linux-kernel - Re: gpiochip_lock_as_irq on pins without FLAG

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Message-ID: <20210702104810.581438d0@gmail.com>
Date:   Fri, 2 Jul 2021 10:48:10 +0000
From:   Vincent Pelletier <plr.vincent@...il.com>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Bartosz Golaszewski <bgolaszewski@...libre.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: gpiochip_lock_as_irq on pins without FLAG_REQUESTED: bug or
 feature ?

On Fri, 2 Jul 2021 02:09:17 +0200, Linus Walleij <linus.walleij@...aro.org> wrote:
> The basic reason is that gpiochips and irqchips are orthogonal.
> You can request an IRQ on a GPIO line without requesting the
> GPIO line for anything else.
> 
> This is also used when drivers want to inspect the state of a GPIO
> line (read the value) while the same line triggers IRQs. This is
> perfectly legal. An extreme example is:
> drivers/media/cec/platform/cec-gpio/cec-gpio.c

Interesting, thank you very much.

> On Mon, Jun 28, 2021 at 5:37 AM Vincent Pelletier <plr.vincent@...il.com> wrote:
> > Also, I notice that both gpiochip_hierarchy_add_domain and
> > gpiochip_reqres_irq call gpiochip_lock_as_irq, and I am surprised I do not
> > get any error about this: in my understanding only the first call on a given pin
> > should succeed, but with my WARN_ON I am seeing both stack traces and
> > no other warning.  
> 
> Hm that may be a subtle bug.
> 
> The state is just a bool so the first to leave will turn out the lights
> for whoever is left in the room :P

Actually my question came from yet another misunderstanding on my side:
I expected this function to act as an exclusive access control (because
of the "lock" in the name), but I then realised my assumption is wrong.

So while this could be a subtle bug indeed (irq_disable without
irq_shutdown is not the exact same state as right after irq_startup),
it's likely not the one I'm chasing - if it leads to any actual issue
at all.

Regards,
-- 
Vincent Pelletier
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