Re: [PATCH 14/16] iio: adc: max1027: Consolidate the end of conversion helper

[Jonathan Cameron]

On Fri, 3 Sep 2021 16:46:50 +0200
Miquel Raynal <miquel.raynal@xxxxxxxxxxx> wrote:

> Hi Nuno,
> 
> Nuno.Sa@xxxxxxxxxx wrote on Fri, 3 Sep 2021 14:28:52 +0000:
> 
> > Hi Miquel,
> >   
> > > From: Miquel Raynal <miquel.raynal@xxxxxxxxxxx>
> > > Sent: Thursday, September 2, 2021 5:13 PM
> > > To: Sa, Nuno <Nuno.Sa@xxxxxxxxxx>
> > > Cc: Jonathan Cameron <jic23@xxxxxxxxxx>; Lars-Peter Clausen
> > > <lars@xxxxxxxxxx>; Thomas Petazzoni
> > > <thomas.petazzoni@xxxxxxxxxxx>; linux-iio@xxxxxxxxxxxxxxx; linux-
> > > kernel@xxxxxxxxxxxxxxx
> > > Subject: Re: [PATCH 14/16] iio: adc: max1027: Consolidate the end of
> > > conversion helper
> > > 
> > > Hi Nuno,
> > > 
> > > "Sa, Nuno" <Nuno.Sa@xxxxxxxxxx> wrote on Mon, 30 Aug 2021
> > > 12:44:48
> > > +0000:
> > >     
> > > > > -----Original Message-----
> > > > > From: Jonathan Cameron <jic23@xxxxxxxxxx>
> > > > > Sent: Monday, August 30, 2021 12:37 PM
> > > > > To: Miquel Raynal <miquel.raynal@xxxxxxxxxxx>
> > > > > Cc: Lars-Peter Clausen <lars@xxxxxxxxxx>; Thomas Petazzoni
> > > > > <thomas.petazzoni@xxxxxxxxxxx>; linux-iio@xxxxxxxxxxxxxxx;    
> > > linux-    
> > > > > kernel@xxxxxxxxxxxxxxx
> > > > > Subject: Re: [PATCH 14/16] iio: adc: max1027: Consolidate the end    
> > > of    
> > > > > conversion helper
> > > > >
> > > > > On Wed, 18 Aug 2021 13:11:37 +0200
> > > > > Miquel Raynal <miquel.raynal@xxxxxxxxxxx> wrote:
> > > > >    
> > > > > > Now that we have a dedicated handler for End Of Conversion    
> > > > > interrupts,    
> > > > > > let's create a second path:
> > > > > > - Situation 1: we are using the external hardware trigger, a    
> > > > > conversion    
> > > > > >   has been triggered and the ADC pushed the data to its FIFO, we    
> > > > > need to    
> > > > > >   retrieve the data and push it to the IIO buffers.
> > > > > > - Situation 2: we are not using the external hardware trigger,    
> > > hence    
> > > > > we    
> > > > > >   are likely waiting in a blocked thread waiting for this interrupt to
> > > > > >   happen: in this case we just wake up the waiting thread.
> > > > > >
> > > > > > Signed-off-by: Miquel Raynal <miquel.raynal@xxxxxxxxxxx>
> > > > > > ---
> > > > > >  drivers/iio/adc/max1027.c | 20 +++++++++++++++++---
> > > > > >  1 file changed, 17 insertions(+), 3 deletions(-)
> > > > > >
> > > > > > diff --git a/drivers/iio/adc/max1027.c    
> > > b/drivers/iio/adc/max1027.c    
> > > > > > index 8d86e77fb5db..8c5995ae59f2 100644
> > > > > > --- a/drivers/iio/adc/max1027.c
> > > > > > +++ b/drivers/iio/adc/max1027.c
> > > > > > @@ -235,6 +235,7 @@ struct max1027_state {
> > > > > >  	struct iio_trigger		*trig;
> > > > > >  	__be16				*buffer;
> > > > > >  	struct mutex			lock;
> > > > > > +	bool				data_rdy;
> > > > > >  	bool				cnvst_trigger;
> > > > > >  	u8				reg ____cacheline_aligned;
> > > > > >  };
> > > > > > @@ -243,12 +244,22 @@ static    
> > > > > DECLARE_WAIT_QUEUE_HEAD(max1027_queue);    
> > > > > >
> > > > > >  static int max1027_wait_eoc(struct iio_dev *indio_dev)
> > > > > >  {
> > > > > > +	struct max1027_state *st = iio_priv(indio_dev);
> > > > > >  	unsigned int conversion_time =    
> > > > > MAX1027_CONVERSION_UDELAY;    
> > > > > > +	int ret;
> > > > > >
> > > > > > -	if (indio_dev->active_scan_mask)
> > > > > > -		conversion_time *= hweight32(*indio_dev-
> > > > > >active_scan_mask);
> > > > > > +	if (st->spi->irq) {
> > > > > > +		ret =    
> > > > > wait_event_interruptible_timeout(max1027_queue,    
> > > > > > +						       st->data_rdy, HZ /    
> > > > > 1000);    
> > > > > > +		st->data_rdy = false;
> > > > > > +		if (ret == -ERESTARTSYS)
> > > > > > +			return ret;
> > > > > > +	} else {
> > > > > > +		if (indio_dev->active_scan_mask)
> > > > > > +			conversion_time *= hweight32(*indio_dev-
> > > > > >active_scan_mask);
> > > > > >
> > > > > > -	usleep_range(conversion_time, conversion_time * 2);
> > > > > > +		usleep_range(conversion_time, conversion_time * 2);
> > > > > > +	}
> > > > > >
> > > > > >  	return 0;
> > > > > >  }
> > > > > > @@ -481,6 +492,9 @@ static irqreturn_t    
> > > > > max1027_eoc_irq_handler(int irq, void *private)    
> > > > > >  	if (st->cnvst_trigger) {
