kms: Try to preserve existing output routing as weston would do

This is basically a backport from weston:
https://github.com/wayland-project/weston/commit/75487c2560dfca033d8d23649914efb4199356dd

It states that:

 - preserving the existing CRTC -> encoder -> connector routing will
   make the startup faster
 - the existing routing may be set according to some device limitations

Since the QtWayland implementation appears to be based off an old
version of weston (maybe pre-2.0), this patch might look different from
the latest weston implementation. But the idea stays the same.

FWIW, this works around the issue I've seen on Renesas R-Car E3 (aka
Ebisu) board. Without this patch, VGA1+HDMI1 setup would fail because
the device reported possible_crtcs=1 (meaning {crtcs[0]}) for the second
screen "HDMI1", but the crtcs[0] was already taken by the first screen
"VGA1".

  No usable crtc/encoder pair for connector "HDMI1"

With this patch, the crtcs[1] is paired with "VGA1" (as it is the
existing routing), so the crtcs[0] can be allocated for "HDMI1".

Change-Id: Ibd304a8b5efbe4a8aa94b2c5697fe2b399386280
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

1 files changed, 16 insertions, 3 deletions

diff --git a/src/platformsupport/kmsconvenience/qkmsdevice.cpp b/src/platformsupport/kmsconvenience/qkmsdevice.cpp
index 8cc7a539b5..1ba32895f8 100644
--- a/src/platformsupport/kmsconvenience/qkmsdevice.cpp
+++ b/src/platformsupport/kmsconvenience/qkmsdevice.cpp
@@ -66,6 +66,8 @@ enum OutputConfiguration {
 
 int QKmsDevice::crtcForConnector(drmModeResPtr resources, drmModeConnectorPtr connector)
 {
+    int candidate = -1;
+
     for (int i = 0; i < connector->count_encoders; i++) {
         drmModeEncoderPtr encoder = drmModeGetEncoder(m_dri_fd, connector->encoders[i]);
         if (!encoder) {
@@ -73,19 +75,30 @@ int QKmsDevice::crtcForConnector(drmModeResPtr resources, drmModeConnectorPtr co
             continue;
         }
 
+        quint32 encoderId = encoder->encoder_id;
+        quint32 crtcId = encoder->crtc_id;
         quint32 possibleCrtcs = encoder->possible_crtcs;
         drmModeFreeEncoder(encoder);
 
         for (int j = 0; j < resources->count_crtcs; j++) {
             bool isPossible = possibleCrtcs & (1 << j);
             bool isAvailable = !(m_crtc_allocator & (1 << j));
-
-            if (isPossible && isAvailable)
+            // Preserve the existing CRTC -> encoder -> connector routing if
+            // any. It makes the initialization faster, and may be better
+            // since we have a very dumb picking algorithm.
+            bool isBestChoice = (!connector->encoder_id ||
+                                 (connector->encoder_id == encoderId &&
+                                  resources->crtcs[j] == crtcId));
+
+            if (isPossible && isAvailable && isBestChoice) {
                 return j;
+            } else if (isPossible && isAvailable) {
+                candidate = j;
+            }
         }
     }
 
-    return -1;
+    return candidate;
 }
 
 static const char * const connector_type_names[] = { // must match DRM_MODE_CONNECTOR_*