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* 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
* 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.
* A service that exchanges time synchronization information between
* a master that defines a timeline and clients that follow the timeline.
#define LOG_TAG "common_time"
#include <utils/Log.h>
#include <inttypes.h>
#include <stdint.h>
#include <common_time/local_clock.h>
#include <assert.h>
#include "clock_recovery.h"
#include "common_clock.h"
#include "diag_thread.h"
// Define log macro so we can make LOGV into LOGE when we are exclusively
// debugging this code.
#define LOG_TS ALOGE
#define LOG_TS ALOGV
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);
tgt_correction_ = 0;
cur_correction_ = 0;
// Precompute the max rate at which we are allowed to change the VCXO
// control.
uint64_t N = 0x10000ull * 1000ull;
uint64_t D = local_clock_->getLocalFreq() * kMinFullRangeSlewChange_mSec;
LinearTransform::reduce(&N, &D);
while ((N > INT32_MAX) || (D > UINT32_MAX)) {
N >>= 1;
D >>= 1;
LinearTransform::reduce(&N, &D);
time_to_cur_slew_.a_to_b_numer = static_cast<int32_t>(N);
time_to_cur_slew_.a_to_b_denom = static_cast<uint32_t>(D);
reset(true, true);
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.");
ClockRecoveryLoop::~ClockRecoveryLoop() {
// 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;
const uint32_t ClockRecoveryLoop::kMinFullRangeSlewChange_mSec = 300;
const int ClockRecoveryLoop::kSlewChangeStepPeriod_mSec = 10;
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;
ALOGE("local=%lld, common=%lld, delta=%lld, rtt=%lld\n",
local_common_time, nominal_common_time,
raw_delta, rtt);
// 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;
if (startup_filter_wr_ == kStartupFilterSize) {
uint32_t min_rtt = findMinRTTNdx(startup_filter_data_,
return true;
int64_t observed_common;
int64_t delta;
float delta_f, dCO;
int32_t tgt_correction;
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",
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;
last_error_est_valid_ = true;
last_error_est_usec_ = delta;
// 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;
// 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.
tgt_correction = CO * 327.66;
// If there was a change in the amt of correction to use, update the
// system.
LOG_TS("clock_loop %" PRId64 " %f %f %f %d\n", raw_delta, delta_f, CO, CObias, tgt_correction);
return true;
int32_t ClockRecoveryLoop::getLastErrorEstimate() {
Mutex::Autolock lock(&lock_);
if (last_error_est_valid_)
return last_error_est_usec_;
return ICommonClock::kErrorEstimateUnknown;
void ClockRecoveryLoop::reset_l(bool position, bool frequency) {
assert(NULL != common_clock_);
if (position) {
startup_filter_wr_ = 0;
if (frequency) {
last_error_est_valid_ = false;
last_error_est_usec_ = 0;
last_delta_f_ = 0.0;
CO = 0.0f;
lastCObias = CObias = 0.0f;
filter_wr_ = 0;
filter_full_ = false;
void ClockRecoveryLoop::setTargetCorrection_l(int32_t tgt) {
// When we make a change to the slew rate, we need to be careful to not
// change it too quickly as it can anger some HDMI sinks out there, notably
// some Sony panels from the 2010-2011 timeframe. From experimenting with
// some of these sinks, it seems like swinging from one end of the range to
// another in less that 190mSec or so can start to cause trouble. Adding in
// a hefty margin, we limit the system to a full range sweep in no less than
// 300mSec.
if (tgt_correction_ != tgt) {
int64_t now = local_clock_->getLocalTime();
tgt_correction_ = tgt;
// Set up the transformation to figure out what the slew should be at
// any given point in time in the future.
time_to_cur_slew_.a_zero = now;
time_to_cur_slew_.b_zero = cur_correction_;
// Make sure the sign of the slope is headed in the proper direction.
bool needs_increase = (cur_correction_ < tgt_correction_);
bool is_increasing = (time_to_cur_slew_.a_to_b_numer > 0);
if (( needs_increase && !is_increasing) ||
(!needs_increase && is_increasing)) {
time_to_cur_slew_.a_to_b_numer = -time_to_cur_slew_.a_to_b_numer;
// Finally, figure out when the change will be finished and start the
// slew operation.
bool ClockRecoveryLoop::applySlew_l() {
bool ret = true;
// If cur == tgt, there is no ongoing sleq rate change and we are already
// finished.
if (cur_correction_ == tgt_correction_)
goto bailout;
if (local_clock_can_slew_) {
int64_t now = local_clock_->getLocalTime();
int64_t tmp;
if (now >= slew_change_end_time_) {
cur_correction_ = tgt_correction_;
} else {
time_to_cur_slew_.doForwardTransform(now, &tmp);
if (tmp > INT16_MAX)
cur_correction_ = INT16_MAX;
else if (tmp < INT16_MIN)
cur_correction_ = INT16_MIN;
cur_correction_ = static_cast<int16_t>(tmp);
ret = false;
} else {
// Since we are not actually changing the rate of a HW clock, we don't
// need to worry to much about changing the slew rate so fast that we
// anger any downstream HDMI devices.
cur_correction_ = tgt_correction_;
// 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);
return ret;
int ClockRecoveryLoop::applyRateLimitedSlew() {
Mutex::Autolock lock(&lock_);
int ret = next_slew_change_timeout_.msecTillTimeout();
if (!ret) {
if (applySlew_l())
ret = next_slew_change_timeout_.msecTillTimeout();
return ret;
} // namespace android