Writing a BSP from downstream kernel sources
As a learning experience, I am trying to create a BSP for a device I own and whose downstream kernel is published. The device in question is the Parrot Skycontroller3, and the sources are available here.
Let me start by sharing my issue. When I build the kernel with `bitbake virtual/kernel`, it fails with errors like:
| AS arch/arm/lib/backtrace.o
| AS arch/arm/lib/bswapsdi2.o
| AS arch/arm/lib/call_with_stack.o
| /tmp/ccz8jKgm.s: Assembler messages:
| /tmp/ccz8jKgm.s:985: Error: .err encountered
| /tmp/ccz8jKgm.s:1033: Error: .err encountered
| /tmp/ccz8jKgm.s:6812: Error: .err encountered
I created it getting inspiration from meta-raspberrypi and meta-qti-bsp, which seems to have the same CPU: apq8009. According to the Parrot sources, my device runs a Qualcomm apq8009/msm89090 cpu. In my repo, I use as a defconfig file the linux.config that is available in the Parrot sources (I copied it in my kernel recipe under `files/defconfig` and added it to SRC_URI).
The Parrot sources also refer to `LINUX_DEFAULT_CONFIG_TARGET := msm8909_defconfig`, so I tried to set `KBUILD_DEFCONFIG = "msm8909_defconfig"`, but that is failing with different errors, such as:
error: 'VM_ARM_DMA_CONSISTENT' undeclared (first use in this function); did you mean 'DMA_ATTR_NON_CONSISTENT'?
error: 'L_PTE_YOUNG' undeclared
As a side note, their `drivers/Kconfig` seemed invalid, so I patched it.
I am a bit lost now, not completely sure where my issues come from. I realize that changing the defconfig has quite some impact (I get different errors), and also that my machine configuration may be completely wrong (I am essentially guessing from the fact that it is an apq8009/msm8909, but for instance the tuning I just copied from meta-qti-bsp, which may be invalid).
I would be glad if I could get hints about debugging this. Again, it is really a learning project: I would like to learn how to create a BSP from a downstream kernel.