linux-kernel - Re: [PATCH v3] Fix IRQ issue by setting IRQ_DISABLE

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Date:   Mon, 1 May 2023 19:45:46 +0900
From:   Masahiro Honda <honda@...hatrax.com>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     Nuno Sá <noname.nuno@...il.com>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Michael Hennerich <Michael.Hennerich@...log.com>,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] Fix IRQ issue by setting IRQ_DISABLE_UNLAZY flag

Hi all,

On Mon, May 1, 2023 at 2:27 AM Jonathan Cameron <jic23@...nel.org> wrote:
>
> On Mon, 24 Apr 2023 11:09:34 +0200
> Nuno Sá <noname.nuno@...il.com> wrote:
>
> > Hi Jonathan,
> >
> > On Sun, 2023-04-23 at 12:15 +0100, Jonathan Cameron wrote:
> > > On Thu, 20 Apr 2023 19:23:16 +0900
> > > Masahiro Honda <honda@...hatrax.com> wrote:
> > >
> > > > The Sigma-Delta ADCs supported by this driver can use SDO as an interrupt
> > > > line to indicate the completion of a conversion. However, some devices
> > > > cannot properly detect the completion of a conversion by an interrupt.
> > > > This is for the reason mentioned in the following commit.
> > > >
> > > > commit e9849777d0e2 ("genirq: Add flag to force mask in
> > > >                       disable_irq[_nosync]()")
> > > >
> > > > A read operation is performed by an extra interrupt before the completion
> > > > of a conversion. This patch fixes the issue by setting IRQ_DISABLE_UNLAZY
> > > > flag.
> > > >
> > > > Signed-off-by: Masahiro Honda <honda@...hatrax.com>
> > > > ---
> > > > v3:
> > > >  - Remove the Kconfig option.
> > > > v2:
> > > > https://lore.kernel.org/linux-iio/20230414102744.150-1-honda@mechatrax.com/
> > > >  - Rework commit message.
> > > >  - Add a new entry in the Kconfig.
> > > >  - Call irq_clear_status_flags(irq, IRQ_DISABLE_UNLAZY) when freeing the
> > > > IRQ.
> > > > v1:
> > > > https://lore.kernel.org/linux-iio/20230306044737.862-1-honda@mechatrax.com/
> > > >
> > > >  drivers/iio/adc/ad_sigma_delta.c | 25 ++++++++++++++++++++-----
> > > >  1 file changed, 20 insertions(+), 5 deletions(-)
> > > >
> > > > diff --git a/drivers/iio/adc/ad_sigma_delta.c
> > > > b/drivers/iio/adc/ad_sigma_delta.c
> > > > index d8570f620..215ecbedb 100644
> > > > --- a/drivers/iio/adc/ad_sigma_delta.c
> > > > +++ b/drivers/iio/adc/ad_sigma_delta.c
> > > > @@ -565,6 +565,14 @@ int ad_sd_validate_trigger(struct iio_dev *indio_dev,
> > > > struct iio_trigger *trig)
> > > >  }
> > > >  EXPORT_SYMBOL_NS_GPL(ad_sd_validate_trigger, IIO_AD_SIGMA_DELTA);
> > > >
> > > > +static void ad_sd_free_irq(void *sd)
> > > > +{
> > > > +       struct ad_sigma_delta *sigma_delta = sd;
> > > > +
> > > > +       irq_clear_status_flags(sigma_delta->spi->irq, IRQ_DISABLE_UNLAZY);
> > > > +       free_irq(sigma_delta->spi->irq, sigma_delta);
> > > > +}
> > >
> > > Don't fuse the two operations unwinding like this.  Just register a callback
> > > that only
> > > does the irq_clear_status_flags immediately after setting them.  Then leave
> >
> > I was the one to propose fusing them together because I thought that we could
> > have issues by clearing the flag after calling free_irq(). After looking again
> > at the IRQ code, I can see that it is not up to free_irq() to free the allocated
> > irq_descs (that might only happen when unmapping the virq) which means we should
> > be fine doing the normal way.
>
> Ah. I'd missed the ordering.  If that had been valid (and I think you are correct
> that it is not required) then a comment to make that clear would be necessary.
>
> Usual case of: When doing something non obvious with ordering, say why.
>
> >
> > That said, looking at the only users that care to clear this flag, it looks like
> > they do it before calling free_irq(). Hence, I'm not sure if there's anything
> > subtle going on. In fact, looking at this line:
> >
> > https://elixir.bootlin.com/linux/latest/source/kernel/irq/manage.c#L1909
> >
> > I'm not so sure we actually need to clear the flag as for these devices, we
> > should only have one consumer/action per IRQ. Anyways, probably for correctness
> > we should still explicitly clear it?
>
> Good question...  Looks to me like a driver shouldn't be clearing this flag
> itself, but it's probably harmless in most cases.
>
> I'd drop the clear of the status flag, perhaps adding a comment that
> the irq core does it for us.
>
> Jonathan
>
>
> >
> > - Nuno Sá
> >
>

I'll remove the callback and just call irq_set_status_flags() with a comment.

Thanks,

Masahiro