Building .dtbo overlay for BBB using bitbake - linux-kernel

Question

I built a yocto distro (rocko) for my BBB using kernel linux-ti-staging-4.14 with a custom device tree. Since I need to port an old project from Debian to Yocto, I also needed to enable the UIO features in the kernel, which works fine too.
Now I need to also create an additonal device-tree-overlay for the UIO PRUSS. I use a dts file from here and extended the kernel recipe via bbappend:
inherit kernel-devicetree
FILESEXTRAPATHS_prepend := "${THISDIR}/files/dts:${THISDIR}/files/beaglebone:"
# Make custom kernel with PRU enabled
SRC_URI += " \
file://bbb-pru-minimal.dts;subdir=git/arch/${ARCH}/boot/dts \
file://AM335X-PRU-UIO-00A0-overlay.dts;subdir=git/arch/${ARCH}/boot/dts/overlays \
file://0001-add-UIO-dtbo.patch \
file://defconfig \
"
KERNEL_DEVICETREE = " \
AM335X-PRU-UIO-00A0.dtbo \
bbb-pru-minimal.dtb \
"
The above listed patch file contains:
diff --git a/arch/arm/boot/dts/Makefile b/arch/arm/boot/dts/Makefile
index 7949c213a434..fe2513074893 100644
--- a/arch/arm/boot/dts/Makefile
+++ b/arch/arm/boot/dts/Makefile
## -610,6 +610,7 ## dtb-$(CONFIG_SOC_TI81XX) += \
dm8168-evm.dtb \
dra62x-j5eco-evm.dtb
dtb-$(CONFIG_SOC_AM33XX) += \
+ AM335X-PRU-UIO-00A0.dtbo \
am335x-baltos-ir2110.dtb \
am335x-baltos-ir3220.dtb \
am335x-baltos-ir5221.dtb \
However, the kernel build fails as it seems there is no rule for .dtbo targets:
| CC scripts/mod/empty.o
| CC scripts/mod/devicetable-offsets.s
| MKELF scripts/mod/elfconfig.h
| HOSTCC scripts/mod/modpost.o
| HOSTCC scripts/mod/sumversion.o
| CHK scripts/mod/devicetable-offsets.h
| HOSTCC scripts/mod/file2alias.o
| HOSTLD scripts/mod/modpost
| NOTE: make -j 24 HOSTCC=gcc -isystem[TOPDIR]/build/tmp/work/beaglebone-poky-linux-gnueabi/linux-ti-staging/4.14.79+gitAUTOINC+3438de3474-c/recipe-sysroot-native/usr/include -O2 -pipe -L[TOPDIR]/build/tmp/work/beaglebone-poky-linux-gnueabi/linux-ti-staging/4.14.79+gitAUTOINC+3438de3474-c/recipe-sysroot-native/usr/lib -L[TOPDIR]/build/tmp/work/beaglebone-poky-linux-gnueabi/linux-ti-staging/4.14.79+gitAUTOINC+3438de3474-c/recipe-sysroot-native/lib -Wl,-rpath-link,[TOPDIR]/build/tmp/work/beaglebone-poky-linux-gnueabi/linux-ti-staging/4.14.79+gitAUTOINC+3438de3474-c/recipe-sysroot-native/usr/lib -Wl,-rpath-link,[TOPDIR]/build/tmp/work/beaglebone-poky-linux-gnueabi/linux-ti-staging/4.14.79+gitAUTOINC+3438de3474-c/recipe-sysroot-native/lib -Wl,-rpath,[TOPDIR]/build/tmp/work/beaglebone-poky-linux-gnueabi/linux-ti-staging/4.14.79+gitAUTOINC+3438de3474-c/recipe-sysroot-native/usr/lib -Wl,-rpath,[TOPDIR]/build/tmp/work/beaglebone-poky-linux-gnueabi/linux-ti-staging/4.14.79+gitAUTOINC+3438de3474-c/recipe-sysroot-native/lib -Wl,-O1 -Wl,--allow-shlib-undefined -Wl,--dynamic-linker=[TOPDIR]/build/tmp/sysroots-uninative/x86_64-linux/lib/ld-linux-x86-64.so.2 HOSTCPP=gcc -E AM335X-PRU-UIO-00A0.dtbo
| CHK scripts/mod/devicetable-offsets.h
| make[3]: *** No rule to make target 'arch/arm/boot/dts/AM335X-PRU-UIO-00A0.dtbo'. Stop.
| arch/arm/Makefile:345: recipe for target 'AM335X-PRU-UIO-00A0.dtbo' failed
| make[2]: *** [AM335X-PRU-UIO-00A0.dtbo] Error 2
| Makefile:146: recipe for target 'sub-make' failed
| make[1]: *** [sub-make] Error 2
| Makefile:24: recipe for target '__sub-make' failed
| make: *** [__sub-make] Error 2
| ERROR: oe_runmake failed
Please note, I replaced the absolute paths in the error message with [TOPDIR] due to privacy reasons.
Since I am fairly new to overlay creation I don't really know what I am missing or how to fix it. Any hints?
Thank you in advance!
Update: Added patch file to description above.

Answer 1

Here is what I found after analyzing Toradex's git projects that can inspire you for a solution:
They have a custom class that handles their device tree files and device tree overlays, here.
The class inherits devicetree that is an official poky class here.
The devicetree class tests if a device tree is an overlay or normal and then it compiles all of them.
So, you can basically use devicetree class in a custom recipe to compile device tree overlays and deploy them manually into your rootfs.
Toradex also has a simple recipe like that here. They have a git project that holds all of their device tree overlays.
That being said, I think Toradex doesn't use the custom recipe, rather they use the custom toradex-devicetree class which inherits from devicetree, compiles all device tree overlays from their device tree overlays git project and then deploy them all into the rootfs.
In that custom class, they append to do_deploy of devicetree class to install the .dtbo files and add them to overlays.txt file under the boot partition.
So finally, you can create a bbappend file for you kernel recipe if you don't have one, and try to follow these steps:
Inherit from devicetree
Add your dts files that are overlays to SRC_URI via local files or URL
Add the path for your overlays to DT_FILES_PATH of the devicetree class
In the kernel recipe, append to do_deploy to add them to rootfs.
Now, here is my advice to achieve this step by step:
Create a custom recipe that uses devicetree class with dts overlay example file
Check if it compiles it correctly
If you understand that, you can then apply the 4 steps above.

Answer 2

Thanks to the hints from #BelHadjSalem I was able to manage the dtbo build.
The class file devicetree.bbclass was introduced in Yocto with release "thud", therefore I updated my whole workspace to the latest Yocto release (dunfell). Afterwards I created a new recipe just to build my dtbo overlay:
inherit devicetree
FILESEXTRAPATHS_prepend := "${THISDIR}/files/dts:"
SRC_URI = " \
file://AM335X-PRU-UIO-00A0.dts;subdir=git/overlays \
"
S = "${WORKDIR}/git/overlays"
COMPATIBLE_MACHINE = ".*(ti33x|beaglebone).*"
The recipe is pretty simple and works in my project as required.
However, one important note: Inheriting the devicetree.bbclass in any kernel recipe or kernel bbappend file, will NOT work! Not sure what exactly happens, but the inheritance of devicetree.bbclass will invalidate the kernel recipe configuration and fail with ERROR: Nothing PROVIDES 'virtual/kernel'