Slow performance using eGPU with lid closed

[Qu Wenruo]

Hi,

When tweaking eGPU under Linux, I hit one strange behavior.

The integrated GPU is Intel HD630, using the DDX driver.
The eGPU is an RX580, using amdgpu driver. Using thunderbolt 3
connection. (4x lanes)

All 2 monitors are connected to the eGPU, using DP and HDMI.

At first glance, it works with some quirks (hot plug doesn't work, hot
unplug crash the kernel), but at least it works without any config.
Also tested DRI_PRIME=1 with offload for games, it works well playing
games in the laptop display.

But when I try to close the lid, the external monitors get very laggy.
(WM is muffer from cinnamon)

My guess is, without any config, the external monitor is still rendered
by the Intel GPU, as all monitor shares the same screen space.
And when the lid is closed, eGPU is forced to its lowest performance.

Is there anyway to force all rendering to be done by eGPU?
It would be nicer if it can be detected automatically (when no eGPU,
still use Intel GPU).

Thanks,
Qu

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