services/common_time/clock_recovery.cpp - platform/frameworks/base - Git at Google

 /*
  * Copyright (C) 2011 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.
  */

 /*
  * A service that exchanges time synchronization information between
  * a master that defines a timeline and clients that follow the timeline.
  */

 #define __STDC_LIMIT_MACROS
 #define LOG_TAG "common_time"
 #include <utils/Log.h>
 #include <stdint.h>

 #include <common_time/local_clock.h>
 #include <assert.h>

 #include "clock_recovery.h"
 #include "common_clock.h"
 #ifdef TIME_SERVICE_DEBUG
 #include "diag_thread.h"
 #endif

 // Define log macro so we can make LOGV into LOGE when we are exclusively
 // debugging this code.
 #ifdef TIME_SERVICE_DEBUG
 #define LOG_TS ALOGE
 #else
 #define LOG_TS ALOGV
 #endif

 namespace android {

 ClockRecoveryLoop::ClockRecoveryLoop(LocalClock* local_clock,
                                      CommonClock* common_clock) {
     assert(NULL != local_clock);
     assert(NULL != common_clock);

     local_clock_  = local_clock;
     common_clock_ = common_clock;

     local_clock_can_slew_ = local_clock_->initCheck() &&
                            (local_clock_->setLocalSlew(0) == OK);

     reset(true, true);

 #ifdef TIME_SERVICE_DEBUG
     diag_thread_ = new DiagThread(common_clock_, local_clock_);
     if (diag_thread_ != NULL) {
         status_t res = diag_thread_->startWorkThread();
         if (res != OK)
             ALOGW("Failed to start A@H clock recovery diagnostic thread.");
     } else
         ALOGW("Failed to allocate diagnostic thread.");
 #endif
 }

 ClockRecoveryLoop::~ClockRecoveryLoop() {
 #ifdef TIME_SERVICE_DEBUG
     diag_thread_->stopWorkThread();
 #endif
 }

 // Constants.
 const float ClockRecoveryLoop::dT = 1.0;
 const float ClockRecoveryLoop::Kc = 1.0f;
 const float ClockRecoveryLoop::Ti = 15.0f;
 const float ClockRecoveryLoop::Tf = 0.05;
 const float ClockRecoveryLoop::bias_Fc = 0.01;
 const float ClockRecoveryLoop::bias_RC = (dT / (2 * 3.14159f * bias_Fc));
 const float ClockRecoveryLoop::bias_Alpha = (dT / (bias_RC + dT));
 const int64_t ClockRecoveryLoop::panic_thresh_ = 50000;
 const int64_t ClockRecoveryLoop::control_thresh_ = 10000;
 const float ClockRecoveryLoop::COmin = -100.0f;
 const float ClockRecoveryLoop::COmax = 100.0f;

 void ClockRecoveryLoop::reset(bool position, bool frequency) {
     Mutex::Autolock lock(&lock_);
     reset_l(position, frequency);
 }

 uint32_t ClockRecoveryLoop::findMinRTTNdx(DisciplineDataPoint* data,
                                           uint32_t count) {
     uint32_t min_rtt = 0;
     for (uint32_t i = 1; i < count; ++i)
         if (data[min_rtt].rtt > data[i].rtt)
             min_rtt = i;

     return min_rtt;
 }

 bool ClockRecoveryLoop::pushDisciplineEvent(int64_t local_time,
                                             int64_t nominal_common_time,
                                             int64_t rtt) {
     Mutex::Autolock lock(&lock_);

     int64_t local_common_time = 0;
     common_clock_->localToCommon(local_time, &local_common_time);
     int64_t raw_delta = nominal_common_time - local_common_time;

 #ifdef TIME_SERVICE_DEBUG
     ALOGE("local=%lld, common=%lld, delta=%lld, rtt=%lld\n",
          local_common_time, nominal_common_time,
          raw_delta, rtt);
 #endif

     // If we have not defined a basis for common time, then we need to use these
     // initial points to do so.  In order to avoid significant initial error
     // from a particularly bad startup data point, we collect the first N data
     // points and choose the best of them before moving on.
     if (!common_clock_->isValid()) {
         if (startup_filter_wr_ < kStartupFilterSize) {
             DisciplineDataPoint& d =  startup_filter_data_[startup_filter_wr_];
             d.local_time = local_time;
             d.nominal_common_time = nominal_common_time;
             d.rtt = rtt;
             startup_filter_wr_++;
         }

         if (startup_filter_wr_ == kStartupFilterSize) {
             uint32_t min_rtt = findMinRTTNdx(startup_filter_data_,
                     kStartupFilterSize);

