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

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Date:   Thu, 11 Feb 2021 19:10:59 -0600
From:   David Lechner <david@...hnology.com>
To:     William Breathitt Gray <vilhelm.gray@...il.com>
Cc:     jic23@...nel.org, kernel@...gutronix.de,
        linux-stm32@...md-mailman.stormreply.com, a.fatoum@...gutronix.de,
        kamel.bouhara@...tlin.com, gwendal@...omium.org,
        alexandre.belloni@...tlin.com, linux-iio@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        syednwaris@...il.com, patrick.havelange@...ensium.com,
        fabrice.gasnier@...com, mcoquelin.stm32@...il.com,
        alexandre.torgue@...com
Subject: Re: [PATCH v7 5/5] counter: 104-quad-8: Add IRQ support for the ACCES
 104-QUAD-8

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.