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GNOME Mutter Adds Debug Option To Override Multi-GPU Copy Mode

([GNOME] 6 Hours Ago MUTTER_DEBUG_MULTI_GPU_FORCE_COPY_MODE)


A new debug option was merged on Wednesday to Mutter Git ahead of the upcoming [1]GNOME 48 release. This new debug option is to override the GPU copy mode handling for multi-GPU setups.

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 [2]this merge for those in need of multi-GPU debugging with GNOME.



[1] https://www.phoronix.com/search/GNOME+48

[2] https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4251



phoronix

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