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>

7 files changed, 215 insertions, 90 deletions

diff --git a/Android.bp b/Android.bp
index 1e78729..0e313ea 100644
--- a/Android.bp
+++ b/Android.bp
@@ -59,6 +59,7 @@ filegroup {
         "bufferinfo/BufferInfoMapperMetadata.cpp",
 
         "compositor/DrmKmsPlan.cpp",
+        "compositor/FlatteningController.cpp",
 
         "drm/DrmAtomicStateManager.cpp",
         "drm/DrmConnector.cpp",
diff --git a/backend/Backend.cpp b/backend/Backend.cpp
index f3156ed..3ca6e92 100644
--- a/backend/Backend.cpp
+++ b/backend/Backend.cpp
@@ -33,27 +33,36 @@ HWC2::Error Backend::ValidateDisplay(HwcDisplay *display, uint32_t *num_types,
   int client_start = -1;
   size_t client_size = 0;
 
-  if (display->ProcessClientFlatteningState(layers.size() <= 1)) {
-    display->total_stats().frames_flattened_++;
-    client_start = 0;
-    client_size = layers.size();
-    MarkValidated(layers, client_start, client_size);
-  } else {
-    std::tie(client_start, client_size) = GetClientLayers(display, layers);
+  auto flatcon = display->GetFlatCon();
+  if (flatcon) {
+    bool should_flatten = false;
+    if (layers.size() <= 1)
+      flatcon->Disable();
+    else
+      should_flatten = flatcon->NewFrame();
+
+    if (should_flatten) {
+      display->total_stats().frames_flattened_++;
+      MarkValidated(layers, 0, layers.size());
+      *num_types = layers.size();
+      return HWC2::Error::HasChanges;
+    }
+  }
 
-    MarkValidated(layers, client_start, client_size);
+  std::tie(client_start, client_size) = GetClientLayers(display, layers);
 
-    auto testing_needed = client_start != 0 || client_size != layers.size();
+  MarkValidated(layers, client_start, client_size);
 
-    AtomicCommitArgs a_args = {.test_only = true};
+  auto testing_needed = client_start != 0 || client_size != layers.size();
 
-    if (testing_needed &&
-        display->CreateComposition(a_args) != HWC2::Error::None) {
-      ++display->total_stats().failed_kms_validate_;
-      client_start = 0;
-      client_size = layers.size();
-      MarkValidated(layers, 0, client_size);
-    }
+  AtomicCommitArgs a_args = {.test_only = true};
+
+  if (testing_needed &&
+      display->CreateComposition(a_args) != HWC2::Error::None) {
+    ++display->total_stats().failed_kms_validate_;
+    client_start = 0;
+    client_size = layers.size();
+    MarkValidated(layers, 0, client_size);
   }
 
   *num_types = client_size;
diff --git a/compositor/FlatteningController.cpp b/compositor/FlatteningController.cpp
new file mode 100644
index 0000000..257f8a0
--- /dev/null
+++ b/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
diff --git a/compositor/FlatteningController.h b/compositor/FlatteningController.h
new file mode 100644
index 0000000..2be6ce4
--- /dev/null
+++ b/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
diff --git a/hwc2_device/HwcDisplay.cpp b/hwc2_device/HwcDisplay.cpp
index cdcf518..13da016 100644
--- a/hwc2_device/HwcDisplay.cpp
+++ b/hwc2_device/HwcDisplay.cpp
@@ -50,32 +50,12 @@ std::string HwcDisplay::DumpDelta(HwcDisplay::Stats delta) {
 }
 
 std::string HwcDisplay::Dump() {
-  std::string flattening_state_str;
-  switch (flattenning_state_) {
-    case ClientFlattenningState::Disabled:
-      flattening_state_str = "Disabled";
-      break;
-    case ClientFlattenningState::NotRequired:
-      flattening_state_str = "Not needed";
-      break;
-    case ClientFlattenningState::Flattened:
-      flattening_state_str = "Active";
-      break;
-    case ClientFlattenningState::ClientRefreshRequested:
-      flattening_state_str = "Refresh requested";
-      break;
-    default:
-      flattening_state_str = std::to_string(flattenning_state_) +
-                             " VSync remains";
-  }
-
   auto connector_name = IsInHeadlessMode()
                             ? std::string("NULL-DISPLAY")
                             : GetPipe().connector->Get()->GetName();
 
   std::stringstream ss;
   ss << "- Display on: " << connector_name << "\n"
-     << "  Flattening state: " << flattening_state_str << "\n"
      << "Statistics since system boot:\n"
      << DumpDelta(total_stats_) << "\n\n"
      << "Statistics since last dumpsys request:\n"
@@ -140,6 +120,10 @@ void HwcDisplay::Deinit() {
 
     current_plan_.reset();
     backend_.reset();
+    if (flatcon_) {
+      flatcon_->StopThread();
+      flatcon_.reset();
+    }
   }
 
