linux-kernel - Re: [PATCH RFC v2 3/4] iio: adc: ad7380: add alert support

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Message-ID: <CAEHHSvb_BRB-ygksULL4+o4eFEPSC4zOs1yBptFd8QzZtS0Dhw@mail.gmail.com>
Date: Tue, 7 Jan 2025 09:51:23 +0100
From: Julien Stephan <jstephan@...libre.com>
To: David Lechner <dlechner@...libre.com>
Cc: Jonathan Cameron <jic23@...nel.org>, Lars-Peter Clausen <lars@...afoo.de>, 
	Michael Hennerich <Michael.Hennerich@...log.com>, Nuno Sá <nuno.sa@...log.com>, 
	Jonathan Corbet <corbet@....net>, linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-doc@...r.kernel.org
Subject: Re: [PATCH RFC v2 3/4] iio: adc: ad7380: add alert support

Le lun. 6 janv. 2025 à 16:29, David Lechner <dlechner@...libre.com> a écrit :
>
> On 12/28/24 8:24 AM, Jonathan Cameron wrote:
> > On Tue, 24 Dec 2024 10:34:32 +0100
> > Julien Stephan <jstephan@...libre.com> wrote:
> >
> >> The alert functionality is an out of range indicator and can be used as an
> >> early indicator of an out of bounds conversion result.
> >>
> >> ALERT_LOW_THRESHOLD and ALERT_HIGH_THRESHOLD registers are common to all
> >> channels.
> >>
> >> When using 1 SDO line (only mode supported by the driver right now), i.e
> >> data outputs only on SDOA, SDOB (or SDOD for 4 channels variants) is
> >> used as an alert pin. The alert pin is updated at the end of the
> >> conversion (set to low if an alert occurs) and is cleared on a falling
> >> edge of CS.
> >>
> >> The ALERT register contains information about the exact alert status:
> >> channel and direction. Unfortunately we can't read this register because
> >> in buffered read we cannot claim for direct mode.
> >>
> >> User can set high/low thresholds and enable event detection using the
> >> regular iio events:
> >>
> >>   events/in_thresh_falling_value
> >>   events/in_thresh_rising_value
> >>   events/thresh_either_en
> >>
> >> If the interrupt properties is present in the device tree, an IIO event
> >> will be generated for each interrupt received.
> >> Because we cannot read ALERT register, we can't determine the exact
> >> channel that triggers the alert, neither the direction (hight/low
> >> threshold violation), so we send and IIO_EV_DIR_EITHER event for all
> >> channels.
> >>
> >> In buffered reads, if input stays out of thresholds limit, an interrupt
> >> will be generated for each sample read, because the alert pin is cleared
> >> on a falling edge of CS (i.e when starting a new conversion). To avoid
> >> generating to much interrupt, we introduce a reset_timeout that can be
> >> used to disable interrupt for a given time (in ms)
> >>
> >>   events/thresh_either_reset_timeout
> >>
> >> When an interrupt is received, interrupts are disabled and re-enabled
> >> after thresh_either_reset_timeout ms. If the reset timeout is set to 0,
> >> interrupt are re-enabled directly.
> >> Note: interrupts are always disabled at least during the handling of the
> >> previous interrupt, because each read triggers 2 transactions, that can
> >> lead to 2 interrupts for a single user read. IRQF_ONESHOT is not enough,
> >> because, it postpones the 2nd irq after the handling of the first one,
> >> which can still trigger 2 interrupts for a single user read.
> >
> > After some of our recent discussions around interrupt handling and
> > the guarantees (that aren't) made, even disabling the interrupt doesn't
> > prevent some irq chips queuing up future interrupts.
> >
> > https://lore.kernel.org/all/io53lznz3qp3jd5rohqsjhosnmdzd6d44sdbwu5jcfrs3rz2a2@orquwgflrtyc/
> >
> > I'm not sure this alert can actually work as a result :(
> > I am struggling to come up with a scheme that will work.
> >
> Would it work if we change it to a level-triggered interrupt instead of edge
> triggered?
>
> Since the main purpose of this is to trigger a hardware shutdown, perhaps we
> could just omit the interrupt/emitting the event and keep the threshold and
> enable attributes if we can't come up with a reasonable way to handle the
> interrupts?
>

Hi Jonathan, and David,

I think this is getting very complicated for something not that useful
in practice.
If needed we can go back on this later to find an appropriate solution.
I sent a non RFC V3 version, removing the interrupt handling? Does
that work for you?

Cheers
Julein