MediaPlayer VideoOutput SIGSEGV gst_v4l2_object_fill_format_list

petero3

With QML MediaPlayer and VideoOutput
and
player.source = "gstreamer-pipeline: v4l2src device=/dev/video2 ! capsfilter caps=video/x-raw,width=720,height=480,framerate=25/1"

We are getting an intermittent SIGSEGV in
gst_v4l2_object_fill_format_list
in /usr/src/debug/gstreamer1.0-plugins-good/1.24.0.imx/sys/v4l2/gstv4l2object.c

It either fails early, or works and keeps working.

By comparison, the following always works:
player.source = "gstreamer-pipeline: videotestsrc ! capsfilter caps=video/x-raw,width=720,height=480,framerate=25/1"

This is on a custom board/custom Yocto
NXP i.MX 8X
ADV7280A-M

JoeCFD

try from command line:
gst-launch-1.0 v4l2src device=/dev/video2 ! capsfilter caps=video/x-raw,width=720,height=480,framerate=25/1 | autovideosink
or
gst-launch-1.0 v4l2src device=/dev/video2 ! capsfilter caps=video/x-raw,width=720,height=480,framerate=25/1 | waylandsink
to see if your pipeline works or not at first.

If not, show the output here.
gst-launch-1.0 -v v4l2src device=/dev/video2 ! capsfilter caps=video/x-raw,width=720,height=480,framerate=25/1 | autovideosink

petero3

Hi @JoeCFD thank you for replying. Yes pipeline works from the command line each time:

gst-launch-1.0 v4l2src device=/dev/video2 ! capsfilter caps=video/x-raw,width=720,height=480,framerate=25/1 ! autovideosink

JoeCFD

@petero3 What is your Qt version?

petero3

@JoeCFD Qt 6.8.3

JoeCFD

@petero3 I guess Qt6 might have removed this feature of running a gstreamer pipeline. This feature is only available in Qt5.

SGaist

@JoeCFD that feature was removed.
Also, unless explicitly selected, the default backend uses ffmpeg.

petero3

@JoeCFD @SGaist

IIRC gstreamer pipelines were removed in the first Qt 6 versions, but are present in later ones.

The following works fine with Qt 6.8.3:

player.source = "gstreamer-pipeline: videotestsrc ! capsfilter caps=video/x-raw,width=720,height=480,framerate=25/1"

As long as source is set dynamically onCompleted as per:
https://bugreports.qt.io/browse/QTBUG-134554

and:

player.source = "gstreamer-pipeline: v4l2src device=/dev/video2 ! capsfilter caps=video/x-raw,width=720,height=480,framerate=25/1"

does not always fail. If gst_v4l2_object_fill_format_list is going to segfault, it does so early. If it doesn't segfault, then it works.

So present, but flaky for some reason.

We are also trying qml6glsink, but so far that just gives a white screen on our device.

JoeCFD

@petero3 What env settings in main() you have?
Show me the code how qml sink is applied.

petero3

@JoeCFD thank you.
Below is the code for the MediaPlayer/VideoOutput with the intermittent segfault.
Will ask my colleague to share the code for the qml6glsink
We also have this gstreamer post re that:
https://discourse.gstreamer.org/t/qmlsink-example-on-imx8qxp-board-doesnt-render-videotstsrc-only-white-screen/4579

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QFile>

int main(int argc, char *argv[])
{   
    qputenv("GST_DEBUG", "4");
    qputenv("GST_DEBUG_DUMP_DOT_DIR", "/appdata/");
    qputenv("QT_LOGGING_RULES", "*.multimedia.*=true");
      
    QGuiApplication app(argc, argv);
    
    QQmlApplicationEngine engine;
    QObject::connect(
        &engine, &QQmlApplicationEngine::objectCreationFailed, &app,
        []() { QCoreApplication::exit(-1); }, Qt::QueuedConnection);
    engine.loadFromModule("video", "Main");
    
    return app.exec();
}

import QtQuick
import QtMultimedia

Window {
    width: 1280
    height: 768
    visible: true
    title: qsTr("Hello World")

