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

[Jonathan Cameron <jic23@kernel.org>]

On Tue, 7 Jan 2025 09:51:23 +0100
Julien Stephan <jstephan@baylibre.com> wrote:

> Le lun. 6 janv. 2025 à 16:29, David Lechner <dlechner@baylibre.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@baylibre.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?

Whilst I'd hope it would I'm not 100% sure.

> >
> > 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?

Works for me.

> 
> Cheers
> Julein
>