             common_clock_->setBasis(
                     startup_filter_data_[min_rtt].local_time,
                     startup_filter_data_[min_rtt].nominal_common_time);
         }

         return true;
     }

     int64_t observed_common;
     int64_t delta;
     float delta_f, dCO;
     int32_t correction_cur;

     if (OK != common_clock_->localToCommon(local_time, &observed_common)) {
         // Since we just checked to make certain that this conversion was valid,
         // and no one else in the system should be messing with it, if this
         // conversion is suddenly invalid, it is a good reason to panic.
         ALOGE("Failed to convert local time to common time in %s:%d",
                 __PRETTY_FUNCTION__, __LINE__);
         return false;
     }

     // Implement a filter which should match NTP filtering behavior when a
     // client is associated with only one peer of lower stratum.  Basically,
     // always use the best of the N last data points, where best is defined as
     // lowest round trip time.  NTP uses an N of 8; we use a value of 6.
     //
     // TODO(johngro) : experiment with other filter strategies.  The goal here
     // is to mitigate the effects of high RTT data points which typically have
     // large asymmetries in the TX/RX legs.  Downside of the existing NTP
     // approach (particularly because of the PID controller we are using to
     // produce the control signal from the filtered data) are that the rate at
     // which discipline events are actually acted upon becomes irregular and can
     // become drawn out (the time between actionable event can go way up).  If
     // the system receives a strong high quality data point, the proportional
     // component of the controller can produce a strong correction which is left
     // in place for too long causing overshoot.  In addition, the integral
     // component of the system currently is an approximation based on the
     // assumption of a more or less homogeneous sampling of the error.  Its
     // unclear what the effect of undermining this assumption would be right
     // now.

     // Two ideas which come to mind immediately would be to...
     // 1) Keep a history of more data points (32 or so) and ignore data points
     //    whose RTT is more than a certain number of standard deviations outside
     //    of the norm.
     // 2) Eliminate the PID controller portion of this system entirely.
     //    Instead, move to a system which uses a very wide filter (128 data
     //    points or more) with a sum-of-least-squares line fitting approach to
     //    tracking the long term drift.  This would take the place of the I
     //    component in the current PID controller.  Also use a much more narrow
     //    outlier-rejector filter (as described in #1) to drive a short term
     //    correction factor similar to the P component of the PID controller.
     assert(filter_wr_ < kFilterSize);
     filter_data_[filter_wr_].local_time           = local_time;
     filter_data_[filter_wr_].observed_common_time = observed_common;
     filter_data_[filter_wr_].nominal_common_time  = nominal_common_time;
     filter_data_[filter_wr_].rtt                  = rtt;
     filter_data_[filter_wr_].point_used           = false;
     uint32_t current_point = filter_wr_;
     filter_wr_ = (filter_wr_ + 1) % kFilterSize;
     if (!filter_wr_)
         filter_full_ = true;

     uint32_t scan_end = filter_full_ ? kFilterSize : filter_wr_;
     uint32_t min_rtt = findMinRTTNdx(filter_data_, scan_end);
     // We only use packets with low RTTs for control. If the packet RTT
     // is less than the panic threshold, we can probably eat the jitter with the
     // control loop. Otherwise, take the packet only if it better than all
     // of the packets we have in the history. That way we try to track
     // something, even if it is noisy.
     if (current_point == min_rtt || rtt < control_thresh_) {
         delta_f = delta = nominal_common_time - observed_common;

         // Compute the error then clamp to the panic threshold.  If we ever
         // exceed this amt of error, its time to panic and reset the system.
         // Given that the error in the measurement of the error could be as
         // high as the RTT of the data point, we don't actually panic until
         // the implied error (delta) is greater than the absolute panic
         // threashold plus the RTT.  IOW - we don't panic until we are
         // absoluely sure that our best case sync is worse than the absolute
         // panic threshold.
         int64_t effective_panic_thresh = panic_thresh_ + rtt;
         if ((delta > effective_panic_thresh) ||
             (delta < -effective_panic_thresh)) {
             // PANIC!!!
             reset_l(false, true);
             return false;
         }

     } else {
         // We do not have a good packet to look at, but we also do not want to
         // free-run the clock at some crazy slew rate. So we guess the
         // trajectory of the clock based on the last controller output and the
         // estimated bias of our clock against the master.
         // The net effect of this is that CO == CObias after some extended
         // period of no feedback.
         delta_f = last_delta_f_ - dT*(CO - CObias);
         delta = delta_f;
     }

     // Velocity form PI control equation.
     dCO = Kc * (1.0f + dT/Ti) * delta_f - Kc * last_delta_f_;
     CO += dCO * Tf; // Filter CO by applying gain <1 here.

