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

From: David Lechner <david@lechnology.com>
To: William Breathitt Gray <vilhelm.gray@gmail.com>
Cc: jic23@kernel.org, kernel@pengutronix.de,
	linux-stm32@st-md-mailman.stormreply.com,
	a.fatoum@pengutronix.de, kamel.bouhara@bootlin.com,
	gwendal@chromium.org, alexandre.belloni@bootlin.com,
	linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, syednwaris@gmail.com,
	patrick.havelange@essensium.com, fabrice.gasnier@st.com,
	mcoquelin.stm32@gmail.com, alexandre.torgue@st.com
Subject: Re: [PATCH v7 5/5] counter: 104-quad-8: Add IRQ support for the ACCES 104-QUAD-8
Date: Thu, 11 Feb 2021 19:10:59 -0600	[thread overview]
Message-ID: <add6a885-e666-c5b7-612c-c2d1b08111a5@lechnology.com> (raw)
In-Reply-To: <YCXEFMJOoOhyhfBu@shinobu>

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.