Re: [PATCH v2 15/16] iio: adc: max1027: Add support for external triggers

[Miquel Raynal <miquel.raynal@bootlin.com>]

Hi Jonathan,

jic23@kernel.org wrote on Sat, 18 Sep 2021 18:13:08 +0100:

> On Wed, 15 Sep 2021 12:18:32 +0200
> Miquel Raynal <miquel.raynal@bootlin.com> wrote:
> 
> > Hi Jonathan, Nuno,
> > 
> > jic23@kernel.org wrote on Sun, 5 Sep 2021 17:10:46 +0100:
> >   
> > > On Thu,  2 Sep 2021 23:14:36 +0200
> > > Miquel Raynal <miquel.raynal@bootlin.com> wrote:
> > >     
> > > > So far the driver only supported to use the hardware cnvst trigger. This
> > > > was purely a software limitation.
> > > > 
> > > > The IRQ handler is already registered as being a poll function and thus
> > > > can be called upon external triggering. In this case, a new conversion
> > > > must be started, and one must wait for the data to be ready before
> > > > reading the samples.
> > > > 
> > > > As the same handler can be called from different places, we check the
> > > > value of the current IRQ with the value of the registered device
> > > > IRQ. Indeed, the first step is to get called with a different IRQ number
> > > > than ours, this is the "pullfunc" version which requests a new      
> > > 
> > > pullfunc?
> > >     
> > > > conversion. During the execution of the handler, we will wait for the
> > > > EOC interrupt to happen. This interrupt is handled by the same
> > > > helper. This time the IRQ number is the one we registered, we can in
> > > > this case call complete() to unlock the primary handler and return. The
> > > > primary handler continues executing by retrieving the data normally and
> > > > finally returns.      
> > > 
> > > Interesting to use the irq number..
> > > 
> > > I'm a little nervous about how this has ended up structured.
> > > I'm not 100% sure my understanding of how you've done it is correct.
> > > 
> > > We should have the following situation:
> > > 
> > > IRQ IN
> > >   |
> > >   v
> > > Trigger IRQ / EOC IRQ  (this is the spi->irq)  (currently iio_trigger_generic_data_poll_ready)
> > >   |              |
> > >   ---------      v
> > >   |        |   complete
> > >   v        v
> > > TrigH1    (TrigH2)   (these are the IRQs below the irq_chip IIO uses to demux triggers)
> > > 
> > > 
> > > So when using it's own trigger we are using an internal interrupt
> > > tree burried inside the IIO core.  When using it only as an EOC interrupt we shouldn't
> > > be anywhere near that internal interrupt chip.
> > > 
> > > So I'm surprised the IRQ matches with the spi->irq as 
> > > those trigH1 and trigH2 will have their own IRQ numbers.
> > > 
> > > For reference I think your architecture is currently
> > > 
> > > IRQ IN
> > >   |
> > >   v
> > >   Trigger IRQ
> > >   |
> > >   v
> > >  TRIG H1
> > >  Either fills the buffer or does the completion.
> > > 
> > > I am a little confused how this works with an external trigger because the Trig H1 interrupt
> > > should be disabled unless we are using the trigger.  That control isn't exposed to the
> > > driver at all.
> > > 
> > > Is my understanding right or have I gotten confused somewhere?    
> > 
> > I think the confusion comes from the fact that in the
> > current implementation, Trigger IRQ and EOC IRQ handlers are the same.
> > This comes from a possible misunderstanding in the previous review,
> > where I understood that you and Nuno wanted to keep using
> > iio_trigger_generic_data_rdy_poll() hand have a single handler in the
> > driver (which I think is far from optimal). I can try to split that
> > handler again to have two distinct paths.  
> That is the right thing to do.  The split should be done a little differently
> than you have it in v3. I've added comments to that patch.
> 
> Data ready triggers are always a little messy because we end up with a split that
> is:
> 
> Trigger side -  Interrupt comes in here...
> 
> --------- GENERIC IIO HANDLING ----- Take the trigger and routes it to the device code --- 
> 
> Device side - We do the data reading here.
> 
> The reason for this is that we may well have other devices using the same trigger
> and we want to keep the model looking the same for all devices.
> 
> A push into an iio buffer should always be on the device side of that boundary.

This is much clearer, I think I have got the main idea.

However I have a question that is specific to the current situation. In
the case of this particular device, I don't really understand how
another device could use the same trigger than the hardware one,
because we have no indication of the trigger being latched. When we get
the information the data is already in the FIFO, this means we get the
information much later than when the hardware transitioned to indicate
a conversion request. Is it that in your model, we should be able to
use the EOC IRQ handler to trigger another IIO device, even though
this implies an additional delay?

Thanks,
Miquèl

> > > I also can't see a path in which the eoc interrupt will get fired for raw_reads.
> > > 
> > > Could you talk me through how that works currently?
> > > 
> > > I suspect part of the confusion here is that this driver happens to be using the
> > > standard core handler iio_trigger_generic_data_rdy_poll which hides away that
> > > there are two interrupt handlers in a normal IIO driver for a device with a
> > > trigger and buffered mode.
> > > 1 for the trigger and 1 for the buffer.  Whether the buffer one is a result
> > > of the trigger one (via iio_poll_trigger) is down to whether the device is
> > > using it's own trigger or not.    
> > 
> > Also, to answer Nuno about the question: is this actually working: IIRC
> > I mentioned it in the cover letter but my hardware does not have the
> > EOC line wired so I am unable to actually test that I am not breaking
> > this. My main goal is to be able to use external triggers (such as a
> > timer) and I am a bit struggling with the constraints of my hardware +
> > the design of this chip.
> > 
> > I will provide a third implementation in v3 and if this still does not
> > fit your mental model please guide me with maybe an untested code
> > snippet just to show me how you think this should be implemented.
> > 
> > Thank you both for the numerous reviews and precious feedback anyway!
> > Miquèl
> >