Comment # 3 on bug 69395 from Alex Deucher

What does the dpm debugfs output show when you are bitmining?  e.g.,
cat /sys/kernel/debug/dri/64/radeon_pm_info

Also does manually forcing the performance level to high help?  E.g.,
echo high > /sys/class/drm/card0/device/power_dpm_force_performance_level

Note, you may need to adjust the paths above depending on your system.