linux-kernel - Re: [PATCH 08/22] gpiolib: cdev: complete the irq/thread timestamp handshake

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Message-ID: <CAMpxmJXbp_JYg8pX6StMy3578O4MZuatDTd-Ki6ydVY8u32+fw@mail.gmail.com>
Date:   Tue, 30 Jun 2020 11:08:37 +0200
From:   Bartosz Golaszewski <bgolaszewski@...libre.com>
To:     Kent Gibson <warthog618@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-gpio <linux-gpio@...r.kernel.org>,
        Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [PATCH 08/22] gpiolib: cdev: complete the irq/thread timestamp handshake

On Thu, Jun 25, 2020 at 12:01 PM Kent Gibson <warthog618@...il.com> wrote:
>
> On Thu, Jun 25, 2020 at 11:44:30AM +0200, Bartosz Golaszewski wrote:
> > On Wed, Jun 24, 2020 at 4:08 PM Kent Gibson <warthog618@...il.com> wrote:
> > >
> > > On Wed, Jun 24, 2020 at 04:00:42PM +0200, Bartosz Golaszewski wrote:
> > > > wt., 23 cze 2020 o 06:02 Kent Gibson <warthog618@...il.com> napisał(a):
> [ snip ]
> > >
> > > I'm not totally sure myself, as my understanding of how interrupts are
> > > shared in the kernel is pretty sketchy, but my concern is that if we
> > > are sharing the irq then whoever we are sharing with may release the irq
> > > and we go from nested to unnested.  Or vice versa.  Not sure if that is
> > > valid, but that was my concern, and it seemed like a minor change to
> > > cover it just in case.
> > >
> >
> > It's my understanding that a shared interrupt must be explicitly
> > requested as shared by all previous users or request_irq() will fail.
> > In this case: we call request_threaded_irq() without the IRQF_SHARED
> > flag so it's never a shared interrupt. Even if someone previously
> > requested it as shared - our call will simply fail.
> >
>
> OK.  Is there a reason not to share the interrupt?
>

If nobody requested this yet, I'd say: let's not touch it. :)

In theory, we check if the line state changed so we should be fine but
in practice this sounds like a can of worms. That being said: I don't
have a reason not to do it. Just a feeling.

> > I still think that resetting the timestamp is fine because it's not
> > being set to 0 in hardirq context. We just need a different
> > explanation.
> >
>
> Or just drop it?

Yes, I think dropping this patch for now is fine.

Bart