   if (vsync_worker_) {
@@ -162,14 +146,10 @@ HWC2::Error HwcDisplay::Init() {
               GetDisplayVsyncPeriod(&period_ns);
               hwc2_->SendVsyncEventToClient(handle_, timestamp, period_ns);
             }
-            if (vsync_flattening_en_) {
-              ProcessFlatenningVsyncInternal();
-            }
             if (vsync_tracking_en_) {
               last_vsync_ts_ = timestamp;
             }
-            if (!vsync_event_en_ && !vsync_flattening_en_ &&
-                !vsync_tracking_en_) {
+            if (!vsync_event_en_ && !vsync_tracking_en_) {
               vsync_worker_->VSyncControl(false);
             }
           },
@@ -192,6 +172,13 @@ HWC2::Error HwcDisplay::Init() {
       ALOGE("Failed to set backend for d=%d %d\n", int(handle_), ret);
       return HWC2::Error::BadDisplay;
     }
+    auto flatcbk = (struct FlatConCallbacks){.trigger = [this]() {
+      if (hwc2_->refresh_callback_.first != nullptr &&
+          hwc2_->refresh_callback_.second != nullptr)
+        hwc2_->refresh_callback_.first(hwc2_->refresh_callback_.second,
+                                       handle_);
+    }};
+    flatcon_ = FlatteningController::CreateInstance(flatcbk);
   }
 
   client_layer_.SetLayerBlendMode(HWC2_BLEND_MODE_PREMULTIPLIED);
@@ -1022,37 +1009,4 @@ void HwcDisplay::set_backend(std::unique_ptr<Backend> backend) {
   backend_ = std::move(backend);
 }
 
-/* returns true if composition should be sent to client */
-bool HwcDisplay::ProcessClientFlatteningState(bool skip) {
-  const int flattenning_state = flattenning_state_;
-  if (flattenning_state == ClientFlattenningState::Disabled) {
-    return false;
-  }
-
-  if (skip) {
-    flattenning_state_ = ClientFlattenningState::NotRequired;
-    return false;
-  }
-
-  if (flattenning_state == ClientFlattenningState::ClientRefreshRequested) {
-    flattenning_state_ = ClientFlattenningState::Flattened;
-    return true;
-  }
-
-  vsync_flattening_en_ = true;
-  vsync_worker_->VSyncControl(true);
-  flattenning_state_ = ClientFlattenningState::VsyncCountdownMax;
-  return false;
-}
-
-void HwcDisplay::ProcessFlatenningVsyncInternal() {
-  if (flattenning_state_ > ClientFlattenningState::ClientRefreshRequested &&
-      --flattenning_state_ == ClientFlattenningState::ClientRefreshRequested &&
-      hwc2_->refresh_callback_.first != nullptr &&
-      hwc2_->refresh_callback_.second != nullptr) {
-    hwc2_->refresh_callback_.first(hwc2_->refresh_callback_.second, handle_);
-    vsync_flattening_en_ = false;
-  }
-}
-
 }  // namespace android
diff --git a/hwc2_device/HwcDisplay.h b/hwc2_device/HwcDisplay.h
index 653aa4a..bf95c3e 100644
--- a/hwc2_device/HwcDisplay.h
+++ b/hwc2_device/HwcDisplay.h
@@ -23,6 +23,7 @@
 #include <sstream>
 
 #include "HwcDisplayConfigs.h"
+#include "compositor/FlatteningController.h"
 #include "compositor/LayerData.h"
 #include "drm/DrmAtomicStateManager.h"
 #include "drm/ResourceManager.h"
@@ -165,10 +166,6 @@ class HwcDisplay {
     return total_stats_;
   }
 
-  /* returns true if composition should be sent to client */
-  bool ProcessClientFlatteningState(bool skip);
-  void ProcessFlatenningVsyncInternal();
-
   /* Headless mode required to keep SurfaceFlinger alive when all display are
    * disconnected, Without headless mode Android will continuously crash.
    * Only single internal (primary) display is required to be in HEADLESS mode
@@ -181,17 +178,11 @@ class HwcDisplay {
 
   void Deinit();
 
- private:
-  enum ClientFlattenningState : int32_t {
-    Disabled = -3,
-    NotRequired = -2,
-    Flattened = -1,
-    ClientRefreshRequested = 0,
-    VsyncCountdownMax = 60, /* 1 sec @ 60FPS */
-  };
-
-  std::atomic_int flattenning_state_{ClientFlattenningState::NotRequired};
+  auto GetFlatCon() {
+    return flatcon_;
+  }
 
+ private:
   HwcDisplayConfigs configs_;
 
   DrmHwcTwo *const hwc2_;
@@ -205,10 +196,10 @@ class HwcDisplay {
   DrmDisplayPipeline *pipeline_{};
 
   std::unique_ptr<Backend> backend_;
+  std::shared_ptr<FlatteningController> flatcon_;
 
   std::shared_ptr<VSyncWorker> vsync_worker_;
   bool vsync_event_en_{};
-  bool vsync_flattening_en_{};
   bool vsync_tracking_en_{};
   int64_t last_vsync_ts_{};
 
diff --git a/meson.build b/meson.build
index 4328a1e..c2e5fb6 100644
--- a/meson.build
+++ b/meson.build
@@ -11,6 +11,7 @@ inc_include = [include_directories('.')]
 
 src_common = files(
     'compositor/DrmKmsPlan.cpp',
+    'compositor/FlatteningController.cpp',
     'backend/BackendManager.cpp',
     'backend/Backend.cpp',
     'backend/BackendClient.cpp',