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* Copyright (C) 2012 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
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* See the License for the specific language governing permissions and
* limitations under the License.
#include <stddef.h>
#include <utils/Mutex.h>
#include <utils/Timers.h>
#include <utils/RefBase.h>
namespace android {
// Ignore present (retire) fences if the device doesn't have support for the
// sync framework, or if all phase offsets are zero. The latter is useful
// because it allows us to avoid resync bursts on devices that don't need
// phase-offset VSYNC events.
static const bool kIgnorePresentFences = true;
static const bool kIgnorePresentFences = false;
class String8;
class Fence;
class DispSyncThread;
// DispSync maintains a model of the periodic hardware-based vsync events of a
// display and uses that model to execute period callbacks at specific phase
// offsets from the hardware vsync events. The model is constructed by
// feeding consecutive hardware event timestamps to the DispSync object via
// the addResyncSample method.
// The model is validated using timestamps from Fence objects that are passed
// to the DispSync object via the addPresentFence method. These fence
// timestamps should correspond to a hardware vsync event, but they need not
// be consecutive hardware vsync times. If this method determines that the
// current model accurately represents the hardware event times it will return
// false to indicate that a resynchronization (via addResyncSample) is not
// needed.
class DispSync {
class Callback: public virtual RefBase {
virtual ~Callback() {};
virtual void onDispSyncEvent(nsecs_t when) = 0;
// reset clears the resync samples and error value.
void reset();
// addPresentFence adds a fence for use in validating the current vsync
// event model. The fence need not be signaled at the time
// addPresentFence is called. When the fence does signal, its timestamp
// should correspond to a hardware vsync event. Unlike the
// addResyncSample method, the timestamps of consecutive fences need not
// correspond to consecutive hardware vsync events.
// This method should be called with the retire fence from each HWComposer
// set call that affects the display.
bool addPresentFence(const sp<Fence>& fence);
// The beginResync, addResyncSample, and endResync methods are used to re-
// synchronize the DispSync's model to the hardware vsync events. The re-
// synchronization process involves first calling beginResync, then
// calling addResyncSample with a sequence of consecutive hardware vsync
// event timestamps, and finally calling endResync when addResyncSample
// indicates that no more samples are needed by returning false.
// This resynchronization process should be performed whenever the display
// is turned on (i.e. once immediately after it's turned on) and whenever
// addPresentFence returns true indicating that the model has drifted away
// from the hardware vsync events.
void beginResync();
bool addResyncSample(nsecs_t timestamp);
void endResync();
// The setPeriod method sets the vsync event model's period to a specific
// value. This should be used to prime the model when a display is first
// turned on. It should NOT be used after that.
void setPeriod(nsecs_t period);
// The getPeriod method returns the current vsync period.
nsecs_t getPeriod();
// setRefreshSkipCount specifies an additional number of refresh
// cycles to skip. For example, on a 60Hz display, a skip count of 1
// will result in events happening at 30Hz. Default is zero. The idea
// is to sacrifice smoothness for battery life.
void setRefreshSkipCount(int count);
// addEventListener registers a callback to be called repeatedly at the
// given phase offset from the hardware vsync events. The callback is
// called from a separate thread and it should return reasonably quickly
// (i.e. within a few hundred microseconds).
status_t addEventListener(nsecs_t phase, const sp<Callback>& callback);
// removeEventListener removes an already-registered event callback. Once
// this method returns that callback will no longer be called by the
// DispSync object.
status_t removeEventListener(const sp<Callback>& callback);
// computeNextRefresh computes when the next refresh is expected to begin.
// The periodOffset value can be used to move forward or backward; an
// offset of zero is the next refresh, -1 is the previous refresh, 1 is
// the refresh after next. etc.
nsecs_t computeNextRefresh(int periodOffset) const;
// dump appends human-readable debug info to the result string.
void dump(String8& result) const;
void updateModelLocked();
void updateErrorLocked();
void resetErrorLocked();
enum { MAX_RESYNC_SAMPLES = 32 };
// mPeriod is the computed period of the modeled vsync events in
// nanoseconds.
nsecs_t mPeriod;
// mPhase is the phase offset of the modeled vsync events. It is the
// number of nanoseconds from time 0 to the first vsync event.
nsecs_t mPhase;
// mError is the computed model error. It is based on the difference
// between the estimated vsync event times and those observed in the
// mPresentTimes array.
nsecs_t mError;
// These member variables are the state used during the resynchronization
// process to store information about the hardware vsync event times used
// to compute the model.
nsecs_t mResyncSamples[MAX_RESYNC_SAMPLES];
size_t mFirstResyncSample;
size_t mNumResyncSamples;
int mNumResyncSamplesSincePresent;
// These member variables store information about the present fences used
// to validate the currently computed model.
sp<Fence> mPresentFences[NUM_PRESENT_SAMPLES];
nsecs_t mPresentTimes[NUM_PRESENT_SAMPLES];
size_t mPresentSampleOffset;
int mRefreshSkipCount;
// mThread is the thread from which all the callbacks are called.
sp<DispSyncThread> mThread;
// mMutex is used to protect access to all member variables.
mutable Mutex mMutex;