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

[Miquel Raynal]

Hi Nuno, Jonathan,

Nuno.Sa@xxxxxxxxxx wrote on Mon, 6 Sep 2021 09:53:15 +0000:

> > -----Original Message-----
> > From: Jonathan Cameron <jic23@xxxxxxxxxx>
> > Sent: Sunday, September 5, 2021 6:11 PM
> > To: Miquel Raynal <miquel.raynal@xxxxxxxxxxx>
> > Cc: Lars-Peter Clausen <lars@xxxxxxxxxx>; linux-iio@xxxxxxxxxxxxxxx;
> > linux-kernel@xxxxxxxxxxxxxxx; Thomas Petazzoni
> > <thomas.petazzoni@xxxxxxxxxxx>; Sa, Nuno <Nuno.Sa@xxxxxxxxxx>
> > Subject: Re: [PATCH v2 15/16] iio: adc: max1027: Add support for
> > external triggers
> > 
> > [External]
> > 
> > On Thu,  2 Sep 2021 23:14:36 +0200
> > Miquel Raynal <miquel.raynal@xxxxxxxxxxx> 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 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 forgot to do this, I think you misunderstood the following patch:
"iio: adc: max1027: Don't just sleep when the EOC interrupt is
available"

As I am having deep troubles reworking this once again, I will try to
explain how this is build below and look for your feedback on it.

> > 
> > 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.
> > 

[...]
 
> I'm also confused by this. Going through the series, I was actually
> thinking that raw_reads were in fact using the EOC IRQ until I realized
> that 'complete()' was being called from the trigger handler... So,
> I'm also not sure how is this supposed to work?

I renamed this handler with a generic name, because it actually serves
several different purposes, see below.

> But I'm probably
> missing something as I guess you tested this and how I'm understanding
> things, you should have gotten timeouts for raw_reads.
> 
> Anyways, as Jonathan said, I was also expecting to see the 'complete()' call
> from the device IRQ handler. Other thing than that is just asking for trouble
> :). 
> 
> - Nuno Sá

So here is how I understand the device:
* During a raw read:
  The IRQ indicates an EOC.
* During a triggered read (internal trigger):
  The driver has no knowledge of the trigger being fired, it only
  sees the IRQ firing at EOC. This means the internal trigger cannot be
  used without the IRQ.

Now here is what I understand from the IIO subsystem and what I tried
to implement:
* Perform a raw read:
  The driver needs to setup the channels, start the operation, wait for
  the end of the conversion, return the value. This is all done in the
  ->raw_read() hook in process context. Raw reads can either use the
  EOC IRQ if wired or just wait for a sufficiently large amount of
  time.
* Perform a triggered read (internal trigger):
  The device will get triggered by a hardware event that is out of
  control from the driver. The driver is only aware of the IRQ firing
  when the conversion is done. It then must push the samples to a set
  of buffers and notify the IIO core.
* Perform a triggered read (external trigger):
  This looks very much like a raw read from the driver point of view,
  the difference being that, when the external trigger fires, the IIO
  core will first call iio_pollfunc_store_time() as hard IRQ handler
  and then calls a threaded handler that is supposed to configure the
  channels, start the conversions, wait for it and again push the
  samples when their are ready. These two handlers are registered by
  devm_iio_triggered_buffer_setup().

There is an additional level of complexity as, my hardware does not
feature this EOC IRQ (it is not wired and thus cannot be used). I want
to be able to also support the absence of IRQ which is not necessary
for any operation but the internal trigger use.

How I ended-up implementing things in v2:
* Raw read:
  Start the conversion.
  Wait for the conversion to be done:
  - With IRQ: use wait_for_completion(), the IRQ will fire, calling
    iio_trigger_generic_data_rdy_poll() [1], calling complete() in this
    situation.
  - Without IRQ: busy wait loop.

[1] Maybe this is the thing that is bothering you, using the internal
IIO trigger interrupt tree logic to handle a regular EOC situation. In
all drivers that I had a chance to look at, it's always done like that
but perhaps it was bad luck and the more I look at it, the less I
understand its use. I will propose an alternative where the hard IRQ
handler really is the EOC handler and eventually calls a threaded
handler in the case of an internal triggering to push the samples.
I don't see the point of using iio_trigger_generic_data_rdy_poll()
anymore (maybe I'm wrong, tell me after reading v3?).

I ended-up writing this driver this way because I thought that I had to
provide a single IRQ handler and use
iio_trigger_generic_data_rdy_poll() but now I think I either
misinterpreted the comments or it was bad advise. I fell that the
driver is much simpler to understand in v3 where there is:
- one hard IRQ handler registered as the primary interrupt handler:
  its purpose is to handle EOC conditions
- one threaded handler registered as the secondary interrupt handler:
  it will only be triggered when using the internal trigger, its purpose
  is to read and push the samples
- one threaded handler registered as the external trigger handler:
  it will start the conversion, wait for EOC (either by busy waiting if
  there is no IRQ or by waiting for completion otherwise - complete()
  being called in the primary IRQ handler), read the data and push it
  to the buffers.

If this implementation does not fit your requirements, I will really
need more advanced advises about what you require, what I should avoid
and perhaps what is wrong in the above explanation :)

Thanks,
Miquèl