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Just to close this: it seems like the gcc-cross-arm used by yocto gatesgarth is too new for that specific downstream kernel (3.18.31).
The goal was to get a proper BSP package for this device for a modern yocto, so I don't think I will try with an older version of Yocto. If I compile an old gcc as part of a custom Yocto layer (on gatesgarth), I am guessing that I will have issues creating a distro that runs both on RPi and on that older device (because RPi will have a newer kernel and gcc, and the skycontroller will use older ones). I also guess that the downstream dts won't work with a modern kernel, and I would not know how to write one myself for that apq8009 chip.
Hence, I'm giving up. Thanks a lot for the help :-).
Thanks a lot for the answer!
It seems like using `KCONFIG_MODE = "--alldefconfig"` with `KBUILD_DEFCONFIG = "msm8909_defconfig"` now ends up with the same kind of errors as when I use the defconfig from the downstream kernel , i.e.:
| /tmp/ccz8jKgm.s: Assembler messages:
| /tmp/ccz8jKgm.s:985: Error: .err encountered
Could it be related to the tuning, e.g. I'm somehow defining a wrong toolchain in my machine configuration  and it fails to build? I was thinking about the tune-cortexa7.inc include, though it seems to me that the apq8009 is a cortexa7 :
> The Qualcomm Snapdragon 212 APQ8009 is an entry level SoC for Android based tablets and smartphones. It contains four ARM Cortex-A7 CPU cores (quad core)
On Sat, Jan 23, 2021 at 11:06 AM Paul Barker <pbarker@...
On Sat, 23 Jan 2021 at 02:29, Jonas Vautherin <jonas.vautherin@...> wrote:
> 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
> My layer is available here.
> 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.
I think setting `KBUILD_DEFCONFIG = "msm8909_defconfig"` is likely the
correct approach here. What you may be missing though is the correct
value for KCONFIG_MODE. By default, the supplied defconfig file is
copied to .config but dependencies between config options aren't
resolved. You need to set `KCONFIG_MODE = "--alldefconfig"` to get the
equivalent of `make msm8909_defconfig` to occur. See
for an idea of how this is done for Raspberry Pi.