SF: add VSyncDispatch
Add a low level timer primitive to arrange for calling a
callback relative to an anticipated vsync time.
This defines vsync offsets in terms of time remaining until the
targeted vsync event, which is a rate independent definition.
Bug: 140301853
Test: 31 new unit tests
Change-Id: I6ac8f05e4c2107755493b2709551773d0e79c17d
diff --git a/services/surfaceflinger/Scheduler/TimeKeeper.h b/services/surfaceflinger/Scheduler/TimeKeeper.h
new file mode 100644
index 0000000..699cd50
--- /dev/null
+++ b/services/surfaceflinger/Scheduler/TimeKeeper.h
@@ -0,0 +1,53 @@
+/*
+ * Copyright 2019 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 <utils/Timers.h>
+#include <functional>
+
+namespace android::scheduler {
+
+/*
+ * TimeKeeper is the interface for a single-shot timer primitive.
+ */
+class TimeKeeper {
+public:
+ virtual ~TimeKeeper();
+
+ /*
+ * Arms callback to fired in time nanoseconds.
+ * There is only one timer, and subsequent calls will reset the callback function and the time.
+ */
+ virtual void alarmIn(std::function<void()> const& callback, nsecs_t time) = 0;
+
+ /*
+ * Cancels an existing pending callback
+ */
+ virtual void alarmCancel() = 0;
+
+ /*
+ * Returns the SYSTEM_TIME_MONOTONIC, used by testing infra to stub time.
+ */
+ virtual nsecs_t now() const = 0;
+
+protected:
+ TimeKeeper(TimeKeeper const&) = delete;
+ TimeKeeper& operator=(TimeKeeper const&) = delete;
+ TimeKeeper() = default;
+};
+
+} // namespace android::scheduler
diff --git a/services/surfaceflinger/Scheduler/VSyncDispatch.cpp b/services/surfaceflinger/Scheduler/VSyncDispatch.cpp
new file mode 100644
index 0000000..c9b2e77
--- /dev/null
+++ b/services/surfaceflinger/Scheduler/VSyncDispatch.cpp
@@ -0,0 +1,285 @@
+/*
+ * Copyright 2019 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.
+ */
+
+#define ATRACE_TAG ATRACE_TAG_GRAPHICS
+#include <utils/Trace.h>
+#include <vector>
+
+#include "TimeKeeper.h"
+#include "VSyncDispatch.h"
+#include "VSyncTracker.h"
+
+namespace android::scheduler {
+
+VSyncTracker::~VSyncTracker() = default;
+TimeKeeper::~TimeKeeper() = default;
+
+impl::VSyncDispatchEntry::VSyncDispatchEntry(std::string const& name,
+ std::function<void(nsecs_t)> const& cb)
+ : mName(name), mCallback(cb), mWorkDuration(0), mEarliestVsync(0) {}
+
+std::optional<nsecs_t> impl::VSyncDispatchEntry::lastExecutedVsyncTarget() const {
+ return mLastDispatchTime;
+}
+
+std::string_view impl::VSyncDispatchEntry::name() const {
+ return mName;
+}
+
+std::optional<nsecs_t> impl::VSyncDispatchEntry::wakeupTime() const {
+ if (!mArmedInfo) {
+ return {};
+ }
+ return {mArmedInfo->mActualWakeupTime};
+}
+
+nsecs_t impl::VSyncDispatchEntry::schedule(nsecs_t workDuration, nsecs_t earliestVsync,
+ VSyncTracker& tracker, nsecs_t now) {
+ mWorkDuration = workDuration;
+ mEarliestVsync = earliestVsync;
+ arm(tracker, now);
+ return mArmedInfo->mActualWakeupTime;
+}
+
+void impl::VSyncDispatchEntry::update(VSyncTracker& tracker, nsecs_t now) {
+ if (!mArmedInfo) {
+ return;
+ }
+ arm(tracker, now);
+}
+
+void impl::VSyncDispatchEntry::arm(VSyncTracker& tracker, nsecs_t now) {
