Re: [PATCH 1/3] dt-bindings: iio: adc: adi,ad7124: Allow specifications of a gpio for irq line

[Jonathan Cameron <jic23@kernel.org>]

On Sun, 27 Oct 2024 22:53:39 +0100
Uwe Kleine-König <u.kleine-koenig@baylibre.com> wrote:

> Hello,
> 
> [dropped Alexandru Tachici from Cc:, the address bounces]
> 
> On Sun, Oct 27, 2024 at 11:54:09AM +0000, Jonathan Cameron wrote:
> > On Thu, 24 Oct 2024 19:17:03 +0200
> > Uwe Kleine-König <u.kleine-koenig@baylibre.com> wrote:
> >   
> > > For the AD7124 chip the logical irq line (̅R̅D̅Y) is physically on the same
> > > pin as the spi MISO output (DOUT) and so reading a register might
> > > trigger an interrupt. For correct operation it's critical that the
> > > actual state of the pin can be read to judge if an interrupt event is a
> > > real one or just a spurious one triggered by toggling the line in its
> > > MISO mode.  
> > 
> > This text should note that this is a limitation with the interrupt controller.
> > The IRQ is disabled when those reads are going on, yet the controller is
> > still detecting the interrupt and reporting it on reenable.
> > I'm not an expert in what the kernel IRQ subsystem requires so maybe
> > this is a valid implementation.  
> 
> This is even the saner option and a controller not triggering might miss
> irqs. Consider the process that triggered a conversion and then calls
> enable_irq() is preempted long enough that the conversion is already
> done when enable_irq() is called. The completion would just time out and
> no measurement reported.

True enough - that race is a possibility, but not all interrupt inputs
are capable of gpio usage whilst setup to received interrupts.

> 
> > > Allow specification of an "interrupt-gpios" property instead of a plain
> > > interrupt. The semantic is that the GPIO's interrupt is to be used as
> > > event source and reading the GPIO can be used to differentiate between a
> > > real event and one triggered by MISO.  
> > 
> > This sort of hack is a bit nasty and if we are going to do it we should
> > allow for double wiring - so to separate GPIO and interrupt pins on the
> > host wired to single pin on the device.
> > 
> > The binding does that by allowing both interrupts and interrupt-gpio
> > but we need to make that explicit in this text. Arguably even when
> > they are the same pin the binding should treat them as independent
> > and the driver should get the gpio from one, and the interrupt from
> > the other.  
> 
> This would also need a code update because currently the interrupt-gpio
> is only used if no interrupt is specified.

Absolutely.  It would require slightly different code.


> 
> > I also definitely need input from Analog Devices folk on this series.  
> 
> Good candidates to comment are still on Cc:
Yeah. I was poking them  :)

> 
> Best regards
> Uwe