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SubjectRe: [RFC PATCH 2/3] iio: allow better control for flushing the hardware fifo
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@intel.com>
> ---
> 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@vger.kernel.org
> +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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