+ auto const nextVsyncTime =
+ tracker.nextAnticipatedVSyncTimeFrom(std::max(mEarliestVsync, now + mWorkDuration));
+ mArmedInfo = {nextVsyncTime - mWorkDuration, nextVsyncTime};
+}
+
+void impl::VSyncDispatchEntry::disarm() {
+ mArmedInfo.reset();
+}
+
+nsecs_t impl::VSyncDispatchEntry::executing() {
+ mLastDispatchTime = mArmedInfo->mActualVsyncTime;
+ disarm();
+ return *mLastDispatchTime;
+}
+
+void impl::VSyncDispatchEntry::callback(nsecs_t t) {
+ {
+ std::lock_guard<std::mutex> lk(mRunningMutex);
+ mRunning = true;
+ }
+
+ mCallback(t);
+
+ std::lock_guard<std::mutex> lk(mRunningMutex);
+ mRunning = false;
+ mCv.notify_all();
+}
+
+void impl::VSyncDispatchEntry::ensureNotRunning() {
+ std::unique_lock<std::mutex> lk(mRunningMutex);
+ mCv.wait(lk, [this]() REQUIRES(mRunningMutex) { return !mRunning; });
+}
+
+VSyncDispatch::VSyncDispatch(std::unique_ptr<TimeKeeper> tk, VSyncTracker& tracker,
+ nsecs_t timerSlack)
+ : mTimeKeeper(std::move(tk)), mTracker(tracker), mTimerSlack(timerSlack) {}
+
+VSyncDispatch::~VSyncDispatch() {
+ std::lock_guard<decltype(mMutex)> lk(mMutex);
+ cancelTimer();
+}
+
+void VSyncDispatch::cancelTimer() {
+ mIntendedWakeupTime = kInvalidTime;
+ mTimeKeeper->alarmCancel();
+}
+
+void VSyncDispatch::setTimer(nsecs_t targetTime, nsecs_t now) {
+ mIntendedWakeupTime = targetTime;
+ mTimeKeeper->alarmIn(std::bind(&VSyncDispatch::timerCallback, this), targetTime - now);
+}
+
+void VSyncDispatch::rearmTimer(nsecs_t now) {
+ rearmTimerSkippingUpdateFor(now, mCallbacks.end());
+}
+
+void VSyncDispatch::rearmTimerSkippingUpdateFor(nsecs_t now,
+ CallbackMap::iterator const& skipUpdateIt) {
+ std::optional<nsecs_t> min;
+ for (auto it = mCallbacks.begin(); it != mCallbacks.end(); it++) {
+ auto& callback = it->second;
+ if (!callback->wakeupTime()) {
+ continue;
+ }
+
+ if (it != skipUpdateIt) {
+ callback->update(mTracker, now);
+ }
+ auto const wakeupTime = *callback->wakeupTime();
+ if (!min || (min && *min > wakeupTime)) {
+ min = wakeupTime;
+ }
+ }
+
+ if (min && (min < mIntendedWakeupTime)) {
+ setTimer(*min, now);
+ } else {
+ cancelTimer();
+ }
+}
+
+void VSyncDispatch::timerCallback() {
+ struct Invocation {
+ std::shared_ptr<impl::VSyncDispatchEntry> callback;
+ nsecs_t timestamp;
+ };
+ std::vector<Invocation> invocations;
+ {
+ std::lock_guard<decltype(mMutex)> lk(mMutex);
+ for (auto it = mCallbacks.begin(); it != mCallbacks.end(); it++) {
+ auto& callback = it->second;
+ auto const wakeupTime = callback->wakeupTime();
+ if (!wakeupTime) {
+ continue;
+ }
+
+ if (*wakeupTime < mIntendedWakeupTime + mTimerSlack) {
+ callback->executing();
+ invocations.emplace_back(
+ Invocation{callback, *callback->lastExecutedVsyncTarget()});
+ }
+ }
+
+ mIntendedWakeupTime = kInvalidTime;
+ rearmTimer(mTimeKeeper->now());
+ }
+
+ for (auto const& invocation : invocations) {
+ invocation.callback->callback(invocation.timestamp);
+ }
+}
+
+VSyncDispatch::CallbackToken VSyncDispatch::registerCallback(
+ std::function<void(nsecs_t)> const& callbackFn, std::string callbackName) {
+ std::lock_guard<decltype(mMutex)> lk(mMutex);
