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

[Lorenzo Bianconi <lorenzo@kernel.org>]

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> On Sun, 8 Nov 2020 19:27:28 +0100
> Lorenzo Bianconi <lorenzo@kernel.org> wrote:
> 
[...]
> 
> So the thing I've been trying to say badly here is that I'm fairly sure the
> issue isn't what you think it is at all.  (Note I've spent a lot of
> time with scopes on interrupt lines looking for similar issues - it's
> not fun).
> 
> I think the actual condition here is that you have an interrupt that is not
> guaranteed to go low for long enough between being cleared and set.  Thus if you are
> read the fifo at almost exactly the moment new data is written you may in theory
> have the interrupt drop, but in practice analog electronics kicks in an you won't
> get an interrupt detected at all. This why the sensor needs to put guarantees
> on that drop time (some do - but I'm not seeing in datasheet for this one).
> On a more mundane note, I'm not sure in this case that there is a guarantee
> it will ever drop even in theory - this buffer could for this short period be
> filling faster than we drain it.

ack, very nice explanation :)

> 
> The reason your change makes this much less likely to happen is that, by checking
> again you are generally much closer to the time of the change of the level in
> the fifo.  Thus, unless you are preempted you should clear it long before it
> would be set again, and thus get a nice clean drop on the interrupt.
> 
> So for some asci art 

very nice :)

> 
> Previously we have
> 
> data samples       |       |       |
>                           _
> Read of fifo   ___________|_____ 
>                     _______ _____________
> interrupt line ____|       |              Interrupt stuck high as edge missed.
>                            ^       
>                            1       
> 
> With your fix
> 
> data samples       |       |       |
>                           _
> Read of fifo   ___________|__|__ 
>                     _______ __
> interrupt line ____|       |  |____|
>                            ^       ^
>                            1       2
> 
> So we would have missed 1, but because we check the fifo level again immediate
> after we would have made it drop, if we hit this unfortunately timing we will
> very quickly pull new data from the sensor and result in a drop well before the
> next interrupt comes in.

in the last case, even if we introduce a little bit of burstiness, I guess it
works because we read both 1 and 2, right?

> 
> 
> > 
> > @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?
> 
> Reenabling it in the thread should work as well.  It is a heavy weight solution
> but it is what you are expected to do in cases like this. 
> 
> I'd be very surprised if that doesn't work.  The normal operation of edge
> interrupt handlers is to reenable in the thread after we are sure we have
> cleared the condition for the original interrupt.  That will take long enough
> (as bus transaction involved) that the interrupt will definitely have dropped
> for long enough to be detected.

agree it should work

> 
> In some similar cases we've just concluded the right option is to not
> support edge interrupts. Do we know if we have boards out there that are using
> it in that mode and is there any chance they would support level interrupts
> as that's going to be a lot simpler and more reliable for this?

I do not know about it, I just received a report about the issue from stm folks.
I am fine to drop support for edge interrupts but do we have a similar issue for
st sensors (acc, magn, gyro) as well? Please consider:
https://elixir.bootlin.com/linux/latest/source/drivers/iio/common/st_sensors/st_sensors_trigger.c#L113

Regards,
Lorenzo

> 
> Jonathan
> > 
> > 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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