From 619e9949557a91392749abc84850edefae485128 Mon Sep 17 00:00:00 2001
From: Ian Douglas Scott <idscott@system76.com>
Date: Fri, 30 May 2025 21:08:04 -0700
Subject: [PATCH] kms: Fix inconsistent selection of primary GPU

Instead of choosing a primary GPU the first time `device_added` is
called (and then not updating it on the next call, even if that should
be the primary GPU), set the primary GPU only after all devices have
been initially added, and on future changes.

Alternately, the `was_empty` test can just be removed, but it's probably
best not to select the primary GPU multiple times each time the
compositor starts on a multi-GPU system.

Fixes https://github.com/pop-os/cosmic-comp/issues/1437.
---
 src/backend/kms/device.rs | 21 +++++++--------------
 src/backend/kms/mod.rs    | 19 +++++++++++++++++--
 2 files changed, 24 insertions(+), 16 deletions(-)

diff --git a/src/backend/kms/device.rs b/src/backend/kms/device.rs
index 5927ae60..806499c3 100644
--- a/src/backend/kms/device.rs
+++ b/src/backend/kms/device.rs
@@ -46,7 +46,7 @@ use std::{
     cell::RefCell,
     collections::{HashMap, HashSet},
     fmt,
-    path::{Path, PathBuf},
+    path::Path,
     sync::{atomic::AtomicBool, mpsc::Receiver, Arc, RwLock},
     time::Duration,
 };
@@ -144,7 +144,7 @@ pub fn init_egl(gbm: &GbmDevice<DrmDeviceFd>) -> Result<EGLInternals> {
 }
 
 impl State {
-    pub fn device_added(&mut self, dev: dev_t, path: PathBuf, dh: &DisplayHandle) -> Result<()> {
+    pub fn device_added(&mut self, dev: dev_t, path: &Path, dh: &DisplayHandle) -> Result<()> {
         if !self.backend.kms().session.is_active() {
             return Ok(());
         }
@@ -337,13 +337,7 @@ impl State {
             // TODO atomic commit all surfaces together and drop surfaces, if it fails due to bandwidth
 
             let kms = self.backend.kms();
-            let was_empty = kms.drm_devices.is_empty();
             kms.drm_devices.insert(drm_node, device);
-            if was_empty {
-                if let Err(err) = kms.select_primary_gpu(dh) {
-                    warn!("Failed to determine a new primary gpu: {}", err);
-                }
-            }
         }
 
         self.common
@@ -497,12 +491,11 @@ impl State {
                 dh.remove_global::<State>(socket.drm_global);
             }
             backend.api.as_mut().remove_node(&device.render_node);
-            let was_primary = *backend.primary_node.read().unwrap() == Some(device.render_node);
-            if was_primary {
-                if let Err(err) = backend.select_primary_gpu(dh) {
-                    warn!("Failed to determine a new primary gpu: {}", err);
-                }
-            }
+            backend
+                .primary_node
+                .write()
+                .unwrap()
+                .take_if(|node| node == &device.render_node);
         }
         self.common
             .output_configuration_state
diff --git a/src/backend/kms/mod.rs b/src/backend/kms/mod.rs
index 2725e3cd..8e0d39db 100644
--- a/src/backend/kms/mod.rs
+++ b/src/backend/kms/mod.rs
@@ -136,6 +136,10 @@ pub fn init_backend(
         }
     }
 
+    if let Err(err) = state.backend.kms().select_primary_gpu(dh) {
+        warn!("Failed to determine primary gpu: {}", err);
+    }
+
     // start x11
     let primary = state.backend.kms().primary_node.read().unwrap().clone();
     state.launch_xwayland(primary);
@@ -254,7 +258,10 @@ fn init_udev(
     let dispatcher = Dispatcher::new(udev_backend, move |event, _, state: &mut State| {
         let dh = state.common.display_handle.clone();
         match match event {
-            UdevEvent::Added { device_id, path } => state
+            UdevEvent::Added {
+                device_id,
+                ref path,
+            } => state
                 .device_added(device_id, path, &dh)
                 .with_context(|| format!("Failed to add drm device: {}", device_id)),
             UdevEvent::Changed { device_id } => state
@@ -265,7 +272,15 @@ fn init_udev(
                 .with_context(|| format!("Failed to remove drm device: {}", device_id)),
         } {
             Ok(()) => {
-                trace!("Successfully handled udev event.")
+                trace!("Successfully handled udev event.");
+                let backend = state.backend.kms();
+                if matches!(event, UdevEvent::Added { .. } | UdevEvent::Removed { .. })
+                    && backend.primary_node.read().unwrap().is_none()
+                {
+                    if let Err(err) = state.backend.kms().select_primary_gpu(&dh) {
+                        warn!("Failed to determine a new primary gpu: {}", err);
+                    }
+                }
             }
             Err(err) => {
                 error!(?err, "Error while handling udev event.")