64 lines
2.2 KiB
C
64 lines
2.2 KiB
C
/*
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* Copyright (C) 2016 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#ifndef AP_HUB_SYNC_H__
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#define AP_HUB_SYNC_H__
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#include <stdio.h>
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#include <stdint.h>
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#include <stdbool.h>
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#ifdef __cplusplus
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extern "C" {
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#endif
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/*
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* This implements an AP-HUB time sync algorithm that is expected to improve time sync accuracy by
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* avoiding communication latency jitter.
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*
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* It uses max of (apTime - hubTime) in a window, which is more consistent than average, to
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* establish mapping between ap timestamp and hub stamp. Additional low pass filtering is added
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* to further lowering jitter (it is not expected for two clocks to drift much in short time).
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*
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* Max is slightly anti-intuitive here because difference is defined as apTime - hubTime. Max of
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* that is equivalent to min of hubTime - apTime, which corresponds to a packet that get delayed
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* by system scheduling minimally (closer to the more consistent hardware related latency).
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*/
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struct ApHubSync {
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uint64_t lastTs; // AP time of previous data point, used for control expiration
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int64_t deltaEstimation; // the estimated delta between two clocks, filtered.
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int64_t windowMax; // track the maximum timestamp difference in a window
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uint64_t windowTimeout; // track window expiration time
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uint8_t state; // internal state of the sync
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};
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// reset data structure
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void apHubSyncReset(struct ApHubSync* sync);
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// add a data point (a pair of apTime and the corresponding hub time).
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void apHubSyncAddDelta(struct ApHubSync* sync, uint64_t apTime, uint64_t hubTime);
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// get the estimation of time delta
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int64_t apHubSyncGetDelta(struct ApHubSync* sync, uint64_t hubTime);
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#ifdef __cplusplus
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}
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#endif
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#endif // AP_HUB_SYNC_H__
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