[Interest] video flickering during play with qmlvideo example on eglfs

[Mauro Salvini]

Hi,

I already posted this message to QT forums but without a response, so
I'm trying here. Sorry if this is an already-answered question but I
found nothing similar.

I'm developing an embedded board based on iMX6SX SOC (that does not
have VPU) with a 7" integrated LCD.

On this board I sucessfully configured, built and installed a minimal
Linux image using Yocto project at rocko release, without graphical
server (X11 nor wayland) but directly on framebuffer, with the OpenGL
ES support.
This image integrates also meta-qt5 layer (version 5.9.6 at last commit
on rocko branch) to develop applications using eglfs platform.

On this board I would play simple short videos from a qml application,
so as a first step I tried the qmlvideo example, but playing a sample
video (h264 software-decoded by libav due to lack of a VPU, 720x480,
25fps) I have a heavy flickering: it seems that video went decoded
well, but went blanked by some black frames, generating a
"stroboscopic" effect with blanking frequency changing between each
reproduction (I can provide a video if needed). Framerate reported by
qmlvideo during reproduction is right (approx. 26fps).

If I reporduce the same video using GStreamer from command-line, video
is shown right (again I can provide a video).

CPU load is approximately the same both with qmlvideo and gstreamer,
and stay low (<20%, due to sw decoding).

I already tried to set QT_EGLFS_IMX6_NO_FB_MULTI_BUFFER=1 to disable
multi-buffering, but without result.
If I set GST_DEBUG=2, logs regarding video stream are quite the same
both in qmlvideo and gstreamer, and without errors.

What could be the problem? Could the problem be on the NXP side of the
SW chain (something about OpenGL, gstreamer, GPU driver or other)? I
googled for a solution, but I haven't found anything.

Thanks in advance, regards.

Mauro