docs/multi-gpu: Document mutter-device-preferred-primary

Add documentation explaining how to change the primary GPU using a udev
rule.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4218>

README.md

@@ -38,6 +38,7 @@ documentation and API references below first.
 - [Building and Running](doc/building-and-running.md)
 - [Debugging](doc/debugging.md)
 - [Monitor configuration](doc/monitor-configuration.md)
+- [Multi-GPU](doc/multi-gpu.md)
 
 ## API Reference
 

doc/multi-gpu.md

@@ -0,0 +1,42 @@
+# Multi-GPU
+
+## Selecting the primary GPU
+
+When multiple GPUs are present in the same system, Mutter applies a series of
+rules to select one of them as primary. However, it is possible to manually
+select a different GPU by using a udev rule.
+
+To illustrate it with an example, I'm going to make the NVIDIA GPU in my system
+to be selected as the primary GPU.
+
+Start by finding the vendor and device IDs of the target GPU:
+
+```bash
+$ lspci -nn
+[...]
+24:00.0 VGA compatible controller [0300]: NVIDIA Corporation AD107 [GeForce RTX 4060] [10de:2882] (rev a1)
+```
+
+In this case, we are looking for the vendor ID (10de) and the product (2882).
+
+Next, create rule to set the desired GPU as primary. Make sure to replace the
+vendor and device IDs:
+
+```bash
+$ cat /etc/udev/rules.d/61-mutter-preferred-primary-gpu.rules
+SUBSYSTEM=="drm", ENV{DEVTYPE}=="drm_minor", ENV{DEVNAME}=="/dev/dri/card[0-9]", SUBSYSTEMS=="pci", ATTRS{vendor}=="0x10de", ATTRS{device}=="0x2882", TAG+="mutter-device-preferred-primary"
+```
+
+Finally, restart and check that the desired GPU was selected:
+
+```bash
+$ journalctl -b0 /usr/bin/gnome-shell
+[...]
+Device '/dev/dri/card1' prefers shadow buffer
+Added device '/dev/dri/card1' (nouveau) using non-atomic mode setting.
+Device '/dev/dri/card2' prefers shadow buffer
+Added device '/dev/dri/card2' (amdgpu) using atomic mode setting.
+Created gbm renderer for '/dev/dri/card1'
+Created gbm renderer for '/dev/dri/card2'
+GPU /dev/dri/card1 selected primary given udev rule
+```