Yocto part III – a custom meta layer

In this part of the series (previous: i, ii) we will have a look at setting up our own meta layer, so that we can make changes without having to fork the Yocto recipes.

The easiest way to create a layer is to use the yocto-layer tool. To fully understand what a layer consists of, I recommend looking at the instructions for creating a layer manually as well. So, long story short:

cd ..
yocto-layer create diet

The name, meta-diet, comes from my goal with this series – to create a minimal fast booting image. Only time will tell how well that goes.

The yocto-layer create command produces an MIT-licensed layer. The MIT license is a weak copyleft, so you can choose to close it. I’ll leave it open for your enjoyment. Also, remember to change the README file. It contains a few references to xxxx and yyyy, which would be you and your project.

As we don’t want the layer to be a part of the layers provided by the Yocto project, we need to separate it out. To do this, change the directory so that you stand in the meta-diet directory and run:

git init

Now you are free to do what you like. I suggest making a commit of what is there and perhaps add a remote so that you can push your changes somewhere.

An empty layer is only so much fun, so lets start by adding a recipe for your core-image-minimal, enhanced with systemd-analyze. First, let’s create a directory for the recipe:

mkdir -p recipes-diet/images

In that directory, create the diet-image.bb file using your favorite, non-emacs, editor. Put the following code in it. This means that we use the core-image-minimal as base, and add systemd-analyze to what we want to install onto the image.

require recipes-core/images/core-image-minimal.bb

IMAGE_INSTALL += "systemd-analyze"

As copying code from a blog post is boring, I’ve put the layer on github as meta-diet. For this installment, use the part-iii tag.

To build the diet image, change directory to the build directory and edit the conf/bblayers.conf file. Simply add the new layer to the BBLAYERS variable. Now you can bitbake the new image as simple as:

bitbake diet-image

So, this installment got us nowhere feature wise, but now we have everything in place to start experimenting!

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