Re: [PATCH] iio: imu: st_lsm6dsx: fix edge-trigger interrupts

[Lorenzo Bianconi <lorenzo@kernel.org>]

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[...]
> > 
> > I guess since edge interrupts run with the line unmasked a new interrupt can fire
> > while the irq thread is still running (so wake_up_process() will just return) but
> > the driver has already read fifo_status register and so it will not read new
> > sample. This case should be fixed reading again the fifo_status register.
> 
> It doesn't actually help because there is always a window after the fifo_status register
> is read before we exit the thread.
> 
> I 'think' what happens (it's been a while since I dug through this stuff) is
> that you end up with the task being added to the runqueue, even though it's
> already running. Upshot the thread gets scheduled gain. 
> 
> If this were not the case there would be a race with any edge based interrupt
> as the thread has to reenable the interrupt and it will always be able to fire
> whilst the thread is still running.

I guess this is the case (race between irq-thread and edge interrupt) since afaik
handle_edge_irq() runs with the irq-line unmasked.
I agree with you this approach does not fix completely the issue but it reduces
the race-surface since now the interrupt can fire while processing the previous one
(the issue occurs if the interrupt fires between the end of hw->settings->fifo_ops.read_fifo()
and the end of the irq-thread) while before the interrupt must always fire before
reading the fifo status register (in fact with the patch applied I am not able
to trigger the issue anymore).

@denis, mario, armando: can you please confirm the hw does not support pulsed
interrupts for fifo-watermark?

If not one possible approach would be to disable the interrupt generation on
the sensor at the beginning of st_lsm6dsx_handler_thread() and schedule a
workqueue at the end of st_lsm6dsx_handler_thread() to re-enable the sensor
interrupt generation. What do you think?