> > > > > >  		ret = max1027_read_scan(indio_dev);
> > > > > >  		iio_trigger_notify_done(indio_dev->trig);
> > > > > > +	} else {
> > > > > > +		st->data_rdy = true;
> > > > > > +		wake_up(&max1027_queue);    
> > > > >
> > > > > I can't see why a queue is appropriate for this.  Use a completion    
> > > and    
> > > > > have
> > > > > one per instance of the device.  No need for the flag etc in that    
> > > case as    
> > > > > complete() means we have had an interrupt.
> > > > >    
> > > >
> > > > In the case that 'st-> cnvst_trigger' is not set but the spi IRQ
> > > > is present, we will wait until we get 'wake_up()' called from here. I    
> > > wonder if    
> > > > that is a good idea as the device own trigger is not being used. FWIW,    
> > > I think this    
> > > > sync logic is a bit confusing... I would still use the normal trigger    
> > > infrastructure    
> > > > ('iio_trigger_generic_data_rdy_poll()') and use the 'cnvst_trigger'    
> > > flag in the    
> > > > trigger handler to manually start conversions + wait till eoc. But I    
> > > might be missing    
> > > > something though.    
> > > 
> > > I implemented it your way, but I think I found a situation that was not
> > > fully handled (the 3rd), which makes the handler very complicated
> > > as we need to handle all the following cases:
> > > 1/ no trigger, irq enabled -> single read EOC interrupt
> > > 2/ external trigger, no irq -> handle the whole conversion process
> > > 3/ external trigger, irq enabled -> handle the whole conversion process
> > >    but also have a dedicated condition to handle the EOC interrupt
> > >    properly (fortunately this is a threaded handler that can be
> > >    preempted): we need to wait from the handler itself that the
> > >    handler gets called again: the first time it is executed as
> > >    "pollfunc", the second time as "EOC interrupt". In the second
> > >    instance, call complete() in order to deliver the first running
> > >    instance of the handler and continue until the reading part.
> > > 4/ cnvst trigger, irq enabled -> only reads the data.
> > > 5/ cnvst trigger, irq disabled -> not possible.
> > > 
> > > I added a lot of comments to make it clearer.
> > >     
> > > > Regarding this handler, I just realized that this is the hard IRQ handler    
> > > which    
> > > > might end up calling 'max1027_read_scan()' which in turn calls    
> > > 'spi_read()'. Am I    
> > > > missing something here?    
> > > 
> > > I renamed it to make it clear, but this is already a threaded handler.
> > >     
> > 
> > Hmm, I think I get what you're trying to do.... FWIW, I think you're just going
> > into a lot of trouble here for scenario 3 (I assume external trigger is something
> > else other the device own one). IMO, I would just assume that if we are using
> > an external trigger we have to wait (sleep) for the end of conversion (i.e, I would
> > not care about the IRQ in this case). It would make things much more simpler and
> > I guess it should be expected that if some user is deliberately not using the
> > device own trigger, will have to wait more for scans.  
> 
> I thought about that, but this means playing with enabling/disabling
> IRQs, which in the end I fear could be almost as verbose :/
> 
> Can you look at the new implementation that I proposed and give me your
> feedback on it?
> [PATCH v2 15/16] iio: adc: max1027: Add support for external triggers
> 
> > I cannot also see a reason why someone would want to use some
> > external trigger if the device one is available... Does it really make sense?  
> 
> I would say that "genericity" is a good enough reason for that, but in
> practice I agree with you.

Synchronising capture across a number of devices is usual reason for
this.  It's not perfect, but particularly if you have a device
with a complex trigger that is not occurring on at an even 'tick' of
time, then in those cases there isn't really any other way of doing
it.  However that irq is unrelated to this driver.  This driver
sees a trigger which it then identifies as not being it's own and
from that fires the conversion and waits for a completion.
The EOC interrupt handler knows we are in this mode and calls
complete() rather than iio_poll_trigger()

Jonathan

> 
> Thanks,
> Miquèl