[mutter] kms-impl-simple: Handle mode set race conditions gracefully

[Jonas Ådahl <jadahl src gnome org>]

commit ce3409b2b7f887ca2e83c939ff73ba4d3bc8c4c0
Author: Jonas Ådahl <jadahl gmail com>
Date:   Tue Jan 14 11:16:09 2020 +0100

    kms-impl-simple: Handle mode set race conditions gracefully
    
    If we end up trying to do a mode set on a DRM state that has already
    changed behind our back without us yet having seen the hotplug event we
    may fail with `EINVAL`. Since the renderer layer doesn't handle mode set
    failure, it'll still try to page flip later on, which will then also
    fail. When failing, it'll try to look up the cached mode set in order to
    retry the mode set later on, as is needed to handle other error
    conditions. However, if the mode set prior to the page flip failed, we
    won't cache the mode set, and the page flip error handling code will get
    confused.
    
    Instead of asserting that a page flip always has a valid cached mode set
    ready to look up, handle it being missing more gracefully by failing to
    mode set. It is expected that things will correct themself as there
    should be a hotplug event waiting around the the corner, to reconfigure
    the monitor configuration setting new modes.
    
    Fixes: https://gitlab.gnome.org/GNOME/mutter/issues/917
    
    https://gitlab.gnome.org/GNOME/mutter/merge_requests/1007

 src/backends/native/meta-kms-impl-simple.c | 39 ++++++++++++++++++++++++------
 1 file changed, 31 insertions(+), 8 deletions(-)
---
diff --git a/src/backends/native/meta-kms-impl-simple.c b/src/backends/native/meta-kms-impl-simple.c
index c4dd31d38..fc8d2929e 100644
--- a/src/backends/native/meta-kms-impl-simple.c
+++ b/src/backends/native/meta-kms-impl-simple.c
@@ -324,6 +324,13 @@ retry_page_flip_data_free (RetryPageFlipData *retry_page_flip_data)
   g_free (retry_page_flip_data);
 }
 
+static CachedModeSet *
+get_cached_mode_set (MetaKmsImplSimple *impl_simple,
+                     MetaKmsCrtc       *crtc)
+{
+  return g_hash_table_lookup (impl_simple->cached_mode_sets, crtc);
+}
+
 static float
 get_cached_crtc_refresh_rate (MetaKmsImplSimple *impl_simple,
                               MetaKmsCrtc       *crtc)
@@ -645,14 +652,30 @@ process_page_flip (MetaKmsImpl      *impl,
 
   if (ret == -EBUSY)
     {
-      float refresh_rate;
-
-      refresh_rate = get_cached_crtc_refresh_rate (impl_simple, crtc);
-      schedule_retry_page_flip (impl_simple,
-                                crtc,
-                                plane_assignment->fb_id,
-                                refresh_rate,
-                                page_flip_data);
+      CachedModeSet *cached_mode_set;
+
+      cached_mode_set = get_cached_mode_set (impl_simple, crtc);
+      if (cached_mode_set)
+        {
+          drmModeModeInfo *drm_mode;
+          float refresh_rate;
+
+          drm_mode = cached_mode_set->drm_mode;
+          refresh_rate = meta_calculate_drm_mode_refresh_rate (drm_mode);
+          schedule_retry_page_flip (impl_simple,
+                                    crtc,
+                                    plane_assignment->fb_id,
+                                    refresh_rate,
+                                    page_flip_data);
+        }
+      else
+        {
+          g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
+                       "Page flip of %u failed, and no mode set available",
+                       meta_kms_crtc_get_id (crtc));
+          meta_kms_page_flip_data_unref (page_flip_data);
+          return FALSE;
+        }
     }
   else if (ret == -EINVAL)
     {