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

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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.

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.


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.
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