1 // Copyright 2021 The Pigweed Authors
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License"); you may not
4 // use this file except in compliance with the License. You may obtain a copy of
5 // the License at
6 //
7 // https://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
11 // WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
12 // License for the specific language governing permissions and limitations under
13 // the License.
14
15 #include "pw_sync/counting_semaphore.h"
16
17 #include <algorithm>
18
19 #include "RTOS.h"
20 #include "pw_assert/check.h"
21 #include "pw_chrono/system_clock.h"
22 #include "pw_chrono_embos/system_clock_constants.h"
23 #include "pw_interrupt/context.h"
24
25 using pw::chrono::SystemClock;
26
27 namespace pw::sync {
28
release(ptrdiff_t update)29 void CountingSemaphore::release(ptrdiff_t update) {
30 for (; update > 0; --update) {
31 // There is no API to atomically detect overflow, however debug builds of
32 // embOS call OS_Error() internally when overflow is detected for the native
33 // token representation. Rather than enter a critical section both due to
34 // cost and potential direct use of the native handle, a lazy check is used
35 // for debug builds which may not trigger on the initial overflow.
36 PW_DCHECK_UINT_LE(OS_GetCSemaValue(&native_type_),
37 CountingSemaphore::max(),
38 "Overflowed counting semaphore.");
39 OS_SignalCSema(&native_type_);
40 }
41 }
42
try_acquire_for(SystemClock::duration timeout)43 bool CountingSemaphore::try_acquire_for(SystemClock::duration timeout) {
44 // Enforce the pw::sync::CountingSemaphore IRQ contract.
45 PW_DCHECK(!interrupt::InInterruptContext());
46
47 // Use non-blocking try_acquire for negative and zero length durations.
48 if (timeout <= SystemClock::duration::zero()) {
49 return try_acquire();
50 }
51
52 // In case the timeout is too long for us to express through the native
53 // embOS API, we repeatedly wait with shorter durations. Note that on a tick
54 // based kernel we cannot tell how far along we are on the current tick, ergo
55 // we add one whole tick to the final duration. However, this also means that
56 // the loop must ensure that timeout + 1 is less than the max timeout.
57 constexpr SystemClock::duration kMaxTimeoutMinusOne =
58 pw::chrono::embos::kMaxTimeout - SystemClock::duration(1);
59 while (timeout > kMaxTimeoutMinusOne) {
60 if (OS_WaitCSemaTimed(&native_type_,
61 static_cast<OS_TIME>(kMaxTimeoutMinusOne.count()))) {
62 return true;
63 }
64 timeout -= kMaxTimeoutMinusOne;
65 }
66 // On a tick based kernel we cannot tell how far along we are on the current
67 // tick, ergo we add one whole tick to the final duration.
68 return OS_WaitCSemaTimed(&native_type_,
69 static_cast<OS_TIME>(timeout.count() + 1));
70 }
71
72 } // namespace pw::sync
73