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Testing Video4Linux2 drivers like a boss

Ezequiel Garcia avatar

Ezequiel Garcia
May 23, 2019

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Note: In September 2021, the GStreamer project merged all its git repositories into a single, unified repository, often called monorepo. The build system referred in this post as "gst-build" is now in the root of this combined/mono repository.

In a previous post, we presented virtme, the awesome QEMU wrapper developed by Andy Lutomirski. virtme allows to run a custom built kernel on top of our running root filesystem, which is quite helpful in the testing phase of some kernel feature or driver.

As an example, virtme is being used by the media subsystem maintainer as part of the regression tests.

On this post, let's explore another example of virtme in action, and see how to test Video4Linux2 drivers on bleeding edge GStreamer builds.

Why GStreamer? Two sound reasons. First, it supports the Video4Linux APIs quite well. Second, it is a very flexible tool, which allows to implement different tests easily. For instance, we can test encode and decode in the same pipeline:

$ gst-launch-1.0 videotestsrc ! v4l2jpegenc ! decodebin ! fakevideosink

There are cases, where you want to test bleeding-edge GStreamer, either to take advantage of the latest support, or to work on an unsupported feature or new plugin. This all means we'd better shape up and build GStreamer from sources.

This may sound troublesome at first, given GStreamer large and modular source base, but it's actually relatively simple using gst-build.

Down the rabbit hole

gst-build is GStreamer's meson-based repository aggregrator. As we will see, it can do much more than that. The README.md has all the info you need to get started, but here's a Debian tl;dr:

Make sure you have meson installed:

$ apt-get install meson

The first step is to clone gst-build and build it. The first time it will clone all the GStreamer repos, and so it will take a lot.

$ git clone https://gitlab.freedesktop.org/gstreamer/gst-build
$ cd gst-build && meson build && ninja -C build/

With gst-build installed, we can now enter a GStreamer sandbox, which GStreamer folks call the Uninstalled Environment. This environment has your bleeding-edge gstreamer. We can enter the environment, and you will see how the prompt is now changed:

$ ninja -C build/ uninstalled
ninja: Entering directory `build/'
[0/1] Running external command uninstalled.
[gst-master] $ 

[gst-master] $ gst-launch-1.0 --version
gst-launch-1.0 version 1.17.0
GStreamer 1.17.0 (GIT)
Unknown package origin

Alternatively, the uninstalled environment can be summoned from your regular environment to run a gstreamer pipeline:

$ ./gst-uninstalled.py gst-launch-1.0 --version
gst-launch-1.0 version 1.17.0
GStreamer 1.17.0 (GIT)
Unknown package origin

Combining gst-build and virtme

With all the pieces in place, the next step is to add some scripting to automate the testing. We'll ask virtme to run a script right after booting, and inside that script we'll put a GStreamer test using the uninstalled environment.

Just as an example, we could have a test directory, with a simple test.sh script inside:


./gst-uninstalled.py gst-launch-1.0 -v v4l2src num-buffers=300 ! v4l2convert ! videoconvert ! kmssink driver-name=virtio_gpu force-modesetting=true

And we'd start virtme with graphic support, specifying the test init directory, and also passing arguments to the vivid driver:

$ virtme-run --cwd ~/gst-build --script-dir ~/gst-build/test/ --kdir ~/builds/virtme-x86_64 --kopt vivid.node_types=0x1 --kopt vivid.n_devs=1 --qemu-opts  -vga virtio  -display gtk,gl=off -m 2000

(Keep in mind, I'm not using vanilla virtme here, but my own fork, which has an extra command to run stuff after booting.)

That's it! This is just an example. From here, we could try many different pipelines here, and take advantage of GStreamer's great flexibility to create complete test suites. Happy testing!

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