https://bugzilla.kernel.org/show_bug.cgi?id=215652
--- Comment #5 from Erhard F. (erhard_f@mailbox.org) --- Ok, changed my config to include the firmware via CONFIG_EXTRA_FIRMWARE="radeon/R520_cp.bin" CONFIG_EXTRA_FIRMWARE_DIR="/lib/firmware"
With CONFIG_DRM_RADEON=y the machine boots now as expected with no radeon error messages.
With CONFIG_DRM_RADEON=m radeon does not load and I still get "modprobe: ERROR: could not insert 'radeon': Unknown symbol in module, or unknown parameter (see dmesg)" trying to load it manually via modprobe -v radeon.