Re: [PATCH v7 5/5] counter: 104-quad-8: Add IRQ support for the ACCES 104-QUAD-8

[David Lechner]

On 2/11/21 5:56 PM, William Breathitt Gray wrote:
> On Wed, Dec 30, 2020 at 11:36:45AM -0600, David Lechner wrote:
> > On 12/25/20 6:15 PM, William Breathitt Gray wrote:
> >
> > > diff --git a/Documentation/ABI/testing/sysfs-bus-counter-104-quad-8 b/Documentation/ABI/testing/sysfs-bus-counter-104-quad-8
> > > index eac32180c40d..0ecba24d43aa 100644
> > > --- a/Documentation/ABI/testing/sysfs-bus-counter-104-quad-8
> > > +++ b/Documentation/ABI/testing/sysfs-bus-counter-104-quad-8
> > > @@ -1,3 +1,28 @@
> > > +What:		/sys/bus/counter/devices/counterX/countY/irq_trigger
> >
> > Do we really need this sysfs attribute? Shouldn't interrupts be configured
> > _only_ by the chrdev interface?
>
> I think this attribute can go away because we can implicitly figure out
> the correct IRQ configuration from the struct counter_watch data when a
> user executes a COUNTER_ADD_WATCH_IOCTL ioctl command.
>
> However, I need some help deciding on an appropriate behavior for
> conflicting counter_watch configurations. Let me give some context
> first.
>
> The 104-QUAD-8 features 8 channels (essentially 8 independent physical
> counters on the device). Each channel can independently issue an event,
> but any particular channel can only be set to a single kind of event
> (COUNTER_EVENT_INDEX, COUNTER_EVENT_OVERFLOW, etc.).
>
> The purpose of the irq_trigger sysfs attribute I introduced in this
> patch is to allow the user to select the event configuration they want
> for a particular channel. We can theoretically figure this out
> implicitly from the struct counter_watch request, so this sysfs
> attribute may not be necessary.
>
> However, how do we handle the case where a user executes two
> COUNTER_ADD_WATCH_IOCTL ioctl commands for the same channel but with
> different event selections? I'm considering three possible behaviors:
>
> * Fail the second ioctl call; event selection of the first struct
>    counter_watch takes precedence and thus second is incompatible.
> * Issue a dev_warn() indicating that the second struct counter_watch
>    event selection will now be the event configuration for that channel.
> * Don't notify the user, just silently reconfigure for the second struct
>    counter_watch event selection.
>
> I'm suspecting the first behavior I listed here (ioctl returning failed)
> is the most appropriate as a user is explicitly made known of this
> particular device's inability to support more than one type of event per
> channel.
>
> What do you think?

I agree that it should return an error instead of adding the watch.
I'm pretty sure that is how I implemented the TI eQEP driver already.