     // Save error terms for later.
     last_delta_f_ = delta_f;
     last_delta_ = delta;

     // Clamp CO to +/- 100ppm.
     if (CO < COmin)
         CO = COmin;
     else if (CO > COmax)
         CO = COmax;

     // Update the controller bias.
     CObias = bias_Alpha * CO + (1.0f - bias_Alpha) * lastCObias;
     lastCObias = CObias;

     // Convert PPM to 16-bit int range. Add some guard band (-0.01) so we
     // don't get fp weirdness.
     correction_cur = CO * 327.66;

     // If there was a change in the amt of correction to use, update the
     // system.
     if (correction_cur_ != correction_cur) {
         correction_cur_ = correction_cur;
         applySlew();
     }

     LOG_TS("clock_loop %lld %f %f %f %d\n", raw_delta, delta_f, CO, CObias, correction_cur);

 #ifdef TIME_SERVICE_DEBUG
     diag_thread_->pushDisciplineEvent(
             local_time,
             observed_common,
             nominal_common_time,
             correction_cur,
             rtt);
 #endif

     return true;
 }

 int32_t ClockRecoveryLoop::getLastErrorEstimate() {
     Mutex::Autolock lock(&lock_);

     if (last_delta_valid_)
         return last_delta_;
     else
         return ICommonClock::kErrorEstimateUnknown;
 }

 void ClockRecoveryLoop::reset_l(bool position, bool frequency) {
     assert(NULL != common_clock_);

     if (position) {
         common_clock_->resetBasis();
         startup_filter_wr_ = 0;
     }

     if (frequency) {
         last_delta_valid_ = false;
         last_delta_ = 0;
         last_delta_f_ = 0.0;
         correction_cur_ = 0x0;
         CO = 0.0f;
         lastCObias = CObias = 0.0f;
         applySlew();
     }

     filter_wr_   = 0;
     filter_full_ = false;
 }

 void ClockRecoveryLoop::applySlew() {
     if (local_clock_can_slew_) {
         local_clock_->setLocalSlew(correction_cur_);
     } else {
         // The SW clock recovery implemented by the common clock class expects
         // values expressed in PPM. CO is in ppm.
         common_clock_->setSlew(local_clock_->getLocalTime(), CO);
     }
 }

 }  // namespace android
	/*
	* Copyright (C) 2011 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.
	*/

	/*
	* A service that exchanges time synchronization information between
	* a master that defines a timeline and clients that follow the timeline.
	*/

	#define __STDC_LIMIT_MACROS
	#define LOG_TAG "common_time"
	#include <utils/Log.h>
	#include <stdint.h>

	#include <common_time/local_clock.h>
	#include <assert.h>

	#include "clock_recovery.h"
	#include "common_clock.h"
	#ifdef TIME_SERVICE_DEBUG
	#include "diag_thread.h"
	#endif

	// Define log macro so we can make LOGV into LOGE when we are exclusively
	// debugging this code.
	#ifdef TIME_SERVICE_DEBUG
	#define LOG_TS ALOGE
	#else
	#define LOG_TS ALOGV
	#endif

	namespace android {

	ClockRecoveryLoop::ClockRecoveryLoop(LocalClock* local_clock,
	CommonClock* common_clock) {
	assert(NULL != local_clock);
	assert(NULL != common_clock);

	local_clock_ = local_clock;
	common_clock_ = common_clock;

	local_clock_can_slew_ = local_clock_->initCheck() &&
	(local_clock_->setLocalSlew(0) == OK);

	reset(true, true);

	#ifdef TIME_SERVICE_DEBUG
	diag_thread_ = new DiagThread(common_clock_, local_clock_);
	if (diag_thread_ != NULL) {
	status_t res = diag_thread_->startWorkThread();
	if (res != OK)
	ALOGW("Failed to start A@H clock recovery diagnostic thread.");
	} else
	ALOGW("Failed to allocate diagnostic thread.");
	#endif
	}

	ClockRecoveryLoop::~ClockRecoveryLoop() {
	#ifdef TIME_SERVICE_DEBUG
	diag_thread_->stopWorkThread();
	#endif
	}