+ return CallbackToken{
+ mCallbacks
+ .emplace(++mCallbackToken,
+ std::make_shared<impl::VSyncDispatchEntry>(callbackName, callbackFn))
+ .first->first};
+}
+
+void VSyncDispatch::unregisterCallback(CallbackToken token) {
+ std::shared_ptr<impl::VSyncDispatchEntry> entry = nullptr;
+ {
+ std::lock_guard<decltype(mMutex)> lk(mMutex);
+ auto it = mCallbacks.find(token);
+ if (it != mCallbacks.end()) {
+ entry = it->second;
+ mCallbacks.erase(it);
+ }
+ }
+
+ if (entry) {
+ entry->ensureNotRunning();
+ }
+}
+
+ScheduleResult VSyncDispatch::schedule(CallbackToken token, nsecs_t workDuration,
+ nsecs_t earliestVsync) {
+ auto result = ScheduleResult::Error;
+ {
+ std::lock_guard<decltype(mMutex)> lk(mMutex);
+
+ auto it = mCallbacks.find(token);
+ if (it == mCallbacks.end()) {
+ return result;
+ }
+ auto& callback = it->second;
+ result = callback->wakeupTime() ? ScheduleResult::ReScheduled : ScheduleResult::Scheduled;
+
+ auto const now = mTimeKeeper->now();
+ auto const wakeupTime = callback->schedule(workDuration, earliestVsync, mTracker, now);
+
+ if (wakeupTime < now - mTimerSlack || callback->lastExecutedVsyncTarget() > wakeupTime) {
+ return ScheduleResult::CannotSchedule;
+ }
+
+ if (wakeupTime < mIntendedWakeupTime - mTimerSlack) {
+ rearmTimerSkippingUpdateFor(now, it);
+ }
+ }
+
+ return result;
+}
+
+CancelResult VSyncDispatch::cancel(CallbackToken token) {
+ std::lock_guard<decltype(mMutex)> lk(mMutex);
+
+ auto it = mCallbacks.find(token);
+ if (it == mCallbacks.end()) {
+ return CancelResult::Error;
+ }
+ auto& callback = it->second;
+
+ if (callback->wakeupTime()) {
+ callback->disarm();
+ mIntendedWakeupTime = kInvalidTime;
+ rearmTimer(mTimeKeeper->now());
+ return CancelResult::Cancelled;
+ }
+ return CancelResult::TooLate;
+}
+
+VSyncCallbackRegistration::VSyncCallbackRegistration(VSyncDispatch& dispatch,
+ std::function<void(nsecs_t)> const& callbackFn,
+ std::string const& callbackName)
+ : mDispatch(dispatch),
+ mToken(dispatch.registerCallback(callbackFn, callbackName)),
+ mValidToken(true) {}
+
+VSyncCallbackRegistration::VSyncCallbackRegistration(VSyncCallbackRegistration&& other)
+ : mDispatch(other.mDispatch),
+ mToken(std::move(other.mToken)),
+ mValidToken(std::move(other.mValidToken)) {
+ other.mValidToken = false;
+}
+
+VSyncCallbackRegistration& VSyncCallbackRegistration::operator=(VSyncCallbackRegistration&& other) {
+ mDispatch = std::move(other.mDispatch);
+ mToken = std::move(other.mToken);
+ mValidToken = std::move(other.mValidToken);
+ other.mValidToken = false;
+ return *this;
+}
+
+VSyncCallbackRegistration::~VSyncCallbackRegistration() {
+ if (mValidToken) mDispatch.get().unregisterCallback(mToken);
+}
+
+ScheduleResult VSyncCallbackRegistration::schedule(nsecs_t workDuration, nsecs_t earliestVsync) {
+ if (!mValidToken) return ScheduleResult::Error;
+ return mDispatch.get().schedule(mToken, workDuration, earliestVsync);
+}
+
+CancelResult VSyncCallbackRegistration::cancel() {
+ if (!mValidToken) return CancelResult::Error;
+ return mDispatch.get().cancel(mToken);
+}
+
+} // namespace android::scheduler
