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Commit 26a922bd authored by Jonas Larsson's avatar Jonas Larsson
Browse files

Optimize and fix light sensor filter 

Optimization: Don't compute the sum of all samples in filter window every run. Instead, keep track of the sum, subtract phased out sample, add new sample and divide new sum by number of valid samples. Massive CPU cycle savings.
Thanks to Jonathan (zenpoy on gmail dot com) for pointing me in the right direction!

Fix: The code for stopping the filter if the sensor has been stable the entire filter window time was only enabled if debug logging was enabled. Oops. CPU cycle savings in steady ambient light.
parent aea51add

services/java/com/android/server/PowerManagerService.java

+30 −21
Original line number Diff line number Diff line
@@ -262,7 +262,9 @@ class PowerManagerService extends IPowerManager.Stub 
    private int mLightFilterSample = -1;

    private int[] mLightFilterSamples;

    private int mLightFilterIndex;

    private int mLightFilterCounter;

    private int mLightFilterSampleCounter;

    private int mLightFilterSum;

    private int mLightFilterEqualCounter;

    private int mLightFilterWindow;

    private int mLightFilterInterval;

    private int mLightFilterReset;
@@ -2214,35 +2216,41 @@ class PowerManagerService extends IPowerManager.Stub 
            synchronized (mLocks) {

                boolean again = false;

                if (mLightFilterSample > 0 && !isScreenTurningOffLocked()) {

                    int discarded = mLightFilterSamples[mLightFilterIndex];

                    mLightFilterSamples[mLightFilterIndex] = mLightFilterSample;

                    mLightFilterIndex = (mLightFilterIndex + 1) %

                            mLightFilterSamples.length;

                    int sum, count, sample;

                    sum = count = sample = 0;

                    for (int i = 0; i < mLightFilterSamples.length; i++) {

                        sample = mLightFilterSamples[i];

                        if (sample < 0) {

                            break;

                        }

                        sum += sample;

                        count++;

                    }

                    mLightFilterSampleCounter = Math.min(mLightFilterSampleCounter + 1,

                             mLightFilterSamples.length);

                    if (mLightFilterSampleCounter < mLightFilterSamples.length) {

                        discarded = 0; // Don't subtract if window isn't full

                    }

                    // Add new value...

                    mLightFilterSum += mLightFilterSample;

                    // ... and subtract discarded value

                    mLightFilterSum -= discarded;

                    // Count can't be zero here

                    int average = Math.round((float)sum / count);

                    int average = Math.round(

                                (float)mLightFilterSum / mLightFilterSampleCounter);

                    if (average != (int)mLightSensorValue) {

                        lightSensorChangedLocked(average);

                    }

                    if ((int)mLightSensorValue != mLightFilterSample) {

                        mLightFilterEqualCounter = 0;

                        again = true;

                        if (mDebugLightSensor) {

                            Slog.d(TAGF, "Tick: " + (int)mLightSensorValue + "::" +

                                    mLightFilterSample);

                                    mLightFilterSample + " sum:" + mLightFilterSum +

                                    " samples:" + mLightFilterSampleCounter);

                        }

                    } else if (mDebugLightSensor) {

                        mLightFilterCounter++;

                        again = mLightFilterCounter < mLightFilterSamples.length;

                    } else {

                        mLightFilterEqualCounter++;

                        again = mLightFilterEqualCounter < mLightFilterSamples.length;

                        if (mDebugLightSensor) {

                            Slog.d(TAGF, "Done: " + (int)mLightSensorValue + " " +

                                    mLightFilterCounter + "/" + mLightFilterSamples.length);

                            mLightFilterEqualCounter + "/" + mLightFilterSamples.length +

                            " sum:" + mLightFilterSum + " samples:" + mLightFilterSampleCounter);

                        }

                    }

                }

                if (again) {
@@ -2264,9 +2272,11 @@ class PowerManagerService extends IPowerManager.Stub 
    }



    private void lightFilterReset(int initial) {

        mLightFilterCounter = 0;

        mLightFilterEqualCounter = 0;

        mLightFilterIndex = 0;

        mLightFilterSamples = new int[(mLightFilterWindow / mLightFilterInterval)];

        mLightFilterSampleCounter = initial == -1 ? 0 : mLightFilterSamples.length;

        mLightFilterSum = initial == -1 ? 0 : initial * mLightFilterSamples.length;

        java.util.Arrays.fill(mLightFilterSamples, initial);

        if (mDebugLightSensor) {

            Slog.d(TAGF, "reset: " + initial);
@@ -3134,9 +3144,8 @@ class PowerManagerService extends IPowerManager.Stub 
                            // process the value immediately if screen has just turned on

                            lightFilterReset(-1);

                            lightSensorChangedLocked(value);

                        } else {

                            // Small changes -> filtered

                            lightFilterReset((int)mLightSensorValue);

                        }

                        if (!mLightFilterRunning) {

                            if (mDebugLightSensor) {

                                Slog.d(TAGF, "start: " + value);

                            }