drm_hwcomposer: Make flattening thread-based instead of vsync-based

Using vsync means consume some CPU time every frame for
IRQ -> Kernel_Thread -> UserSpace_listener thread transitions.

Framework tries to reduce vsync usage to the minimum, by using timelines
instead.

New flattening controller thread wakes-up only once per second.

This commit also removes flattening info from dumpsys. Practice shows
that it is almost useless. Instead debugging can be done using ALOGV
dumps.

Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>

compositor/FlatteningController.cpp

@@ -0,0 +1,98 @@
+/*
+ * Copyright (C) 2023 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/*
+ * Usually, display controllers do not use intermediate buffer for composition
+ * results. Instead, they scan-out directly from the input buffers, composing
+ * the planes on the fly every VSYNC.
+ *
+ * Flattening is a technique that reduces memory bandwidth and power consumption
+ * by converting non-updating multi-plane composition into a single-plane.
+ * Additionally, flattening also makes more shared planes available for use by
+ * other CRTCs.
+ *
+ * If the client is not updating layers for 1 second, FlatCon triggers a
+ * callback to refresh the screen. The compositor should mark all layers to be
+ * composed by the client into a single framebuffer using GPU.
+ */
+
+#define LOG_TAG "hwc-flatcon"
+
+#include "FlatteningController.h"
+
+#include "utils/log.h"
+
+namespace android {
+
+auto FlatteningController::CreateInstance(FlatConCallbacks &cbks)
+    -> std::shared_ptr<FlatteningController> {
+  auto fc = std::shared_ptr<FlatteningController>(new FlatteningController());
+
+  fc->cbks_ = cbks;
+
+  std::thread(&FlatteningController::ThreadFn, fc).detach();
+
+  return fc;
+}
+
+/* Compositor should call this every frame */
+bool FlatteningController::NewFrame() {
+  bool wake_it = false;
+  auto lock = std::lock_guard<std::mutex>(mutex_);
+
+  if (flatten_next_frame_) {
+    flatten_next_frame_ = false;
+    return true;
+  }
+
+  sleep_until_ = std::chrono::system_clock::now() + kTimeout;
+  if (disabled_) {
+    wake_it = true;
+    disabled_ = false;
+  }
+
+  if (wake_it)
+    cv_.notify_all();
+
+  return false;
+}
+
+void FlatteningController::ThreadFn(
+    const std::shared_ptr<FlatteningController> &fc) {
+  for (;;) {
+    std::unique_lock<std::mutex> lock(fc->mutex_);
+    if (fc.use_count() == 1 || !fc->cbks_.trigger)
+      break;
+
+    if (fc->sleep_until_ <= std::chrono::system_clock::now() &&
+        !fc->disabled_) {
+      fc->disabled_ = true;
+      fc->flatten_next_frame_ = true;
+      ALOGV("Timeout. Sending an event to compositor");
+      fc->cbks_.trigger();
+    }
+
+    if (fc->disabled_) {
+      ALOGV("Wait");
+      fc->cv_.wait(lock);
+    } else {
+      ALOGV("Wait_until");
+      fc->cv_.wait_until(lock, fc->sleep_until_);
+    }
+  }
+}
+
+}  // namespace android

compositor/FlatteningController.h

@@ -0,0 +1,71 @@
+/*
+ * Copyright (C) 2023 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <chrono>
+#include <condition_variable>
+#include <functional>
+#include <thread>
+
+namespace android {
+
+// NOLINTNEXTLINE(misc-unused-using-decls): False positive
+using std::chrono_literals::operator""s;
+
+struct FlatConCallbacks {
+  std::function<void()> trigger;
+};
+
+class FlatteningController {
+ public:
+  static auto CreateInstance(FlatConCallbacks &cbks)
+      -> std::shared_ptr<FlatteningController>;
+
+  void Disable() {
+    auto lock = std::lock_guard<std::mutex>(mutex_);
+    flatten_next_frame_ = false;
+    disabled_ = true;
+  }
+
+  /* Compositor should call this every frame */
+  bool NewFrame();
+
+  auto ShouldFlatten() const {
+    return flatten_next_frame_;
+  }
+
+  void StopThread() {
+    auto lock = std::lock_guard<std::mutex>(mutex_);
+    cbks_ = {};
+    cv_.notify_all();
+  }
+
+  static constexpr auto kTimeout = 1s;
+
+ private:
+  FlatteningController() = default;
+  static void ThreadFn(const std::shared_ptr<FlatteningController> &fc);
+
+  bool flatten_next_frame_{};
+  bool disabled_{};
+  decltype(std::chrono::system_clock::now()) sleep_until_{};
+  std::mutex mutex_;
+  std::condition_variable cv_;
+  FlatConCallbacks cbks_;
+};
+
+}  // namespace android