    Item {
        anchors.fill: parent

        MediaPlayer {
            id: player
            videoOutput: videoOutput
        }

        VideoOutput {
            id: videoOutput
            anchors.fill: parent
        }

        Component.onCompleted: {
            // works
            //player.source = "gstreamer-pipeline: videotestsrc ! capsfilter caps=video/x-raw,width=720,height=480,framerate=25/1"

            // segfaults intermittently
            player.source = "gstreamer-pipeline: v4l2src device=/dev/video2 ! capsfilter caps=video/x-raw,width=720,height=480,framerate=25/1"

            player.play()
        }
    }
}

petero3

PS. For the qml6glsink white screen issue, the code is the standard qmlsink example:
https://gitlab.freedesktop.org/gstreamer/gstreamer/-/tree/main/subprojects/gst-plugins-good/tests/examples/qt6/qmlsink?ref_type=heads
https://discourse.gstreamer.org/t/qmlsink-example-on-imx8qxp-board-doesnt-render-videotstsrc-only-white-screen/4579

JoeCFD

@petero3 does your code work on Ubuntu or Redhat?
Is wayland applied in Yocto? If yes, the following envs are needed.

#ifdef YOCTO
    setenv( "DISPLAY",         ":0",      1 );
    setenv( "GDK_BACKEND",     "wayland", 1 );
    setenv( "QT_QPA_PLATFORM", "wayland", 1 );

    setenv( "XDG_RUNTIME_DIR", "/run/user/1000",            1 );
    setenv( "WAYLAND_DISPLAY", "wayland-1",                 1 );
    setenv( "QT_QPA_FONTDIR",  "/usr/share/fonts/truetype", 1 );
#endif

petero3

In case anyone encounters the same issue, what worked for us is to use:
https://gitlab.freedesktop.org/gstreamer/gstreamer/-/blob/main/subprojects/gst-plugins-good/tests/examples/qt6/qmlsink/main.cpp
but change:
QQuickWindow::BeforeSynchronizingStage to QQuickWindow::NoStage
Not sure why that was necessary, but that's what worked.
https://discourse.gstreamer.org/t/qmlsink-example-on-imx8qxp-board-doesnt-render-videotstsrc-only-white-screen/4579/5

petero3

With
player.source = "gstreamer-pipeline:v4l2src device=/dev/video4 ! capsfilter caps=video/x-raw,width=720,height=480,framerate=30/1"

Segfault
1 format_cmp_func gstv4l2object.c 1328 0xffffcf22524c
2 g_slist_insert_sorted_real gslist.c 862 0xfffff6143070
3 g_slist_insert_sorted gslist.c 913 0xfffff6143a5c
4 gst_v4l2_object_fill_format_list gstv4l2object.c 1376 0xffffcf2243a4
5 gst_v4l2_object_get_format_list gstv4l2object.c 1425 0xffffcf224874
6 gst_v4l2_object_probe_caps gstv4l2object.c 5446 0xffffcf230414
7 gst_v4l2_object_get_caps gstv4l2object.c 5635 0xffffcf230a74
8 gst_base_src_default_query gstbasesrc.c 1373 0xffffe16fae8c
9 gst_pad_query gstpad.c 4252 0xffffe15c0d28
10 gst_pad_query_caps gstutils.c 3117 0xffffe160af8c
11 gst_v4l2src_negotiate gstv4l2src.c 788 0xffffcf23b584
12 gst_base_src_negotiate_unlocked gstbasesrc.c 3502 0xffffe16f543c
13 gst_base_src_loop gstbasesrc.c 2920 0xffffe16f7780
14 gst_task_func gsttask.c 399 0xffffe15fb354
15 g_thread_pool_thread_proxy gthreadpool.c 336 0xfffff615325c
16 g_thread_proxy gthread.c 890 0xfffff6152b40
17 start_thread pthread_create.c 448 0xfffff656085c
18 thread_start clone3.S 76 0xfffff65c40cc