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

Sakari Ailus sakari.ailus at iki.fi
Thu Mar 16 15:15:35 PDT 2017

Hi Steve,

On Tue, Mar 14, 2017 at 09:43:09AM -0700, Steve Longerbeam wrote:
> On 03/14/2017 09:21 AM, Nicolas Dufresne wrote:
> >Le lundi 13 mars 2017 à 10:45 +0000, Russell King - ARM Linux a écrit :
> >>On Mon, Mar 13, 2017 at 11:02:34AM +0100, Hans Verkuil wrote:
> >>>On 03/11/2017 07:14 PM, Steve Longerbeam wrote:
> >>>>The event must be user visible, otherwise the user has no indication
> >>>>the error, and can't correct it by stream restart.
> >>>In that case the driver can detect this and call vb2_queue_error. It's
> >>>what it is there for.
> >>>
> >>>The event doesn't help you since only this driver has this issue. So nobody
> >>>will watch this event, unless it is sw specifically written for this SoC.
> >>>
> >>>Much better to call vb2_queue_error to signal a fatal error (which this
> >>>apparently is) since there are more drivers that do this, and vivid supports
> >>>triggering this condition as well.
> >>So today, I can fiddle around with the IMX219 registers to help gain
> >>an understanding of how this sensor works.  Several of the registers
> >>(such as the PLL setup [*]) require me to disable streaming on the
> >>sensor while changing them.
> >>
> >>This is something I've done many times while testing various ideas,
> >>and is my primary way of figuring out and testing such things.
> >>
> >>Whenever I resume streaming (provided I've let the sensor stop
> >>streaming at a frame boundary) it resumes as if nothing happened.  If I
> >>stop the sensor mid-frame, then I get the rolling issue that Steve
> >>reports, but once the top of the frame becomes aligned with the top of
> >>the capture, everything then becomes stable again as if nothing happened.
> >>
> >>The side effect of what you're proposing is that when I disable streaming
> >>at the sensor by poking at its registers, rather than the capture just
> >>stopping, an error is going to be delivered to gstreamer, and gstreamer
> >>is going to exit, taking the entire capture process down.
> >Indeed, there is no recovery attempt in GStreamer code, and it's hard
> >for an higher level programs to handle this. Nothing prevents from
> >adding something of course, but the errors are really un-specific, so
> >it would be something pretty blind. For what it has been tested, this
> >case was never met, usually the error is triggered by a USB camera
> >being un-plugged, a driver failure or even a firmware crash. Most of
> >the time, this is not recoverable.
> >
> >My main concern here based on what I'm reading, is that this driver is
> >not even able to notice immediately that a produced frame was corrupted
> >(because it's out of sync). From usability perspective, this is really
> >bad.
> First, this is an isolated problem, specific to bt.656 and it only
> occurs when disrupting the analog video source signal in some
> way (by unplugging the RCA cable from the ADV718x connector
> for example).
> Second, there is no DMA status support in i.MX6 to catch these
> shifted bt.656 codes, and the ADV718x does not provide any
> status indicators of this event either. So monitoring frame intervals
> is the only solution available, until FSL/NXP issues a new silicon rev.
> >  Can't the driver derive a clock from some irq and calculate for
> >each frame if the timing was correct ?
> That's what is being done, essentially.
> >  And if not mark the buffer with
> I prefer to keep the private event, V4L2_BUF_FLAG_ERROR is too
> unspecific.

Is the reason you prefer an event that you have multiple drivers involved,
or that the error flag is, well, only telling there was an error with a
particular frame?

Returning -EIO (by calling vb2_queue_error()) would be a better choice as it
is documented behaviour.


Sakari Ailus
e-mail: sakari.ailus at iki.fi	XMPP: sailus at retiisi.org.uk

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