diff --git a/services/surfaceflinger/Scheduler/VSyncDispatch.h b/services/surfaceflinger/Scheduler/VSyncDispatch.h
new file mode 100644
index 0000000..0050495
--- /dev/null
+++ b/services/surfaceflinger/Scheduler/VSyncDispatch.h
@@ -0,0 +1,230 @@
+/*
+ * Copyright 2019 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 <android-base/thread_annotations.h>
+#include <utils/Timers.h>
+#include <functional>
+#include <memory>
+#include <mutex>
+#include <string>
+#include <string_view>
+#include <unordered_map>
+
+#include "StrongTyping.h"
+
+namespace android::scheduler {
+class TimeKeeper;
+class VSyncTracker;
+
+enum class ScheduleResult { Scheduled, ReScheduled, CannotSchedule, Error };
+enum class CancelResult { Cancelled, TooLate, Error };
+
+namespace impl {
+
+// VSyncDispatchEntry is a helper class representing internal state for each entry in VSyncDispatch
+// hoisted to public for unit testing.
+class VSyncDispatchEntry {
+public:
+ // This is the state of the entry. There are 3 states, armed, running, disarmed.
+ // Valid transition: disarmed -> armed ( when scheduled )
+ // Valid transition: armed -> running -> disarmed ( when timer is called)
+ // Valid transition: armed -> disarmed ( when cancelled )
+ VSyncDispatchEntry(std::string const& name, std::function<void(nsecs_t)> const& fn);
+ std::string_view name() const;
+
+ // Start: functions that are not threadsafe.
+ // Return the last vsync time this callback was invoked.
+ std::optional<nsecs_t> lastExecutedVsyncTarget() const;
+
+ // This moves the state from disarmed->armed and will calculate the wakeupTime.
+ nsecs_t schedule(nsecs_t workDuration, nsecs_t earliestVsync, VSyncTracker& tracker,
+ nsecs_t now);
+ // This will update armed entries with the latest vsync information. Entry remains armed.
+ void update(VSyncTracker& tracker, nsecs_t now);
+
+ // This will return empty if not armed, or the next calculated wakeup time if armed.
+ // It will not update the wakeupTime.
+ std::optional<nsecs_t> wakeupTime() const;
+
+ // This moves state from armed->disarmed.
+ void disarm();
+
+ // This moves the state from armed->running.
+ // Store the timestamp that this was intended for as the last called timestamp.
+ nsecs_t executing();
+ // End: functions that are not threadsafe.
+
+ // Invoke the callback with the timestamp, moving the state from running->disarmed.
+ void callback(nsecs_t timestamp);
+ // Block calling thread while the callback is executing.
+ void ensureNotRunning();
+
+private:
+ void arm(VSyncTracker& tracker, nsecs_t now);
+ std::string const mName;
+ std::function<void(nsecs_t)> const mCallback;
+
+ nsecs_t mWorkDuration;
+ nsecs_t mEarliestVsync;
+
+ struct ArmingInfo {
+ nsecs_t mActualWakeupTime;
+ nsecs_t mActualVsyncTime;
+ };
+ std::optional<ArmingInfo> mArmedInfo;
+ std::optional<nsecs_t> mLastDispatchTime;
+
+ std::mutex mRunningMutex;
+ std::condition_variable mCv;
+ bool mRunning GUARDED_BY(mRunningMutex) = false;
+};
+
+} // namespace impl
+
+/*
+ * VSyncDispatch is a class that will dispatch callbacks relative to system vsync events.
