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docs/multi-gpu: Document copy modes

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In preparation to allow to select the copy mode, document them.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4251>
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José Expósito 2025-02-05 19:10:20 +01:00 committed by Marge Bot
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doc/multi-gpu.md
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@ -40,3 +40,23 @@ Created gbm renderer for '/dev/dri/card1'
Created gbm renderer for '/dev/dri/card2'
GPU /dev/dri/card1 selected primary given udev rule
```
## Copy modes
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.