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// Copyright 2020 The Pigweed Authors
// 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, 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.
#include "pw_chrono/system_clock.h"
#include <atomic>
#include <chrono>
#include <limits>
#include <mutex>
#include "pw_sync/spin_lock.h"
#include "tx_api.h"
namespace pw::chrono::backend {
namespace {
#if defined(TX_NO_TIMER) && TX_NO_TIMER
#error "This backend is not compatible with TX_NO_TIMER"
#endif // defined(TX_NO_TIMER) && TX_NO_TIMER
// Extension wrapping pw::SpinLock to allow an atomic to be used to determine
// whether it has been constructed and is ready to be used.
class ConstructionSignalingSpinLock : public sync::SpinLock {
ConstructionSignalingSpinLock(std::atomic<bool>& constructed_signal)
: sync::SpinLock() {
// Note that the memory order is relaxed because C++ global static
// construction is a single threaded environment., std::memory_order_relaxed);
// This is POD, meaning this atomic is available before static C++ global
// constructors are run.
std::atomic<bool> system_clock_spin_lock_constructed = {false};
ConstructionSignalingSpinLock system_clock_spin_lock(
int64_t overflow_tick_count = 0;
ULONG native_tick_count = 0;
"global state is not atomic nor double buferred");
// The tick count resets to 0, ergo the overflow count is the max count + 1.
constexpr int64_t kNativeOverflowTickCount =
static_cast<int64_t>(std::numeric_limits<ULONG>::max()) + 1;
} // namespace
int64_t GetSystemClockTickCount() {
// Note that the memory order is relaxed because C++ global static
// construction is a single threaded environment.
if (!system_clock_spin_lock_constructed.load(std::memory_order_relaxed)) {
return tx_time_get();
std::lock_guard lock(system_clock_spin_lock);
const ULONG new_native_tick_count = tx_time_get();
// WARNING: This must be called more than once per overflow period!
if (new_native_tick_count < native_tick_count) {
// Native tick count overflow detected!
overflow_tick_count += kNativeOverflowTickCount;
native_tick_count = new_native_tick_count;
return overflow_tick_count + native_tick_count;
} // namespace pw::chrono::backend