Comment 4 for bug 1923153
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| Daniel van Vugt (vanvugt) wrote Re: Display output laggy from iGPU when operating on desktop with attaching AMD R7 430 graphic (dGPU) | #4 |
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(Get the code!)
Xorg actually doesn't have support for synchronizing multiple screens very well. The compositor can only paint all screens at once, and then the Xorg drivers have to output their areas at separate times later.
The 'modeset' Xorg driver will just sync to one and tear on the others. And I am not sure how radeon/amdgpu are expected to interplay. If they support a TearFree feature then that means at least one of them has to delay each frame, which you might see as lag. Also, if the monitors have different refresh rates then there's no guarantee Xorg will sync to the fastest one -- it might sync to the slowest, which has to be worked around in the drivers.
I suggest the first thing to investigate here is to see if you can get both GPUs using the same driver (from the kernel onward). Because using both 'radeon' and 'amdgpu' at the same time might be causing this.
GNOME Wayland sessions don't have this problem at all because there is no Xorg and it schedules each display separately.