Re: Questions: iio: accel: kionix-kx022a: timestamp when using the data-rdy trigger?

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Fri, 17 Feb 2023 13:59:16 +0200
Matti Vaittinen <mazziesaccount@gmail.com> wrote:

> On 2/17/23 13:43, Jonathan Cameron wrote:
> > On Fri, 17 Feb 2023 07:56:22 +0200
> > Matti Vaittinen <mazziesaccount@gmail.com> wrote:
> >   
> >> Hi Mehdi,
> >>
> >> On 2/16/23 22:22, Mehdi Djait wrote: >>> And here are the relevant steps after an IRQ occurs :  
> >>> 1. IRQ context --> kx022a_irq_handler() --> gets the current timestamp
> >>> with "data->timestamp = iio_get_time_ns(idev);" and returns
> >>> IRQ_WAKE_THREAD
> >>>
> >>> 2. kx022a_irq_thread_handler() -> checks that the trigger is enabled  
> >>> --> iio_trigger_poll_chained() --> handle_nested_irq(): which will only  
> >>> call the bottom half of the pollfuncs  
> >>
> >> I don't get the kx022a at my hands until next week to test this, but it
> >> seems to me your reasoning is right. iio_pollfunc_store_time() is
> >> probably not called. I just wonder why I didn't see zero timestamps when
> >> testing this. (OTOH, I had somewhat peculiar IRQ handling at first -
> >> maybe I broke this along the way).  
> > 
> > This is a common problem.  So far we've always solved it in the driver
> > by using the pf->timestamp only if it's been set - otherwise fallback
> > to grabbing a new one to pass into iio_push_to_buffer_with_timestamp()
> > in the threaded handler.
> > 
> > It might be possible to solve in a generic fashion but it's a bit
> > fiddly so I don't think anyone has ever looked at it.  
> 
> I agree it's "fiddly" :) I played with a though of conditionally adding 
> the timestamp in the iio_trigger_poll_chained() if the timestamp is zero 
> there. This, however, would require clearing the timestamp when it is 
> read - which gets "fiddly" soon. Hence I just suggested adding a note in 
> kerneldoc.
> 
> >>  
> >>> Question 2: If the change proposed in question 1 is wrong, would this
> >>> one be better iio_push_to_buffers_with_timestamp(idev, data->buffer,
> >>> iio_get_time_ns(idev)). There is some delay between the IRQ occuring
> >>> and trigger_handler being called but that is better than getting all 0
> >>> timestamps like suggested in [2]  
> >>
> >> Please, use the data->timestamp as you suggested.  
> > 
> > I'd suggest a bit of both.  If you have a timestamp from the irq handler
> > use it. If it's not available then grab one locally in the threaded handler.  
> 
> Hm. I don't think we will end up in the kx022a threaded handler so that 
> the data->timestamp is not populated in the IRQ handler. I am _far_ from 
> an IIO expert - but I guess the only way would be that some other 
> trigger invoked the threaded handler(?) Shouldn't the 
> kx022a_validate_trigger() prevent this?
Ah.  I'd missed this one restricted what triggers could be used.
We'll have to pay attention to this if that particular condition is ever
relaxed.
> 
> Please, follow Jonathan's guidance if he does not tell othervice. You 
> clearly should not trust a random guy who obviously does not know how to 
> write these drivers in the first place XD

You were right here :)

> 
> >>  
> >>> I hope that I'm understating this correctly or at least not totally
> >>> off :) If yes, I will send a patch.  
> >>
> >> Thanks Mehdi! I think this was a great catch! Maybe - while at it - you
> >> could also send a patch adding a small kerneldoc to the
> >> iio_trigger_poll_chained() mentioning this particular issue. Yes, I
> >> guess it should be obvious just by reading the function name *_chained()
> >> - but I did fall on this trap (and according to your reference [2] so
> >> has someone else).
> >>  
> >>> [1] https://lore.kernel.org/linux-iio/4FDB33CD.2090805@metafoo.de/
> >>> [2] https://lore.kernel.org/linux-iio/20201205182659.7cd23d5b@archlinux/
> >>> [3] https://lore.kernel.org/linux-iio/20220126191606.00003f37@Huawei.com/  
> >>
> >> Yours,
> >> 	-- Matti
> >>  
> >   
>