	// Constants.
	const float ClockRecoveryLoop::dT = 1.0;
	const float ClockRecoveryLoop::Kc = 1.0f;
	const float ClockRecoveryLoop::Ti = 15.0f;
	const float ClockRecoveryLoop::Tf = 0.05;
	const float ClockRecoveryLoop::bias_Fc = 0.01;
	const float ClockRecoveryLoop::bias_RC = (dT / (2 * 3.14159f * bias_Fc));
	const float ClockRecoveryLoop::bias_Alpha = (dT / (bias_RC + dT));
	const int64_t ClockRecoveryLoop::panic_thresh_ = 50000;
	const int64_t ClockRecoveryLoop::control_thresh_ = 10000;
	const float ClockRecoveryLoop::COmin = -100.0f;
	const float ClockRecoveryLoop::COmax = 100.0f;

	void ClockRecoveryLoop::reset(bool position, bool frequency) {
	Mutex::Autolock lock(&lock_);
	reset_l(position, frequency);
	}

	uint32_t ClockRecoveryLoop::findMinRTTNdx(DisciplineDataPoint* data,
	uint32_t count) {
	uint32_t min_rtt = 0;
	for (uint32_t i = 1; i < count; ++i)
	if (data[min_rtt].rtt > data[i].rtt)
	min_rtt = i;

	return min_rtt;
	}

	bool ClockRecoveryLoop::pushDisciplineEvent(int64_t local_time,
	int64_t nominal_common_time,
	int64_t rtt) {
	Mutex::Autolock lock(&lock_);

	int64_t local_common_time = 0;
	common_clock_->localToCommon(local_time, &local_common_time);
	int64_t raw_delta = nominal_common_time - local_common_time;

	#ifdef TIME_SERVICE_DEBUG
	ALOGE("local=%lld, common=%lld, delta=%lld, rtt=%lld\n",
	local_common_time, nominal_common_time,
	raw_delta, rtt);
	#endif

	// If we have not defined a basis for common time, then we need to use these
	// initial points to do so. In order to avoid significant initial error
	// from a particularly bad startup data point, we collect the first N data
	// points and choose the best of them before moving on.
	if (!common_clock_->isValid()) {
	if (startup_filter_wr_ < kStartupFilterSize) {
	DisciplineDataPoint& d = startup_filter_data_[startup_filter_wr_];
	d.local_time = local_time;
	d.nominal_common_time = nominal_common_time;
	d.rtt = rtt;
	startup_filter_wr_++;
	}

	if (startup_filter_wr_ == kStartupFilterSize) {
	uint32_t min_rtt = findMinRTTNdx(startup_filter_data_,
	kStartupFilterSize);

	common_clock_->setBasis(
	startup_filter_data_[min_rtt].local_time,
	startup_filter_data_[min_rtt].nominal_common_time);
	}

	return true;
	}

	int64_t observed_common;
	int64_t delta;
	float delta_f, dCO;
	int32_t correction_cur;

	if (OK != common_clock_->localToCommon(local_time, &observed_common)) {
	// Since we just checked to make certain that this conversion was valid,
	// and no one else in the system should be messing with it, if this
	// conversion is suddenly invalid, it is a good reason to panic.
	ALOGE("Failed to convert local time to common time in %s:%d",
	__PRETTY_FUNCTION__, __LINE__);
	return false;
	}

	// Implement a filter which should match NTP filtering behavior when a
	// client is associated with only one peer of lower stratum. Basically,
	// always use the best of the N last data points, where best is defined as
	// lowest round trip time. NTP uses an N of 8; we use a value of 6.
	//
	// TODO(johngro) : experiment with other filter strategies. The goal here
	// is to mitigate the effects of high RTT data points which typically have
	// large asymmetries in the TX/RX legs. Downside of the existing NTP
	// approach (particularly because of the PID controller we are using to
	// produce the control signal from the filtered data) are that the rate at
	// which discipline events are actually acted upon becomes irregular and can
	// become drawn out (the time between actionable event can go way up). If
	// the system receives a strong high quality data point, the proportional
	// component of the controller can produce a strong correction which is left
	// in place for too long causing overshoot. In addition, the integral
	// component of the system currently is an approximation based on the
	// assumption of a more or less homogeneous sampling of the error. Its
	// unclear what the effect of undermining this assumption would be right
	// now.

