kms: Process impl idle callbacks before pre dispatch flush

mode_set_fallback() schedules a call to mode_set_fallback_feedback_idle(), but
it is possible for Mutter to repaint before the idle callbacks are dispatched.
If that happens, mode_set_fallback_feedback_idle() does not get called before
Mutter enters wait_for_pending_flips(), leading to a deadlock.

Add the needed interfaces so that meta_kms_device_dispatch_sync() can flush all
the implementation idle callbacks before it checks if any "events" are
available. This prevents the deadlock by ensuring
mode_set_fallback_feedback_idle() does get called before potentially waiting
for actual DRM events.

Presumably this call would not be needed if the implementation was running in
its own thread, since it would eventually dispatch its idle callbacks before
going to sleep polling on the DRM fd. This call might even be unnecessary
overhead in that case, synchronizing with the implementation thread needlessly.
But the thread does not exist yet, so this is needed for now.

This is part 1 of 2 fixing a complete desktop freeze when drmModePageFlip()
fails with EINVAL and the fallback to drmModeSetCrtc() succeeds but the success
is not registered correctly as completed "flip". The freeze occurs under
wait_for_pending_flips() which calls down into meta_kms_impl_device_dispatch()
which ends up poll()'ing the DRM fd even though drmModeSetCrtc() will not
produce a DRM event, hence the poll() never returns. The freeze was observed
when hotplugging a DisplayLink dock for the first time on Ubuntu 19.10.

CC stable: gnome-3-34

https://gitlab.gnome.org/GNOME/mutter/merge_requests/953

src/backends/native/meta-kms-device.c

@@ -1,5 +1,6 @@
 /*
  * Copyright (C) 2019 Red Hat
+ * Copyright (C) 2019 DisplayLink (UK) Ltd.
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License as
@@ -149,12 +150,28 @@ dispatch_in_impl (MetaKmsImpl  *impl,
   return meta_kms_impl_device_dispatch (impl_device, error);
 }
 
+static gboolean
+dispatch_idle_in_impl (MetaKmsImpl  *impl,
+                       gpointer      user_data,
+                       GError      **error)
+{
+  meta_kms_impl_dispatch_idle (impl);
+
+  return TRUE;
+}
+
 int
 meta_kms_device_dispatch_sync (MetaKmsDevice  *device,
                                GError        **error)
 {
   int callback_count;
 
+  if (!meta_kms_run_impl_task_sync (device->kms,
+                                    dispatch_idle_in_impl,
+                                    device->impl_device,
+                                    error))
+    return -1;
+
   callback_count = meta_kms_flush_callbacks (device->kms);
   if (callback_count > 0)
     return TRUE;

src/backends/native/meta-kms-impl-simple.c

@@ -1,5 +1,6 @@
 /*
  * Copyright (C) 2018-2019 Red Hat
+ * Copyright (C) 2019 DisplayLink (UK) Ltd.
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License as
@@ -806,6 +807,15 @@ meta_kms_impl_simple_discard_pending_page_flips (MetaKmsImpl *impl)
                    g_source_destroy);
 }
 
+static void
+meta_kms_impl_simple_dispatch_idle (MetaKmsImpl *impl)
+{
+  MetaKmsImplSimple *impl_simple = META_KMS_IMPL_SIMPLE (impl);
+
+  if (impl_simple->mode_set_fallback_feedback_source)
+    mode_set_fallback_feedback_idle (impl_simple);
+}
+
 static void
 meta_kms_impl_simple_finalize (GObject *object)
 {
@@ -845,4 +855,5 @@ meta_kms_impl_simple_class_init (MetaKmsImplSimpleClass *klass)
   impl_class->process_update = meta_kms_impl_simple_process_update;
   impl_class->handle_page_flip_callback = meta_kms_impl_simple_handle_page_flip_callback;
   impl_class->discard_pending_page_flips = meta_kms_impl_simple_discard_pending_page_flips;
+  impl_class->dispatch_idle = meta_kms_impl_simple_dispatch_idle;
 }

src/backends/native/meta-kms-impl.c

@@ -1,5 +1,6 @@
 /*
  * Copyright (C) 2018 Red Hat
+ * Copyright (C) 2019 DisplayLink (UK) Ltd.
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License as
@@ -65,6 +66,12 @@ meta_kms_impl_discard_pending_page_flips (MetaKmsImpl *impl)
   META_KMS_IMPL_GET_CLASS (impl)->discard_pending_page_flips (impl);
 }
 
+void
+meta_kms_impl_dispatch_idle (MetaKmsImpl *impl)
+{
+  META_KMS_IMPL_GET_CLASS (impl)->dispatch_idle (impl);
+}
+
 static void
 meta_kms_impl_set_property (GObject      *object,
                             guint         prop_id,

src/backends/native/meta-kms-impl.h

@@ -1,5 +1,6 @@
 /*
  * Copyright (C) 2018 Red Hat
+ * Copyright (C) 2019 DisplayLink (UK) Ltd.
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License as
@@ -38,6 +39,7 @@ struct _MetaKmsImplClass
   void (* handle_page_flip_callback) (MetaKmsImpl         *impl,
                                       MetaKmsPageFlipData *page_flip_data);
   void (* discard_pending_page_flips) (MetaKmsImpl *impl);
+  void (* dispatch_idle) (MetaKmsImpl *impl);
 };
 
 MetaKms * meta_kms_impl_get_kms (MetaKmsImpl *impl);
@@ -51,4 +53,6 @@ void meta_kms_impl_handle_page_flip_callback (MetaKmsImpl         *impl,
 
 void meta_kms_impl_discard_pending_page_flips (MetaKmsImpl *impl);
 
+void meta_kms_impl_dispatch_idle (MetaKmsImpl *impl);
+
 #endif /* META_KMS_IMPL_H */