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/*
* Copyright (C) 2022 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.
*/
#pragma once
#include <hardware/sensors.h>
#include <algorithm>
#include <deque>
// A utility class that reads sensor samples from a direct channel buffer.
// Direct channel operates in a lockless manner and uses an atomic counter for
// synchronization. This class implements the counter based synchronization
// protocol and therefore guarantees data consistency. See
// https://developer.android.com/reference/android/hardware/SensorDirectChannel
// for more details on the atomic counter.
//
// Besides reading samples, the reader also supports keeping track of recently
// obtained samples.
//
// DirectChannelBufferReader is not thread safe. It's the caller's responsibility
// to serialize the calls, including the access to the returned sample
// container.
//
// Example usage:
// DirectChannelBufferReader reader(buf, 100);
//
// int num_samples = reader.Read();
// const std::deque<sensors_event_t>& samples = reader.GetSampleContainer();
// for (auto it = samples.end() - num_samples; it != samples.end(); it++) {
// HandleNewSamples(*it);
// }
//
// int num_samples_skipped;
// reader.Read(&num_samples_skipped);
// if (num_samples_skipped > 0) {
// ReportMissedSamples(num_samples_skipped);
// }
//
//
// Another example:
//
// DirectChannelBufferReader reader(buf, 100);
//
// std::vector<sensors_event_t> Query(int start_time, int end_time) {
// reader.Read();
// std::vector<sensors_event_t> output;
// for (auto& sample : reader_.GetSampleContainer()) {
// if (sample.timestamp >= start_time && sample.timestamp < end_time) {
// output.push_back(sample);
// }
// }
// return output;
// }
class DirectChannelBufferReader {
public:
static constexpr int kErrorHeadOfBufferNotFound = -1;
// Constructor
// direct_channel_buffer: Pointer to the shared buffer where sensor samples
// are written into.
// buffer_size_samples: The size of direct_channel_buffer in number of
// samples.
DirectChannelBufferReader(const sensors_event_t* direct_channel_buffer,
int buffer_size_samples);
virtual ~DirectChannelBufferReader() {}
// Attempts to read samples from the direct channel buffer. Returns
// the number of samples read, or kErrorHeadOfBufferNotFound if the reader
// can not find the write head e.g. due to corrupted data in the buffer.
// The function is non-blocking and returns 0 if new samples are not available.
// The caller should control its polling based on external factors like
// events in a different subsystem (e.g. camera frame ready)
// After the call completes, the caller can use GetSampleContainer() to
// access the samples. Sometimes it may be possible for one or more samples
// in the direct channel buffer to be overwritten by the writter before the
// reader has a chance to read it, e.g. when the reader does not keep up
// with the writer. The number of samples that were lost / skipped is
// written to <num_samples_skipped>, if the argument is not null.
int Read(int* num_samples_skipped = nullptr);
// Returns the container that holds recent samples. New samples are appended
// to the end of the container when Read() is called. Samples from previous
// rounds of Read() are kept around in the container, except when the total
// samples exceeds <buffer_size_samples> - 1, in which case older samples
// would be truncated. The caller is free to remove samples from the
// container, e.g. after the samples are consumed.
//
// Calls to the returned container must be synchronized with calls to this
// instance of DirectChannelBufferReader.
std::deque<sensors_event_t>& GetSampleContainer() { return buffer_; }
protected:
// For test only.
virtual const sensors_event_t ReadOneSample(int index);
private:
// Truncates the head of <buffer_> until its size <= buffer_size_samples - 1.
void TruncateBuffer();
// Points to the direct channel buffer where the sensor writes samples into.
const volatile sensors_event_t* direct_channel_buffer_;
// The number of samples that <direct_channel_buffer_> is able to hold.
const int buffer_size_samples_;
// The atomic counter value of the last valid sample.
int64_t last_atomic_counter_ = 0;
// The index into <direct_channel_buffer_> that should be read next time.
int index_ = 0;
// The number of successive sensors_event_t reads with consecutive atomic
// counters values.
// E.g. 1 => streak_ = 1
// 5 6 7 => streak_ = 3
// 1 2 3 14 => streak_ = 1
// 1 2 3 14 15 => streak_ = 2
int streak_ = 0;
// The buffer holding recent samples.
std::deque<sensors_event_t> buffer_;
};
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