Re: [RFC PATCH 2/3] iio: allow better control for flushing the hardware fifo

[Lars-Peter Clausen]

On 05/03/2015 08:11 AM, Octavian Purdila wrote:
> On Sat, May 2, 2015 at 8:42 PM, Lars-Peter Clausen <lars@xxxxxxxxxx> wrote:
> > On 04/29/2015 01:18 PM, Octavian Purdila wrote:
> > > Some applications need to be able to flush [1] the hardware fifo of
> > > the device and to receive events of when that happened [2] so that it
> > > can ignore stale data.
> > >
> > > This patch adds a new event (IIO_EV_TYPE_HWFIFO_FLUSHED) that should
> > > be sent to userspace when a flush has been completed. The application
> > > will be able to identify which are the samples to ignore based on the
> > > timestamp of the event.
> > >
> > > To allow applications to accurately generate a hardware fifo flush on
> > > demand, this patch also adds a new sysfs entry that triggers a
> > > hardware fifo flush when written to.
> > >
> > > [1]
> > > https://source.android.com/devices/sensors/hal-interface.html#flush_sensor
> > > [2]
> > > https://source.android.com/devices/sensors/hal-interface.html#metadata_flush_complete_events
> >
> >
> > Since there is no asynchronous queue for commands to be executed in IIO
> > adding a asynchronous completion event doesn't make too much sense. This is
> > something that needs to be handled at the HAL level.
> >
> > The HAL needs to have a queue of commands that need to be executed where new
> > events can be added asynchronously, then has a loop which goes through the
> > commands in the queue and executes them, and once executed generated the
> > appropriate completion event.
>
> Hi Lars,
>
> Thanks for the review.
>
> We can't do this at the HAL level because the needed information is
> only available at the HAL level. At the HAL level each received sample
> from the driver is converted to an event. When doing a flush the HAL
> must add a special event (flush complete) after the last sample in the
> hardware fifo. But the HAL does not know how many samples are in the
> hardware fifo, how many are in the device buffer, etc.

Ok, so that's what you need the timestamp for I presume? So the signature of 
the of the sync function is basically.

timestamp sync(device)

where timestamp is greater or equal to the timestamp of all samples before 
the sync and smaller or equal to all samples after the sync.

What your implementation does is implement a synchronous API to flush the 
FIFO but delivers the result of the operation asynchronously via a rather 
arbitrary delivery mechanism. That is in my opinion not a good API/ABI and 
might even have some race condition issues.

If you do a flush, then read as much data as available you know that this 
data is from before the flush and any data read at a later point is after 
the flush.

> > I really wish that document would specify what is actually meant by flush.
> > Copy the FIFO content to a software buffer or discard the FIFO content.
>
> It does say: "... and flushes the FIFO; those events are delivered as
> usual (i.e.: as if the maximum reporting latency had expired) ..."

Missed that, thanks.

> > > Signed-off-by: Octavian Purdila <octavian.purdila@xxxxxxxxx>
> > > ---
> > >    Documentation/ABI/testing/sysfs-bus-iio | 11 +++++++++++
> > >    include/linux/iio/sysfs.h               |  3 +++
> > >    include/uapi/linux/iio/types.h          |  1 +
> > >    3 files changed, 15 insertions(+)
> > >
> > > diff --git a/Documentation/ABI/testing/sysfs-bus-iio
> > > b/Documentation/ABI/testing/sysfs-bus-iio
> > > index 866b4ec..bb4d8de 100644
> > > --- a/Documentation/ABI/testing/sysfs-bus-iio
> > > +++ b/Documentation/ABI/testing/sysfs-bus-iio
> > > @@ -1375,3 +1375,14 @@ Description:
> > >                  The emissivity ratio of the surface in the field of view
> > > of the
> > >                  contactless temperature sensor.  Emissivity varies from 0
> > > to 1,
> > >                  with 1 being the emissivity of a black body.
> > > +
> > > +What:          /sys/bus/iio/devices/iio:deviceX/buffer/hwfifo_flush
> > > +KernelVersion: 4.2
> > > +Contact:       linux-iio@xxxxxxxxxxxxxxx
> > > +Description:
> > > +               Write only entry that accepts a single strictly positive
> > > integer
> > > +               specifying the number of samples to flush from the
> > > hardware fifo
> > > +               to the device buffer. When the flush is completed an
> > > +               IIO_EV_TYPE_HWFIFO_FLUSHED event is generated. The event
> > > has the
> > > +               timestamp equal with the timestamp of last sample that was
> > > +               flushed from the hardware fifo.
> >
> >
> > I'd prefer this to be handled through the normal read() API rather than
> > having a side channel for it. Big question is how though. We could specify
> > that reading in O_NONBLOCK mode will always read data if it is available and
> > not only if it is above the watermark threshold.
>
> Do you mean to try and flush when the available data in the device
> buffer is less then the requested size? That should work and hopefully
> the ABI change does not matter since the hwfifo stuff has not been
> released yet.

Basically only let poll() and blocking read() block when not enough data is 
available. But for non-blocking read return as much data as possible if data 
is available.

> I prefer the explicit flush though. I think it is better to have the
> ABIs clearly visible instead of being buried in the details.

Yea, it's a bit tricky. What should be used for this sysfs, IOCTL, read()...

- Lars
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