ebiten/audio/internal/oboe/oboe_common_StabilizedCallback_android.cpp

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/*
* Copyright (C) 2018 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.
*/
#include "oboe_oboe_StabilizedCallback_android.h"
#include "oboe_common_AudioClock_android.h"
#include "oboe_common_Trace_android.h"
constexpr int32_t kLoadGenerationStepSizeNanos = 20000;
constexpr float kPercentageOfCallbackToUse = 0.8;
using namespace oboe;
StabilizedCallback::StabilizedCallback(AudioStreamCallback *callback) : mCallback(callback){
Trace::initialize();
}
/**
* An audio callback which attempts to do work for a fixed amount of time.
*
* @param oboeStream
* @param audioData
* @param numFrames
* @return
*/
DataCallbackResult
StabilizedCallback::onAudioReady(AudioStream *oboeStream, void *audioData, int32_t numFrames) {
int64_t startTimeNanos = AudioClock::getNanoseconds();
if (mFrameCount == 0){
mEpochTimeNanos = startTimeNanos;
}
int64_t durationSinceEpochNanos = startTimeNanos - mEpochTimeNanos;
// In an ideal world the callback start time will be exactly the same as the duration of the
// frames already read/written into the stream. In reality the callback can start early
// or late. By finding the delta we can calculate the target duration for our stabilized
// callback.
int64_t idealStartTimeNanos = (mFrameCount * kNanosPerSecond) / oboeStream->getSampleRate();
int64_t lateStartNanos = durationSinceEpochNanos - idealStartTimeNanos;
if (lateStartNanos < 0){
// This was an early start which indicates that our previous epoch was a late callback.
// Update our epoch to this more accurate time.
mEpochTimeNanos = startTimeNanos;
mFrameCount = 0;
}
int64_t numFramesAsNanos = (numFrames * kNanosPerSecond) / oboeStream->getSampleRate();
int64_t targetDurationNanos = static_cast<int64_t>(
(numFramesAsNanos * kPercentageOfCallbackToUse) - lateStartNanos);
Trace::beginSection("Actual load");
DataCallbackResult result = mCallback->onAudioReady(oboeStream, audioData, numFrames);
Trace::endSection();
int64_t executionDurationNanos = AudioClock::getNanoseconds() - startTimeNanos;
int64_t stabilizingLoadDurationNanos = targetDurationNanos - executionDurationNanos;
Trace::beginSection("Stabilized load for %lldns", stabilizingLoadDurationNanos);
generateLoad(stabilizingLoadDurationNanos);
Trace::endSection();
// Wraparound: At 48000 frames per second mFrameCount wraparound will occur after 6m years,
// significantly longer than the average lifetime of an Android phone.
mFrameCount += numFrames;
return result;
}
void StabilizedCallback::generateLoad(int64_t durationNanos) {
int64_t currentTimeNanos = AudioClock::getNanoseconds();
int64_t deadlineTimeNanos = currentTimeNanos + durationNanos;
// opsPerStep gives us an estimated number of operations which need to be run to fully utilize
// the CPU for a fixed amount of time (specified by kLoadGenerationStepSizeNanos).
// After each step the opsPerStep value is re-calculated based on the actual time taken to
// execute those operations.
auto opsPerStep = (int)(mOpsPerNano * kLoadGenerationStepSizeNanos);
int64_t stepDurationNanos = 0;
int64_t previousTimeNanos = 0;
while (currentTimeNanos <= deadlineTimeNanos){
for (int i = 0; i < opsPerStep; i++) cpu_relax();
previousTimeNanos = currentTimeNanos;
currentTimeNanos = AudioClock::getNanoseconds();
stepDurationNanos = currentTimeNanos - previousTimeNanos;
// Calculate exponential moving average to smooth out values, this acts as a low pass filter.
// @see https://en.wikipedia.org/wiki/Moving_average#Exponential_moving_average
static const float kFilterCoefficient = 0.1;
auto measuredOpsPerNano = (double) opsPerStep / stepDurationNanos;
mOpsPerNano = kFilterCoefficient * measuredOpsPerNano + (1.0 - kFilterCoefficient) * mOpsPerNano;
opsPerStep = (int) (mOpsPerNano * kLoadGenerationStepSizeNanos);
}
}