Using bbclass based on conditions

When you are working with YOCTO build system, you might be aware of the construct .bbclass.

For those who are not: these are code snippets which will be injected into the recipe itself. Mostly they add new tasks or provide some generalization for things you often. A good example might be pypi.bbclass. In your recipe you write usually something likes this

SRC_URI = "https://www.foo.org/foo.tar.gz"

When it comes to python packages a way more elegant way is to use pypi.org.
And here does the pypi.bbclass provide some magic - you recipe will just look like this

inherit pypi 

PYPI_PACKAGE = "foo"

the bbclass pypi will automatically translate the variable name PYPI_PACKAGE into a valid URL to fetch the package. Also it will set some internal variables such as HOMEPAGE or S to the correct settings. You see life can be convenient when you know how to do it.

If you want to apply a bbclass you can either insert

inherit foo.bbclass

into each recipe or you can put it into your conf/local.conf. E.g. like this

#   - 'buildstats' collect build statistics#   - 'image-mklibs' to reduce shared library files size for an image#   - 'image-prelink' in order to prelink the filesystem image# NOTE: if listing mklibs & prelink both, then make sure mklibs is before prelink# NOTE: mklibs also needs to be explicitly enabled for a given image, see local.conf.extendedUSER_CLASSES ?= "buildstats image-mklibs image-prelink"INHERIT += "rm_work"

Now imagine you're having a layer construction like this (cat conf/bblayers.conf)

# POKY_BBLAYERS_CONF_VERSION is increased each time build/conf/bblayers.conf# changes incompatiblyPOKY_BBLAYERS_CONF_VERSION = "2"
BBPATH = "${TOPDIR}"BBFILES ?= ""
BBLAYERS ?= " \  /media/disk/meta-sca \  /media/disk/meta-buildutils \  /media/disk/meta-clang \  /media/disk/poky/meta \  /media/disk/poky/meta-poky \  /media/disk/poky/meta-yocto-bsp \  /media/disk/poky/build/workspace \  "

which means hundreds of recipes, but most of these are written and maintained by somebody else.
So what can you do if you want a bbclass

• applied only to your layer?
• only applied to recipes of a certain license?
• only applied if the recipe is depending on a certain packages?

When I got into this problem I started digging through the bitbake-code itself and found a nice little function called "inherit" to be found at

from bb.parse.parse_py import BBHandler

this combined with

python __anonymous() {
}

you can create magic. The function is executed on parsing the recipe by bitbake, so it would be obviously a good place to check if another class should be inherited here or not.

I wrote a small class here which simplifies this even further.

Now you can decide by global rules were to include some extra bbclass-magic.
E.g. (from conf/local.conf)

# NOTE: if listing mklibs & prelink both, then make sure mklibs is before prelink# NOTE: mklibs also needs to be explicitly enabled for a given image, see local.conf.extendedUSER_CLASSES ?= "buildstats image-mklibs image-prelink"INHERIT += "rm_work auto-inherit"AUTO_INHERIT_CONF = "BBClass=foo;props=[auto_inherit_is_at_path(d,'meta-foo/recipes-foo/',False)]"

do include a bbclass named "foo.bbclass" into every recipe which is located under "meta-foo/recipes-foo" relative to your project root.

Another example

# NOTE: if listing mklibs & prelink both, then make sure mklibs is before prelink# NOTE: mklibs also needs to be explicitly enabled for a given image, see local.conf.extendedUSER_CLASSES ?= "buildstats image-mklibs image-prelink"INHERIT += "rm_work auto-inherit"AUTO_INHERIT_CONF = "BBClass=bar;props=[auto_inherit_license(d,'GPL.*')]"

would inherit the bbclass "bar.bblass" into each recipe which is licensed under GPL.

Last example

# NOTE: if listing mklibs & prelink both, then make sure mklibs is before prelink# NOTE: mklibs also needs to be explicitly enabled for a given image, see local.conf.extendedUSER_CLASSES ?= "buildstats image-mklibs image-prelink"INHERIT += "rm_work auto-inherit"AUTO_INHERIT_CONF = "BBClass=bar;props=[auto_inherit_contains_package(d,'python3')]"

this would inherit bar.bbclass into each recipe which requires python3.

Also you can combine these rules to make even more complex scenarios.

So I hope this will help you using more of the power from bbclasses.