> On Sun, 8 Nov 2020 19:27:28 +0100 > Lorenzo Bianconi <lorenzo@xxxxxxxxxx> 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@xxxxxxxxxx> > > > > > > > > --- > > > > > > > > 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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