Yocto find the recipe or class that defines a task - yocto

Question

I am a yocto noob, trying to decipher how the device tree is built from a Xilinx hardware definition (.hdf) file. But my question is more general.
Is there a yocto way to find the source of task?
Given a task name is it possible to find where the tasks source code lives? (presumably in a recipe or class)
As an example, where is the source for the Python task do_create_yaml which is called by recipes in the meta-xilinx-bsp layer that compile the device tree blob?
bitbake -e device-tree
Will dump the python source for do_create_yaml (amongst the rest of it prodigious output) but how can I find where that is coming from?

Answer 1

Device tree is part of Linux Kernel. In Yocto, this is compiled from KERNEL_DEVICETREE variable value either defined as part of Linux Kernel recipe or machine configuration.
For example, for cubieboard7 as defined here,
KERNEL_DEVICETREE = "s700_cb7_linux.dtb"
instructs the compilation to use this dts file for compilation. This is done by yocto by using various classes.
In our example, we inherit kernel.bbclass which in turn inherits kernel-devicetree.bbclass, in this class (copied from kernel-devicetree.bbclass),
do_compile_append() {
for dtbf in ${KERNEL_DEVICETREE}; do
dtb=`normalize_dtb "$dtbf"`
oe_runmake $dtb
done
}
do_install_append() {
for dtbf in ${KERNEL_DEVICETREE}; do
dtb=`normalize_dtb "$dtbf"`
dtb_ext=${dtb##*.}
dtb_base_name=`basename $dtb .$dtb_ext`
dtb_path=`get_real_dtb_path_in_kernel "$dtb"`
install -m 0644 $dtb_path ${D}/${KERNEL_IMAGEDEST}/$dtb_base_name.$dtb_ext
done
}
do_deploy_append() {
for dtbf in ${KERNEL_DEVICETREE}; do
dtb=`normalize_dtb "$dtbf"`
this appends tasks to compile, install and deploy tasks. So defining KERNEL_DEVICETREE enables the automatic build of dtb.

Answer 2

I found that the datastore contains the filename for tasks as a VarFlag,
from a devpyshell
pydevshell> d.getVarFlags("do_create_yaml")
gives
{'filename': '.....yocto/sources/core/../meta-xilinx-tools/classes/xsctyaml.bbclass', 'lineno': '61', 'func': 1, 'task': 1, 'python': '1', 'deps': ['do_prepare_recipe_sysroot']}
So for the example in my question the active definition for the do_create_yaml task is in xsctyaml.bbclass.