[gst-devel] gstV4L2src: Issue with blocking VIDIOC_QBUF

Stefan Kost ensonic at hora-obscura.de
Thu Aug 23 20:04:40 CEST 2007

Amal Prabhu wrote:
> Hi, 
> I created the following pipeline: 
> gstV4L2src ! tee ! queue ! videosink 

its v4l2src :)

> The videosink is an experimental one. 
> The gstV4L2src is launched with default configuration.
> This pipeline works like a viewfinder. However I am
> seeing some problem once the queue is introduced. 
> It seems that very frequently the queue accumulates a
> bunch of frames and then sends them to the videosink.
> The problem happens because the gst_pad_push() call
> seems to be blocked. 

Why are you using the queue? The v4l2src has a kind of internal queue (number of
enqueued buffers). Please retry without.

> The gst_pad_push() renders the buffer in the videosink
> and then unrefs the buffer. The unref call ends up
> calling VIDIOC_QBUF of the v4l2 camera driver. 
> At some point during data flow the gstv4l2src switches
> to a mode where it does a memcpy before sending out
> buffer downstream. This is when the number of buffers
> de-queued is equal to the total number of buffers
> allocated by the driver. 
> In this mode gstv4l2src immediately calls VIDIOC_QBUF.
> So the pipeline gets into a situation where two
> threads are calling VIDIOC_QBUF at the same time. 
> The problem I am seeing is that the VIDIOC_QBUF called
> by the unref in the videosink seems to block for a
> while. I see a multiple of the other VIDIOC_QBUF calls
> happening during this time. Due to this the buffers
> get pushed downstream, the queue gets full at some
> point and this unblocks the blocked VIDIOC_QBUF. Hence
> a bunch of the queued raw frames get displayed in a
> spurt. 

which version of gst-plugins-good are you using? Could you perhaps test the
latest (CVS). There have been some changes.

> Is this a problem with the implementation of the
> driver I am using? or is it a general V4L2 issue? Does
> it happen due to priorities of the two threads? 
It could be a driver issue. Unfortunately there is one detail not exactly
defined in the specs regarding DQBUF.


> Thank you
> Best Regards
> Amal
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