linux-kernel - Re: [PATCH v5 15/39] [media] v4l2: add a frame interval error event

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Date:   Sat, 11 Mar 2017 00:30:09 +0100
From:   Pavel Machek <pavel@....cz>
To:     Steve Longerbeam <slongerbeam@...il.com>
Cc:     Hans Verkuil <hverkuil@...all.nl>, robh+dt@...nel.org,
        mark.rutland@....com, shawnguo@...nel.org, kernel@...gutronix.de,
        fabio.estevam@....com, linux@...linux.org.uk, mchehab@...nel.org,
        nick@...anahar.org, markus.heiser@...marIT.de,
        p.zabel@...gutronix.de, laurent.pinchart+renesas@...asonboard.com,
        bparrot@...com, geert@...ux-m68k.org, arnd@...db.de,
        sudipm.mukherjee@...il.com, minghsiu.tsai@...iatek.com,
        tiffany.lin@...iatek.com, jean-christophe.trotin@...com,
        horms+renesas@...ge.net.au, niklas.soderlund+renesas@...natech.se,
        robert.jarzmik@...e.fr, songjun.wu@...rochip.com,
        andrew-ct.chen@...iatek.com, gregkh@...uxfoundation.org,
        shuah@...nel.org, sakari.ailus@...ux.intel.com,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-media@...r.kernel.org,
        devel@...verdev.osuosl.org
Subject: Re: [PATCH v5 15/39] [media] v4l2: add a frame interval error event

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?
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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