	// Two ideas which come to mind immediately would be to...
	// 1) Keep a history of more data points (32 or so) and ignore data points
	// whose RTT is more than a certain number of standard deviations outside
	// of the norm.
	// 2) Eliminate the PID controller portion of this system entirely.
	// Instead, move to a system which uses a very wide filter (128 data
	// points or more) with a sum-of-least-squares line fitting approach to
	// tracking the long term drift. This would take the place of the I
	// component in the current PID controller. Also use a much more narrow
	// outlier-rejector filter (as described in #1) to drive a short term
	// correction factor similar to the P component of the PID controller.
	assert(filter_wr_ < kFilterSize);
	filter_data_[filter_wr_].local_time = local_time;
	filter_data_[filter_wr_].observed_common_time = observed_common;
	filter_data_[filter_wr_].nominal_common_time = nominal_common_time;
	filter_data_[filter_wr_].rtt = rtt;
	filter_data_[filter_wr_].point_used = false;
	uint32_t current_point = filter_wr_;
	filter_wr_ = (filter_wr_ + 1) % kFilterSize;
	if (!filter_wr_)
	filter_full_ = true;

	uint32_t scan_end = filter_full_ ? kFilterSize : filter_wr_;
	uint32_t min_rtt = findMinRTTNdx(filter_data_, scan_end);
	// We only use packets with low RTTs for control. If the packet RTT
	// is less than the panic threshold, we can probably eat the jitter with the
	// control loop. Otherwise, take the packet only if it better than all
	// of the packets we have in the history. That way we try to track
	// something, even if it is noisy.
	if (current_point == min_rtt \|\| rtt < control_thresh_) {
	delta_f = delta = nominal_common_time - observed_common;

	// Compute the error then clamp to the panic threshold. If we ever
	// exceed this amt of error, its time to panic and reset the system.
	// Given that the error in the measurement of the error could be as
	// high as the RTT of the data point, we don't actually panic until
	// the implied error (delta) is greater than the absolute panic
	// threashold plus the RTT. IOW - we don't panic until we are
	// absoluely sure that our best case sync is worse than the absolute
	// panic threshold.
	int64_t effective_panic_thresh = panic_thresh_ + rtt;
	if ((delta > effective_panic_thresh) \|\|
	(delta < -effective_panic_thresh)) {
	// PANIC!!!
	reset_l(false, true);
	return false;
	}

	} else {
	// We do not have a good packet to look at, but we also do not want to
	// free-run the clock at some crazy slew rate. So we guess the
	// trajectory of the clock based on the last controller output and the
	// estimated bias of our clock against the master.
	// The net effect of this is that CO == CObias after some extended
	// period of no feedback.
	delta_f = last_delta_f_ - dT*(CO - CObias);
	delta = delta_f;
	}

	// Velocity form PI control equation.
	dCO = Kc * (1.0f + dT/Ti) * delta_f - Kc * last_delta_f_;
	CO += dCO * Tf; // Filter CO by applying gain <1 here.

	// Save error terms for later.
	last_delta_f_ = delta_f;
	last_delta_ = delta;

	// Clamp CO to +/- 100ppm.
	if (CO < COmin)
	CO = COmin;
	else if (CO > COmax)
	CO = COmax;

	// Update the controller bias.
	CObias = bias_Alpha * CO + (1.0f - bias_Alpha) * lastCObias;
	lastCObias = CObias;

	// Convert PPM to 16-bit int range. Add some guard band (-0.01) so we
	// don't get fp weirdness.
	correction_cur = CO * 327.66;

	// If there was a change in the amt of correction to use, update the
	// system.
	if (correction_cur_ != correction_cur) {
	correction_cur_ = correction_cur;
	applySlew();
	}

	LOG_TS("clock_loop %lld %f %f %f %d\n", raw_delta, delta_f, CO, CObias, correction_cur);

	#ifdef TIME_SERVICE_DEBUG
	diag_thread_->pushDisciplineEvent(
	local_time,
	observed_common,
	nominal_common_time,
	correction_cur,
	rtt);
	#endif

	return true;
	}

	int32_t ClockRecoveryLoop::getLastErrorEstimate() {
	Mutex::Autolock lock(&lock_);

	if (last_delta_valid_)
	return last_delta_;
	else
	return ICommonClock::kErrorEstimateUnknown;
	}

	void ClockRecoveryLoop::reset_l(bool position, bool frequency) {
	assert(NULL != common_clock_);

	if (position) {
	common_clock_->resetBasis();
	startup_filter_wr_ = 0;
	}

	if (frequency) {
	last_delta_valid_ = false;
	last_delta_ = 0;
	last_delta_f_ = 0.0;
	correction_cur_ = 0x0;
	CO = 0.0f;
	lastCObias = CObias = 0.0f;
	applySlew();
	}

	filter_wr_ = 0;
	filter_full_ = false;
	}

	void ClockRecoveryLoop::applySlew() {
	if (local_clock_can_slew_) {
	local_clock_->setLocalSlew(correction_cur_);
	} else {
	// The SW clock recovery implemented by the common clock class expects
	// values expressed in PPM. CO is in ppm.
	common_clock_->setSlew(local_clock_->getLocalTime(), CO);
	}
	}

	} // namespace android