+ */
+class VSyncDispatch {
+public:
+ using CallbackToken = StrongTyping<size_t, class CallbackTokenTag, Compare>;
+
+ /* creates a VsyncDispatch.
+ * \param [in] a timekeeper object for dispatching events.
+ * \param [in] a tracker object that is monitoring expected vsync events.
+ * \param [in] a tunable in nanoseconds that indicates when events that fall close together
+ * should be dispatched in one timer wakeup.
+ */
+ explicit VSyncDispatch(std::unique_ptr<TimeKeeper> tk, VSyncTracker& tracker,
+ nsecs_t timerSlack);
+ ~VSyncDispatch();
+
+ /*
+ * Registers a callback that will be called at designated points on the vsync timeline.
+ * The callback can be scheduled, rescheduled targeting vsync times, or cancelled.
+ * The token returned must be cleaned up via unregisterCallback.
+ *
+ * \param [in] callbackFn A function to schedule for callback. The resources needed to invoke
+ * callbackFn must have lifetimes encompassing the lifetime of the
+ * CallbackToken returned.
+ * \param [in] callbackName A human-readable, unique name to identify the callback.
+ * \return A token that can be used to schedule, reschedule, or cancel the
+ * invocation of callbackFn.
+ *
+ */
+ CallbackToken registerCallback(std::function<void(nsecs_t)> const& callbackFn,
+ std::string callbackName);
+
+ /*
+ * Unregisters a callback.
+ *
+ * \param [in] token The callback to unregister.
+ *
+ */
+ void unregisterCallback(CallbackToken token);
+
+ /*
+ * Schedules the registered callback to be dispatched.
+ *
+ * The callback will be dispatched at 'workDuration' nanoseconds before a vsync event.
+ *
+ * The caller designates the earliest vsync event that should be targeted by the earliestVsync
+ * parameter.
+ * The callback will be scheduled at (workDuration - predictedVsync), where predictedVsync
+ * is the first vsync event time where ( predictedVsync >= earliestVsync ).
+ *
+ * If (workDuration - earliestVsync) is in the past, or if a callback has already been
+ * dispatched for the predictedVsync, an error will be returned.
+ *
+ * It is valid to reschedule a callback to a different time.
+ *
+ * \param [in] token The callback to schedule.
+ * \param [in] workDuration The time before the actual vsync time to invoke the callback
+ * associated with token.
+ * \param [in] earliestVsync The targeted display time. This will be snapped to the closest
+ * predicted vsync time after earliestVsync.
+ * \return A ScheduleResult::Scheduled if callback was scheduled.
+ * A ScheduleResult::ReScheduled if callback was rescheduled.
+ * A ScheduleResult::CannotSchedule
+ * if (workDuration - earliestVsync) is in the past, or
+ * if a callback was dispatched for the predictedVsync already.
+ * A ScheduleResult::Error if there was another error.
+ */
+ ScheduleResult schedule(CallbackToken token, nsecs_t workDuration, nsecs_t earliestVsync);
+
+ /* Cancels a scheduled callback, if possible.
+ *
+ * \param [in] token The callback to cancel.
+ * \return A CancelResult::TooLate if the callback was already dispatched.
+ * A CancelResult::Cancelled if the callback was successfully cancelled.
+ * A CancelResult::Error if there was an pre-condition violation.
