I am on an Intel(R) Core(TM)2 Duo CPU T5450 @ 1.66GHz laptop, running Debian 10.1 i686 with a custom-compiled 5.4.0-rc6 kernel (config attached). The laptop has a Radeon video card; the relevant output of 'lspci -k' is
01:05.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] RC410M [Mobility Radeon Xpress 200M]
Subsystem: ASUSTeK Computer Inc. RC410M [Mobility Radeon Xpress 200M]
Kernel driver in use: radeon
Kernel modules: radeon
rc6 is giving me traces in dmesg complaining about drm_vblank (the trace is attached). The issue was not present in rc5.
I am sending the message to the addresses listed in the MAINTAINERS file of the current Linux tree under the headings
DRM DRIVERS
DRM DRIVERS AND MISC GPU PATCHES
RADEON and AMDGPU DRM DRIVERS
apologies if this is overly broad.