drm_hwcomposer: HWC3: Reduce graphics memory consumption

Implement [1].

[1]: https://source.android.com/docs/core/graphics/reduce-consumption
Change-Id: I28436aae3e1ce0be296536366a6961ef91d5f58b
Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>

hwc3/ComposerClient.cpp

diff --git a/hwc3/ComposerClient.cpp b/hwc3/ComposerClient.cpp
index 2a7a1f6..c8cb37c 100644
--- a/hwc3/ComposerClient.cpp
+++ b/hwc3/ComposerClient.cpp
@@ -425,6 +425,24 @@
     return lp;
   }
 
+  [[maybe_unused]]
+  auto HandleClearSlot(int32_t slot_id)
+      -> std::optional<HwcLayer::LayerProperties> {
+    if (slots_.count(slot_id) == 0) {
+      return std::nullopt;
+    }
+
+    slots_.erase(slot_id);
+
+    auto lp = HwcLayer::LayerProperties{};
+    lp.slot_buffer = {
+        .slot_id = slot_id,
+        .bi = std::nullopt,
+    };
+
+    return lp;
+  }
+
  private:
   std::map<int32_t /*slot*/, std::shared_ptr<Hwc3BufferHandle>> slots_;
 };
@@ -555,6 +573,22 @@
     return;
   }
 
+#if __ANDROID_API__ >= 34
+  /* https://source.android.com/docs/core/graphics/reduce-consumption */
+  if (command.bufferSlotsToClear) {
+    auto hwc3_layer = GetHwc3Layer(*layer);
+    for (const auto& slot : *command.bufferSlotsToClear) {
+      auto lp = hwc3_layer->HandleClearSlot(slot);
+      if (!lp) {
+        cmd_result_writer_->AddError(hwc3::Error::kBadLayer);
+        return;
+      }
+
+      layer->SetLayerProperties(lp.value());
+    }
+  }
+#endif
+
   HwcLayer::LayerProperties properties;
   if (command.buffer) {
     auto hwc3_layer = GetHwc3Layer(*layer);