For those with multi-GPU systems and working to debug issues around performance or other issues particularly when driving a display with GNOME from the secondary GPU, the new "MUTTER_DEBUG_MULTI_GPU_FORCE_COPY_MODE" environment variable override may be useful.
MUTTER_DEBUG_MULTI_GPU_FORCE_COPY_MODE allows for changing the copy mode for buffers from the primary GPU to secondary GPU. The documentation on this newly-merged debug code explains:
"Mutter composites the buffer to be displayed on all displays on the primary GPU, regardless of which GPU the display is connected to.
Therefore, when a display is connected to a secondary GPU, the contents to be displayed on that display need to be copied from the primary GPU to the secondary GPU.
There are 3 copy modes available:
- Secondary GPU copy mode: The copy is performed by the secondary GPU. This is the default copy mode.
- Zero-copy mode: The primary GPU exports a framebuffer and the secondary GPU imports it. This mode is tried if the secondary GPU copy mode fails.
- Primary GPU copy mode: The primary GPU copies its contents to a dumb buffer and the secondary GPU scan-outs from it. First, the GPU is used to perform the copy and, if it fails, the copy is perform by the CPU. This mode is used if the zero-copy mode fails.
For debug purposes, it is possible to force the copy mode by setting the environment variable `MUTTER_DEBUG_MULTI_GPU_FORCE_COPY_MODE` to `zero-copy`, `primary-gpu-gpu` or `primary-gpu-cpu`."
More details via this merge for those in need of multi-GPU debugging with GNOME.