commit dd21494e0099851d6632ffaa3719da411311695d
author Roman Stratiienko <roman.o.stratiienko@globallogic.com> Tue May 03 18:24:49 2022 +0300
committer Roman Stratiienko <roman.o.stratiienko@globallogic.com> Tue May 10 11:17:32 2022 +0300
tree c7cd6882fe5e0fb0bfa1d9db0d05dc7645bb54e4
parent 5f21dbc6f381d54b483a904799d850f6f9571a8e

drm_hwcomposer: Fix HwcLayer::GetReleaseFences()

GetReleaseFences() should return release fence for the prior buffer
(not for the one assigned to layer at the moment of GetReleaseFences call).

Once not provided, old front buffer can be damaged before new buffer
presented. (Such issues start to appear once we started using
non-blocking DRM/KMS commits).

Using present (out) fence is a perfect solution, since it is
signaled once old buffer replaced with the new one.

Signed-off-by: Roman Stratiienko <roman.o.stratiienko@globallogic.com>

hwc2_device/HwcDisplay.cpp

diff --git a/hwc2_device/HwcDisplay.cpp b/hwc2_device/HwcDisplay.cpp
index 3956fc9..d94c9bc 100644
--- a/hwc2_device/HwcDisplay.cpp
+++ b/hwc2_device/HwcDisplay.cpp
@@ -391,6 +391,9 @@
 
 /* Find API details at:
  * https://cs.android.com/android/platform/superproject/+/android-11.0.0_r3:hardware/libhardware/include/hardware/hwcomposer2.h;l=1767
+ *
+ * Called after PresentDisplay(), CLIENT is expecting release fence for the
+ * prior buffer (not the one assigned to the layer at the moment).
  */
 HWC2::Error HwcDisplay::GetReleaseFences(uint32_t *num_elements,
                                          hwc2_layer_t *layers,
@@ -402,8 +405,13 @@
 
   uint32_t num_layers = 0;
 
-  for (std::pair<const hwc2_layer_t, HwcLayer> &l : layers_) {
+  for (auto &l : layers_) {
+    if (!l.second.GetPriorBufferScanOutFlag() || !present_fence_) {
+      continue;
+    }
+
     ++num_layers;
+
     if (layers == nullptr || fences == nullptr)
       continue;
 
@@ -413,9 +421,10 @@
     }
 
     layers[num_layers - 1] = l.first;
-    fences[num_layers - 1] = l.second.GetReleaseFence().Release();
+    fences[num_layers - 1] = UniqueFd::Dup(present_fence_.Get()).Release();
   }
   *num_elements = num_layers;
+
   return HWC2::Error::None;
 }
 
@@ -520,9 +529,9 @@
 /* Find API details at:
  * https://cs.android.com/android/platform/superproject/+/android-11.0.0_r3:hardware/libhardware/include/hardware/hwcomposer2.h;l=1805
  */
-HWC2::Error HwcDisplay::PresentDisplay(int32_t *present_fence) {
+HWC2::Error HwcDisplay::PresentDisplay(int32_t *out_present_fence) {
   if (IsInHeadlessMode()) {
-    *present_fence = -1;
+    *out_present_fence = -1;
     return HWC2::Error::None;
   }
   HWC2::Error ret{};
@@ -537,13 +546,14 @@
 
   if (ret == HWC2::Error::BadLayer) {
     // Can we really have no client or device layers?
-    *present_fence = -1;
+    *out_present_fence = -1;
     return HWC2::Error::None;
   }
   if (ret != HWC2::Error::None)
     return ret;
 
-  *present_fence = a_args.out_fence.Release();
+  this->present_fence_ = UniqueFd::Dup(a_args.out_fence.Get());
+  *out_present_fence = a_args.out_fence.Release();
 
   ++frame_no_;
   return HWC2::Error::None;
@@ -690,6 +700,16 @@
     *num_types = *num_requests = 0;
     return HWC2::Error::None;
   }
+
+  /* In current drm_hwc design in case previous frame layer was not validated as
+   * a CLIENT, it is used by display controller (Front buffer). We have to store
+   * this state to provide the CLIENT with the release fences for such buffers.
+   */
+  for (auto &l : layers_) {
+    l.second.SetPriorBufferScanOutFlag(l.second.GetValidatedType() !=
+                                       HWC2::Composition::Client);
+  }
+
   return backend_->ValidateDisplay(this, num_types, num_requests);
 }