like to first try to determine what's wrong
> > > before reverting those, as they are correct in theory but maybe the
> > > implementation missed something.
> > >
> > > Have you tried tracing the execution on your platform in order to see
> > > what the driver is doing?
> >
> > Looking at commits that have related Fixes tags:
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=bf781869e5cf3e4ec1a47dad69b6f0df97629cbd
> > https://git.kernel.org/pub/scm/linux/kernel/git/linusw/linux-pinctrl.git/commit/?id=e8f24c58d1b69ecf410a673c22f546dc732bb879
> >
> > Gr{oetje,eeting}s,
> >
> >                         Geert
> >
> > --
> > Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
> >
> > In personal conversations with technical people, I call myself a hacker. But
> > when I'm talking to journalists I just say "programmer" or something like that.
> >                                 -- Linus Torvalds
>
> Hi Marcelo,
>
> Thanks for reporting this issue. I can give you a little context on
> why commit 6dbbf84603961 ("gpiolib: Don't free if pin ranges are not
> defined") was created. We were seeing a refcounting issue on Pixel 6.
> In our kernel CONFIG_PINCTRL is defined. Basically, the camera kernel
> module requests for a GPIO on sensor enable (when the camera sensor is
> turned on) and releases that GPIO on sensor disable (when the camera
> sensor is turned off). Before commit 6dbbf84603961, if we constantly
> switched between the front and back camera eventually we would hit the
> below error in drivers/pinctrl/pinmux.c:pin_request():
>
>     E samsung-pinctrl 10840000.pinctrl: could not increase module
> refcount for pin 134
>
> In our kernel the sensor GPIOs don't have pin_ranges defined. So you
> would get these call stacks:
>
> Sensor Enable:
>   gpiochip_generic_request()
>   -> return 0
>
> Sensor Disable:
>   gpiochip_generic_free()
>   -> pinctrl_gpio_free()
>
> This led to an imbalance of request vs free calls leading to the
> refcounting error. When we added commit 6dbbf84603961 ("gpiolib: Don't
> free if pin ranges are not defined"), this issue was resolved. My
> recommendation would be to drill down into your driver to figure out
> what happens in these functions to see why you're getting the results
> you reported.

Thanks for your reply.

Commit 6dbbf84603961 ("gpiolib: Don't free if pin ranges are not
defined") is perfectly fine in the context and fixes a serious issue.
But to revert the original patch we need to revert this patch too, for
the same reason, i.e., in order to not generate a *_free() imbalance.

In my case the imbalance causes problems as soon as the test script is
run a second time.

>
> --Will

Regards,
Marcelo.