Re: [PATCH v3] Fix IRQ issue by setting IRQ_DISABLE_UNLAZY flag

[Jonathan Cameron <jic23@kernel.org>]

On Mon, 24 Apr 2023 11:09:34 +0200
Nuno Sá <noname.nuno@gmail.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@mechatrax.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@mechatrax.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
>