+ */
+ CancelResult cancel(CallbackToken token);
+
+private:
+ VSyncDispatch(VSyncDispatch const&) = delete;
+ VSyncDispatch& operator=(VSyncDispatch const&) = delete;
+
+ using CallbackMap = std::unordered_map<size_t, std::shared_ptr<impl::VSyncDispatchEntry>>;
+
+ void timerCallback();
+ void setTimer(nsecs_t, nsecs_t) REQUIRES(mMutex);
+ void rearmTimer(nsecs_t now) REQUIRES(mMutex);
+ void rearmTimerSkippingUpdateFor(nsecs_t now, CallbackMap::iterator const& skipUpdate)
+ REQUIRES(mMutex);
+ void cancelTimer() REQUIRES(mMutex);
+
+ static constexpr nsecs_t kInvalidTime = std::numeric_limits<int64_t>::max();
+ std::unique_ptr<TimeKeeper> const mTimeKeeper;
+ VSyncTracker& mTracker;
+ nsecs_t const mTimerSlack;
+
+ std::mutex mutable mMutex;
+ size_t mCallbackToken GUARDED_BY(mMutex) = 0;
+
+ CallbackMap mCallbacks GUARDED_BY(mMutex);
+ nsecs_t mIntendedWakeupTime GUARDED_BY(mMutex) = kInvalidTime;
+};
+
+/*
+ * Helper class to operate on registered callbacks. It is up to user of the class to ensure
+ * that VsyncDispatch lifetime exceeds the lifetime of VSyncCallbackRegistation.
+ */
+class VSyncCallbackRegistration {
+public:
+ VSyncCallbackRegistration(VSyncDispatch&, std::function<void(nsecs_t)> const& callbackFn,
+ std::string const& callbackName);
+ VSyncCallbackRegistration(VSyncCallbackRegistration&&);
+ VSyncCallbackRegistration& operator=(VSyncCallbackRegistration&&);
+ ~VSyncCallbackRegistration();
+
+ // See documentation for VSyncDispatch::schedule.
+ ScheduleResult schedule(nsecs_t workDuration, nsecs_t earliestVsync);
+
+ // See documentation for VSyncDispatch::cancel.
+ CancelResult cancel();
+
+private:
+ VSyncCallbackRegistration(VSyncCallbackRegistration const&) = delete;
+ VSyncCallbackRegistration& operator=(VSyncCallbackRegistration const&) = delete;
+
+ std::reference_wrapper<VSyncDispatch> mDispatch;
+ VSyncDispatch::CallbackToken mToken;
+ bool mValidToken;
+};
+
+} // namespace android::scheduler
diff --git a/services/surfaceflinger/Scheduler/VSyncTracker.h b/services/surfaceflinger/Scheduler/VSyncTracker.h
new file mode 100644
index 0000000..97b9620
--- /dev/null
+++ b/services/surfaceflinger/Scheduler/VSyncTracker.h
@@ -0,0 +1,56 @@
+/*
+ * Copyright 2019 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 <utils/Timers.h>
+#include "VSyncDispatch.h"
+
+namespace android::scheduler {
+/*
+ * VSyncTracker is an interface for providing estimates on future Vsync signal times based on
+ * historical vsync timing data.
+ */
+class VSyncTracker {
+public:
+ virtual ~VSyncTracker();
+
+ /*
+ * Adds a known timestamp from a vsync timing source (HWVsync signal, present fence)
+ * to the model.
+ *
+ * \param [in] timestamp The timestamp when the vsync signal was.
+ */
+ virtual void addVsyncTimestamp(nsecs_t timestamp) = 0;
+
+ /*
+ * Access the next anticipated vsync time such that the anticipated time >= timePoint.
+ * This will always give the best accurate at the time of calling; multiple
+ * calls with the same timePoint might give differing values if the internal model
+ * is updated.
+ *
+ * \param [in] timePoint The point in time after which to estimate a vsync event.
+ * \return A prediction of the timestamp of a vsync event.
+ */
+ virtual nsecs_t nextAnticipatedVSyncTimeFrom(nsecs_t timePoint) const = 0;
+
+protected:
+ VSyncTracker(VSyncTracker const&) = delete;
+ VSyncTracker& operator=(VSyncTracker const&) = delete;
+ VSyncTracker() = default;
+};
+
+} // namespace android::scheduler