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.

gst-build is one of the main build systems used by the community to develop the GStreamer platform. In my last blog post, I presented gst-build and explained how to get started with it. Now, let's get straight to the point regarding cross-compilation.

For this example, we will target an AArch64 CPU for a Xilinx reference board, the Zynq UltraScale+ MPSoC ZCU106 Evaluation Kit. As you'll see, cross compiling can be very useful when you want to save time when working with GStreamer, or when you want to be able to work on both the host and target with the same base code.

Prerequisites

• Toolchain (aarch64-linux-gnu-gcc) + sysroot (optional)
• Meson cross file
• Meson > 0.54.1

First we'll need here to have a proper toolchain to cross-build. In my case I used the regular toolchain provided by Ubuntu installing the packages:

$ sudo apt install gcc-aarch64-linux-gnu g++-aarch64-linux-gnu libc6-dev-arm64-cross

This is installing a minimal toolchain in /usr/aarch64-linux-gnu/

Cross file generated with generate-cross-file.py

Below the cross file used to build for aarch64, this file has been generated with this helper script allowing to generated the cross file for other target as well. As you can see, here, we don't use a dedicated rootfs because gst-build will build all that we need for the GStreamer essentials (including libffi, glib etc.).

$ ./generate-cross-file.py

[host_machine]
system = 'linux'
cpu_family = 'aarch64'
cpu = 'arm64'
endian = 'little'

[properties]
c_args = []
cpp_args = []
objc_args = []
objcpp_args = []
c_link_args = []
cpp_link_args = []
objc_link_args = []
objcpp_link_args = []
pkg_config_libdir = ['__unknown_sysroot__']


[binaries]
c = ['aarch64-linux-gnu-gcc']
cpp = ['aarch64-linux-gnu-g++']
ar = ['aarch64-linux-gnu-ar']
pkgconfig = 'pkg-config'
strip = ['aarch64-linux-gnu-strip']

Here Meson will use aarch64-linux-gnu-gxx to compile with the given arguments setup above. As Meson does not recommend to use environment variables, the cross file contains hard-coded path to the sysroot to provide package config. Indeed since Meson > 0.54, you can define pkg_config_libdir which will help pkg-config to search for the package configuration files for the given target. You can also tell the path to the pkg-config wrapper by modifying the pkgconfig variables as well. Predefined cross file can also be found in gst-build/data/cross-files

Configuring the project for Zynq UltraScale+ MPSoC ZCU106 Evaluation Board

When the cross file ready, we can now configure gst-build in order to have a dedicated build for our platform. Here I'm disabling some unnecessary options of gst-build such as libav, vaapi or gtk_doc.

⚠ Note: Make sure that you are running Meson 0.54.1 which has the necessary patch for complete support of cross-compilation, otherwise gst-build will take glib from the system (pkg_config_libdir prerequisite).

Notice that on this platform, we use gst-omx, so we also give some options specific to this platform, in particular the path to the Xilinx OpenMAX headers .

$ /path/to/meson_0_54 build-cross-arm64 --cross-file=my-meson-cross-file.txt -D omx=enabled -D sharp=disabled -D gst-omx:header_path=/opt/allegro-vcu-omx-il/omx_header -D gst-omx:target=zynqultrascaleplus -D libav=disabled -D rtsp_server=disabled -D vaapi=disabled -D disable_gst_omx=false -Dugly=disabled -Dgtk_doc=disabled -Dglib:libmount=disabled -Dintrospection=disabled

After this step, you should be able to build with ninja.

$ ninja -C build-cross-arm64

Installing

Last but not the least, you need to install the artifacts in a folder to deploy on the device, for example, by mounting it on your target with NFS. You have to provide a DESTDIR variable to ninja and it will install in $DESTDIR/usr/local/ as install prefix.

$ DESTDIR=/opt/gst-build-cross-artifacts/linux_arm64 ninja -C build-cross-arm64 install

Running the binaries on target

After mounting the folder or copying it to your target, you have to set up a few variables to be able to run GStreamer pipelines:

    PATH=$DESTDIR/usr/local/bin:$PATH
    LD_LIBRARY_PATH=$DESTDIR/usr/local/lib:$LD_LIBRARY_PATH
    GST_PLUGIN_PATH=$DESTDIR/usr/local/lib/gstreamer-1.0
    GST_OMX_CONFIG_DIR=$DESTDIR/usr/local/etc/xdg

A python script as shell script is also available to setup the correct environment variables for your target.

$ /path_to/cross-gst-uninstalled.py /opt/gst-build-cross-artifacts/linux_arm64

Building wavpack in gst-plugins-good

To build a plugin such as wavpack which depends on the 3rd party wavpack library. You'll need to get a proper sysroot with this new library and its dependencies (if needed).

Regarding a root file-system with wavpack, I generated one with cerbero where cross compiling could be described in a next blog post :) But you should normally have it as part of your sysroot.

$ cd /opt
$ git clone https://gitlab.freedesktop.org/gstreamer/cerbero
$ cd cerbero
$ ./cerbero-uninstalled -c config/cross-lin-arm64.cbc bootstrap
$ ./cerbero-uninstalled -c config/cross-lin-arm64.cbc build wavpack

This should have generated a minimal root file-system in /opt/cerbero/build/dist/linux_arm64 which can used then with gst-build as a base root file-system.

You can now generate a new cross file with the given root file-system as parameter.

$ ./generate-cross-file.py --sysroot /opt/cerbero/build/dist/linux_arm64/ --no-include-sysroot

Here I define a sysroot to be be used but I'm disabling the use of sys_root in the cross file to avoid Meson to tell pkg-config to prefix every path with this value. cerbero is generating pkg-config files with the sysroot path already in each pc files.

[host_machine]
system = 'linux'
cpu_family = 'aarch64'
cpu = 'arm64'
endian = 'little'

[properties]
c_args = []
cpp_args = []
objc_args = []
objcpp_args = []
c_link_args = ['-L/opt/cerbero/build/dist/linux_arm64', '-Wl,-rpath-link=/opt/cerbero/build/dist/linux_arm64']
cpp_link_args = ['-L/opt/cerbero/build/dist/linux_arm64', '-Wl,-rpath-link=/opt/cerbero/build/dist/linux_arm64']
objc_link_args = ['-L/opt/cerbero/build/dist/linux_arm64', '-Wl,-rpath-link=/opt/cerbero/build/dist/linux_arm64']
objcpp_link_args = ['-L/opt/cerbero/build/dist/linux_arm64', '-Wl,-rpath-link=/opt/cerbero/build/dist/linux_arm64']
pkg_config_libdir = ['/opt/cerbero/build/dist/linux_arm64/pkgconfig:/opt/NFS/cerbero_rootfs/linux_arm64//usr/share/pkgconfig']


[binaries]
c = ['aarch64-linux-gnu-gcc']
cpp = ['aarch64-linux-gnu-g++']
ar = ['aarch64-linux-gnu-ar']
pkgconfig = 'pkg-config'
strip = ['aarch64-linux-gnu-strip']

Now you should be able to go back to the configure/build/install step and get the wavpack in your plugins registry.

I hope you'll enjoy the use of gst-build, which is for me a very powerful and flexible tool. A lot of options can be found in the gst-build README such as the update or the use of GStreamer branches.

If you would like to learn more about gst-build or any other parts of GStreamer, please contact us!