Re: [PATCH v1 1/1] iio: adc: tsc2046: rework the trigger state machine

[Oleksij Rempel <o.rempel@pengutronix.de>]

On Sun, Jan 09, 2022 at 03:44:04PM +0000, Jonathan Cameron wrote:
> On Fri,  7 Jan 2022 08:40:17 +0100
> Oleksij Rempel <o.rempel@pengutronix.de> wrote:
> 
> > Initially this was designed to:
> > | Fix sleeping in atomic context warning and a deadlock after iio_trigger_poll()
> > | call
> > |
> > | If iio_trigger_poll() is called after IRQ was disabled, we will call
> > | reenable_trigger() directly from hard IRQ or hrtimer context instead of
> > | IRQ thread. In this case we will run in to multiple issue as sleeping in atomic
> > | context and a deadlock.
> > |
> > | To avoid this issue, rework the trigger to use state machine. All state
> > | changes are done over the hrtimer, so it allows us to drop fsleep() and
> > | avoid the deadlock.
> > 
> > This issue was fixed by: 9020ef659885 ("iio: trigger: Fix a scheduling
> > whilst atomic issue seen on tsc2046").
> > 
> > Even if the root cause of this issue probably will and can be fixed in the iio
> > core, this patch can be seen as clean-up to provide better internal state
> > machine.
> 
> Probably want to update this text?

ack

> A few comments below.
> > 
> > Fixes: 9374e8f5a38d ("iio: adc: add ADC driver for the TI TSC2046 controller")
> 
> From above isn't this now fixed?  The cleanup here is just making things easier
> to follow I think...

done

> > Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
> > ---
> >  drivers/iio/adc/ti-tsc2046.c | 102 ++++++++++++++++++++---------------
> >  1 file changed, 58 insertions(+), 44 deletions(-)
> > 
> > diff --git a/drivers/iio/adc/ti-tsc2046.c b/drivers/iio/adc/ti-tsc2046.c
> > index d84ae6b008c1..91f6bd5effe7 100644
> > --- a/drivers/iio/adc/ti-tsc2046.c
> > +++ b/drivers/iio/adc/ti-tsc2046.c
> > @@ -123,14 +123,21 @@ struct tsc2046_adc_ch_cfg {
> >  	unsigned int oversampling_ratio;
> >  };
> >  
> > +enum tsc2046_state {
> > +	TSC2046_STATE_STANDBY,
> > +	TSC2046_STATE_ENABLE_IRQ_POLL,
> > +	TSC2046_STATE_POLL,
> > +	TSC2046_STATE_ENABLE_IRQ,
> > +};
> > +
> >  struct tsc2046_adc_priv {
> >  	struct spi_device *spi;
> >  	const struct tsc2046_adc_dcfg *dcfg;
> >  
> >  	struct iio_trigger *trig;
> >  	struct hrtimer trig_timer;
> > -	spinlock_t trig_lock;
> > -	unsigned int trig_more_count;
> > +	enum tsc2046_state state;
> > +	spinlock_t state_lock;
> >  
> >  	struct spi_transfer xfer;
> >  	struct spi_message msg;
> > @@ -411,21 +418,47 @@ static const struct iio_info tsc2046_adc_info = {
> >  	.update_scan_mode = tsc2046_adc_update_scan_mode,
> >  };
> >  
> > -static enum hrtimer_restart tsc2046_adc_trig_more(struct hrtimer *hrtimer)
> > +static enum hrtimer_restart tsc2046_adc_timer(struct hrtimer *hrtimer)
> >  {
> >  	struct tsc2046_adc_priv *priv = container_of(hrtimer,
> >  						     struct tsc2046_adc_priv,
> >  						     trig_timer);
> >  	unsigned long flags;
> >  
> > -	spin_lock_irqsave(&priv->trig_lock, flags);
> > -
> > -	disable_irq_nosync(priv->spi->irq);
> > -
> > -	priv->trig_more_count++;
> > -	iio_trigger_poll(priv->trig);
> > -
> > -	spin_unlock_irqrestore(&priv->trig_lock, flags);
> > +	spin_lock_irqsave(&priv->state_lock, flags);
> > +	switch (priv->state) {
> > +	case TSC2046_STATE_ENABLE_IRQ_POLL:
> > +		/*
> > +		 * IRQ handler called iio_trigger_poll() to sample ADC.
> > +		 * Here we
> > +		 * - re-enable IRQs
> > +		 * - start hrtimer for timeout if no IRQ will occur
> > +		 */
> > +		priv->state = TSC2046_STATE_POLL;
> > +		enable_irq(priv->spi->irq);
> 
> I comment on this below, but I'm not sure why you don't move the enable_irq()
> here out of this timer function and then have the first entry of the timer
> go directly to TSC2046_STATE_POLL after a longer initial wait.

Hm... yes. You are right.

> It's been a long time since I looked at this, so perhaps I'm missing the
> point.  What you have here works as far as I can see, it just seems to push
> more than necessary into the state machine.

The IRQ line is a level shifter connected to one of channels muxed to the core
ADC. If we switch internal muxer to different channel, the IRQ line will
change the state.

So, we need a trigger which:
- do not triggers if we do ADC readings.
- keeps triggering as long as we have some state changes on the IRQ line
- trigger only with specific rate
- still triggers for some amount of time after last interrupt event was
  detected. Current implementation is doing only one extra read.

I reworked it a bit, will send new patch soon.

> > +		hrtimer_start(&priv->trig_timer,
> > +			      ns_to_ktime(priv->scan_interval_us *
> > +					  NSEC_PER_USEC),
> > +			      HRTIMER_MODE_REL_SOFT);
> > +		break;


> > -
> > -	spin_unlock_irqrestore(&priv->trig_lock, flags);
> > +	tim = ns_to_ktime((priv->scan_interval_us - priv->time_per_scan_us) *
> > +			  NSEC_PER_USEC);
> > +	hrtimer_start(&priv->trig_timer, tim, HRTIMER_MODE_REL_SOFT);
> 
> This moves enabling the irq to the first instance of the timer - is that ever too late?

no, we should disable IRQ if we do readings anyway.

> >  }
> >  
> >  static int tsc2046_adc_set_trigger_state(struct iio_trigger *trig, bool enable)
> > @@ -493,8 +506,8 @@ static int tsc2046_adc_set_trigger_state(struct iio_trigger *trig, bool enable)
> >  	if (enable) {
> >  		enable_irq(priv->spi->irq);
> >  	} else {
> > +		hrtimer_cancel(&priv->trig_timer);
> 
> So this will wait for the callback to finish.  However, is there a chance
> of an interrupt just after this but before disable_irq that ends up
> starting the timer again?

Yes, you are right. I'll rework it.

Regards,
Oleksij
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