eglfs: kms: Allow overriding plane ids per crtc

With atomic enabled (QT_QPA_EGLFS_KMS_ATOMIC=1) the plane chosen for
a crtc becomes important. The current logic is pretty broken when there
are multiple planes available with many of them marked as being supported
by multiple CRTCs. Choosing the same plane for multiple crtcs results in
failing the atomic commit. This happens with a RPi4 with two screens
connected, for example. (because there are 2*3 planes: one primary, one
overlay, one cursor for each screen, but Qt is trying to use the same
primary plane with both CRTCs)

The issue does not surface in special environments where there is one
(dedicated) plane per crtc exposed by drm, but becomes apparent on a PC
or on the RPi where the setup described above is the common case.

As a temporary solution allow doing the following:

export QT_QPA_EGLFS_KMS_PLANES_FOR_CRTCS="49,28:78,57"

This would then force using plane id 28 for crtc id 49, and plane id 57
for crtc id 78. (the ids can be discovered either from the eglfs debug
logs, or by checking /sys/kernel/debug/dri/<N>/state)

This is to be complemented with a fix for picking a unique primary plane
for all CRTCs but that's going to be a separate patch. Allowing a manual
override is important regardless since it helps troubleshooting.

Task-number: QTBUG-74953
Change-Id: Ie03f80dac31813f2c4489235d435769dd3d3883e
Reviewed-by: Johan Helsing <johan.helsing@qt.io>

1 files changed, 24 insertions, 2 deletions

diff --git a/src/platformsupport/kmsconvenience/qkmsdevice.cpp b/src/platformsupport/kmsconvenience/qkmsdevice.cpp
index 6121faf362..263c02682d 100644
--- a/src/platformsupport/kmsconvenience/qkmsdevice.cpp
+++ b/src/platformsupport/kmsconvenience/qkmsdevice.cpp
@@ -490,8 +490,30 @@ QPlatformScreen *QKmsDevice::createScreenForConnector(drmModeResPtr resources,
         }
     }
 
-    if (output.eglfs_plane)
-        qCDebug(qLcKmsDebug, "Output eglfs plane is: %d", output.eglfs_plane->id);
+    // A more useful version: allows specifying "crtc_id,plane_id:crtc_id,plane_id:..."
+    // in order to allow overriding the plane used for a given crtc.
+    if (qEnvironmentVariableIsSet("QT_QPA_EGLFS_KMS_PLANES_FOR_CRTCS")) {
+        const QString val = qEnvironmentVariable("QT_QPA_EGLFS_KMS_PLANES_FOR_CRTCS");
+        qCDebug(qLcKmsDebug, "crtc_id:plane_id override list: %s", qPrintable(val));
+        const QStringList crtcPlanePairs = val.split(QLatin1Char(':'));
+        for (const QString &crtcPlanePair : crtcPlanePairs) {
+            const QStringList values = crtcPlanePair.split(QLatin1Char(','));
+            if (values.count() == 2 && uint(values[0].toInt()) == output.crtc_id) {
+                uint planeId = values[1].toInt();
+                for (const QKmsPlane &kmsplane : qAsConst(m_planes)) {
+                    if (kmsplane.id == planeId) {
+                        output.eglfs_plane = (QKmsPlane*)&kmsplane;
+                        break;
+                    }
+                }
+            }
+        }
+    }
+
+    if (output.eglfs_plane) {
+        qCDebug(qLcKmsDebug, "Chose plane %u for output %s (crtc id %u) (may not be applicable)",
+                output.eglfs_plane->id, connectorName.constData(), output.crtc_id);
+    }
 
     m_crtc_allocator |= (1 << output.crtc_index);