Regards,
Lorenzo

> 
> Jonathan
> 
> 
> 
> 
> > 
> > Regards,
> > Lorenzo
> > 
> > >   
> > > >   
> > > > > 
> > > > > 
> > > > > Hmm. Having had a look at one of the datasheets, I'm far from convinced these
> > > > > parts truely support edge interrupts.  I can't see anything about minimum
> > > > > off periods etc that you need for true edge interrupts. Otherwise they are
> > > > > going to be prone to races.    
> > > > 
> > > > @mario, denis, armando: any pointer for this?
> > > >   
> > > > > 
> > > > > So I think the following can happen.
> > > > > 
> > > > > A) We drain the fifo and it stays under the limit. Hence once that
> > > > >    is crossed in future we will interrupt as normal.
> > > > > 
> > > > > B) We drain the fifo but it either has a very low watermark, or is
> > > > >    filling very fast.   We manage to drain enough to get the interrupt
> > > > >    to fire again, so all is fine if less than ideal.  With you loop we
> > > > >    may up entering the interrupt handler when we don't actually need to.
> > > > >    If you want to avoid that you would need to disable the interrupt,
> > > > >    then drain the fifo and finally do a dance to successfully reenable
> > > > >    the interrupt, whilst ensuring no chance of missing by checking it
> > > > >    should not have fired (still below the threshold)
> > > > > 
> > > > > C) We try to drain the fifo, but it is actually filling fast enough that
> > > > >    we never get it under the limit, so no interrupt ever fires.
> > > > >    With new code, we'll keep spinning to 0 so might eventually drain it.
> > > > >    That needs a timeout so we just give up eventually.
> > > > > 
> > > > > D) watershed is one sample, we drain low enough to successfully get down
> > > > >    to zero at the moment of the read, but very very soon after that we get
> > > > >    one sample again. There is a window in which the interrupt line dropped
> > > > >    but analogue electronics etc being what they are, it may not have been
> > > > >    detectable.  Hence we miss an interrupt...  What you are doing is reducing
> > > > >    the chance of hitting this.  It is nasty, but you might be able to ensure
> > > > >    a reasonable period by widening this window.  Limit the watermark to 2
> > > > >    samples?  
> > > > > 
> > > > > Also needs a fixes tag :)    
> > > > 
> > > > ack, I will add them in v2
> > > > 
> > > > Regards,
> > > > Lorenzo  
> > > > >     
> > > > > > 
> > > > > > Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> > > > > > ---
> > > > > >  drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c | 33 +++++++++++++++-----
> > > > > >  1 file changed, 25 insertions(+), 8 deletions(-)
> > > > > > 
> > > > > > diff --git a/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c b/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c
> > > > > > index 5e584c6026f1..d43b08ceec01 100644
> > > > > > --- a/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c
> > > > > > +++ b/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c
> > > > > > @@ -2457,22 +2457,36 @@ st_lsm6dsx_report_motion_event(struct st_lsm6dsx_hw *hw)
> > > > > >  	return data & event_settings->wakeup_src_status_mask;
> > > > > >  }
> > > > > >  
> > > > > > +static irqreturn_t st_lsm6dsx_handler_irq(int irq, void *private)
> > > > > > +{
> > > > > > +	return IRQ_WAKE_THREAD;
> > > > > > +}
> > > > > > +
> > > > > >  static irqreturn_t st_lsm6dsx_handler_thread(int irq, void *private)
> > > > > >  {
> > > > > >  	struct st_lsm6dsx_hw *hw = private;
> > > > > > +	int fifo_len = 0, len = 0;
> > > > > >  	bool event;
> > > > > > -	int count;
> > > > > >  
> > > > > >  	event = st_lsm6dsx_report_motion_event(hw);
> > > > > >  
> > > > > >  	if (!hw->settings->fifo_ops.read_fifo)
> > > > > >  		return event ? IRQ_HANDLED : IRQ_NONE;
> > > > > >  
> > > > > > -	mutex_lock(&hw->fifo_lock);
> > > > > > -	count = hw->settings->fifo_ops.read_fifo(hw);
> > > > > > -	mutex_unlock(&hw->fifo_lock);
> > > > > > +	/*
> > > > > > +	 * If we are using edge IRQs, new samples can arrive while
> > > > > > +	 * processing current IRQ and those may be missed unless we
> > > > > > +	 * pick them here, so let's try read FIFO status again
> > > > > > +	 */
> > > > > > +	do {
> > > > > > +		mutex_lock(&hw->fifo_lock);
> > > > > > +		len = hw->settings->fifo_ops.read_fifo(hw);
> > > > > > +		mutex_unlock(&hw->fifo_lock);
> > > > > > +
> > > > > > +		fifo_len += len;
> > > > > > +	} while (len > 0);
> > > > > >  
> > > > > > -	return count || event ? IRQ_HANDLED : IRQ_NONE;
> > > > > > +	return fifo_len || event ? IRQ_HANDLED : IRQ_NONE;
> > > > > >  }
> > > > > >  
> > > > > >  static int st_lsm6dsx_irq_setup(struct st_lsm6dsx_hw *hw)
> > > > > > @@ -2488,10 +2502,14 @@ static int st_lsm6dsx_irq_setup(struct st_lsm6dsx_hw *hw)
> > > > > >  
> > > > > >  	switch (irq_type) {
> > > > > >  	case IRQF_TRIGGER_HIGH:
> > > > > > +		irq_type |= IRQF_ONESHOT;
> > > > > > +		fallthrough;
> > > > > >  	case IRQF_TRIGGER_RISING:
> > > > > >  		irq_active_low = false;
> > > > > >  		break;
> > > > > >  	case IRQF_TRIGGER_LOW:
> > > > > > +		irq_type |= IRQF_ONESHOT;
> > > > > > +		fallthrough;
> > > > > >  	case IRQF_TRIGGER_FALLING:
> > > > > >  		irq_active_low = true;
> > > > > >  		break;
> > > > > > @@ -2520,10 +2538,9 @@ static int st_lsm6dsx_irq_setup(struct st_lsm6dsx_hw *hw)
> > > > > >  	}
> > > > > >  
> > > > > >  	err = devm_request_threaded_irq(hw->dev, hw->irq,
> > > > > > -					NULL,
> > > > > > +					st_lsm6dsx_handler_irq,
> > > > > >  					st_lsm6dsx_handler_thread,
> > > > > > -					irq_type | IRQF_ONESHOT,
> > > > > > -					"lsm6dsx", hw);
> > > > > > +					irq_type, "lsm6dsx", hw);
> > > > > >  	if (err) {
> > > > > >  		dev_err(hw->dev, "failed to request trigger irq %d\n",
> > > > > >  			hw->irq);    
> > > > >     
> > > >   
> > >   
> 

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