On 08/05/14 17:17, Srinivas Pandruvada wrote:
On 05/07/2014 01:03 PM, Jonathan Cameron wrote:
On May 6, 2014 5:56:00 PM GMT+01:00, Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxxxxxx> wrote:
Hi Jonathan,
The Android user space has some capability to ask the supported
hardware
to enable buffering in hardware.
I don't think that we can achieve this by current ABI. Do you want me
to propose new ABI?
This is closely related to watershed events on buffers, both software and hardware. We
had these back in the early days but the interface was fiddly. It used a couple of iio
events to tell user space the watershed was passed.
One suggestion from Arnd Bergmann was to use one of the less commonly used poll
types to indicate this to user space. It was in a long system wide review he did not long
after we entered staging. Looked like a neat idea as could coexist nicely with existing
interfaces on the same buffer. Would definitely require a fair bit of documentation.
Thread in question is around about:
https://lkml.org/lkml/2011/3/16/190
Also note we already have hardware buffered devices pushing into software
buffers (without a trigger) which effectively handle the same use case using existing interfaces.
See the ti_am335x_adc driver.
There is definitely room for something more controllable but it shouldn't be too focused
on hardware buffering as makes sense for software buffers too!
So to take a stab in the air we need some means of setting the watershed level
(and a callback to pass this on to the hardware if that makes sense).
I think so. We need a watermark level and an event. We can use poll flags to allow prioritized event.
Yes. That is pretty much it.
The fiddly cases are going to be the corner cases such as when the length changes.
what do you think?
"
Android batch mode:
batch(int handle, int flags, int64_t period_ns, int64_t
max_report_latency)
Enabling batch mode for a given sensor sets the delay between events.
max_report_latency sets the maximum time by which events can be delayed
and batched together before being reported to the applications. A value
of zero disables batch mode for the given sensor. The period_ns
parameter is equivalent to calling setDelay() -- this function both
enables or disables the batch mode AND sets the event's period in
nanoseconds. See setDelay() for a detailed explanation of the period_ns
parameter.
Hmm. Max latency would just be a timeout on the poll. Period is a trigger characteristic
or a hardware one if no explicit trigger is present.
Correct. But this value can be used to infer the watermark level.
True enough. That can be done in userspace though rather than necessarily
in the kernel interface.
There is clearly ad
In non-batch mode, all sensor events must be reported as soon as they
are detected. For example, an accelerometer activated at 50Hz will
trigger interrupts 50 times per second.
While in batch mode, sensor events do not need to be reported as soon
as
they are detected. They can be temporarily stored and reported in
batches, as long as no event is delayed by more than
maxReportingLatency
nanoseconds. That is, all events since the previous batch are recorded
and returned at once. This reduces the amount of interrupts sent to the
SoC and allows the SoC to switch to a lower power mode (idle) while the
sensor is capturing and batching data.
setDelay() is not affected and it behaves as usual.
Each event has a timestamp associated with it. The timestamp must be
accurate and correspond to the time at which the event physically
happened.
Batching does not modify the behavior of poll(): batches from different
sensors can be interleaved and split. As usual, all events from the
same
sensor are time-ordered.
"
I will do some experiments with one device which I will get with a large Fifo.
Cool. There quite a few simple ADCs that support up to 16 element buffers but
we have never put any effort into supporting this. Will be interesting to see
if there are advantages in that case as well.
Looking forward to seeing how you get on with this. There was a small
surge in devices with reasonable sized buffers about 4 years ago, then they
disappeared, but seem to be making a come back. Also odd cases such as the
vibration sensors that we could handle better. These tend to use some
threshold type event to trigger very high rate sampling for a short period.
Anyhow, lots of things to consider! Good luck.
Thanks,
Srinivas
Thanks,
Srinivas
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