From: Jonathan Cameron <jic23@kernel.org>
To: Lorenzo Bianconi <lorenzo@kernel.org>
Cc: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>,
Jonathan Cameron <Jonathan.Cameron@huawei.com>,
linux-iio@vger.kernel.org, mario.tesi@st.com,
denis.ciocca@st.com, armando.visconti@st.com
Subject: Re: [PATCH] iio: imu: st_lsm6dsx: fix edge-trigger interrupts
Date: Sat, 14 Nov 2020 17:31:00 +0000 [thread overview]
Message-ID: <20201114173100.0d6ce33e@archlinux> (raw)
In-Reply-To: <20201114164840.GA3993@lore-desk>
On Sat, 14 Nov 2020 17:48:40 +0100
Lorenzo Bianconi <lorenzo@kernel.org> wrote:
> > 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?
We should always be fine, because the extra check must take a bit of time. Either
the event happens after that time (in which case the interrupt will have been low
long enough) or it doesn't and we will catch it.
>
> >
> >
> > >
> > > @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
It was a part now supported by that driver that I hit this issue on
years ago. As a side note, there is a bug in there though, be it one we
probably can't hit? stat_drdy has to be defined, if not the while loop will get
a negative back (which is true) and loop for ever.
https://elixir.bootlin.com/linux/latest/source/drivers/iio/common/st_sensors/st_sensors_trigger.c#L36
Probably want's to return 0 but print an error message. Whilst there even better
if that function just returns a boolean so we cant accidentally put such a bug
back in again in future.
Lets go with your fix, but perhaps we should add a note to the dt binding to
say level interrupts preferred? Saving a check or two in the common case is
definitely beneficial if the host supports level interrupts.
If you can do a v3 with updated explanation and comments that would be great.
Thanks,
Jonathan
>
> 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);
> > > > > > > >
> > > > > > >
> > > > > >
> > > >
> >
next prev parent reply other threads:[~2020-11-14 17:31 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-22 9:26 [PATCH] iio: imu: st_lsm6dsx: fix edge-trigger interrupts Lorenzo Bianconi
2020-11-01 16:33 ` Jonathan Cameron
2020-11-02 10:15 ` Lorenzo Bianconi
2020-11-02 17:44 ` Jonathan Cameron
2020-11-02 18:18 ` Lorenzo Bianconi
2020-11-08 16:49 ` Jonathan Cameron
2020-11-08 18:27 ` Lorenzo Bianconi
2020-11-14 15:06 ` Jonathan Cameron
2020-11-14 16:48 ` Lorenzo Bianconi
2020-11-14 17:31 ` Jonathan Cameron [this message]
2020-11-14 17:58 ` Lorenzo Bianconi
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