On 03/10/2017 03:30 PM, Pavel Machek wrote:
On Fri 2017-03-10 10:37:21, Steve Longerbeam wrote:
Hi Hans,
On 03/10/2017 04:03 AM, Hans Verkuil wrote:
On 10/03/17 05:52, Steve Longerbeam wrote:
Add a new FRAME_INTERVAL_ERROR event to signal that a video capture or
output device has measured an interval between the reception or transmit
completion of two consecutive frames of video that is outside the nominal
frame interval by some tolerance value.
Reading back what was said on this I agree with Sakari that this doesn't
belong here.
Userspace can detect this just as easily (if not easier) with a timeout.
Unfortunately measuring frame intervals from userland is not accurate
enough for i.MX6.
The issue here is that the IPUv3, specifically the CSI unit, can
permanently lose vertical sync if there are truncated frames sent
on the bt.656 bus. We have seen a single missing line of video cause
loss of vertical sync. The only way to correct this is to shutdown
the IPU capture hardware and restart, which can be accomplished
simply by restarting streaming from userland.
There are no other indicators from the sensor about these short
frame events (believe me, we've exhausted all avenues with the ADV718x).
And the IPUv3 DMA engine has no status indicators for short frames
either. So the only way to detect them is by measuring frame intervals.
The intervals have to be able to resolve a single line of missing video.
With a PAL video source that requires better than 58 usec accuracy.
There is too much uncertainty to resolve this at user level. The
driver is able to resolve this by measuring intervals between hardware
interrupts as long as interrupt latency is reasonably low, and we
have another method using the i.MX6 hardware input capture support
that can measure these intervals very accurately with no errors
introduced by interrupt latency.
Requiring < 58 usec interrupt latency for correct operation is a
little too optimistic, no?
No it's not too optimistic, from experience the imx6 kernel has irq
latency less than 10 usec under normal system load. False events can be
generated if the latency gets bad, it's true, and that's why there is
the imx6 timer input capture approach.
Steve
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