The recovery time from deep sleep of STM32U5 is too slow

Last year, when I was adding support for NUCLEO-U545RE-Q and NUCLEO-U5A5ZJ-Q boards, there are 3 failed tests (https://github.com/mbed-ce/mbed-os/pull/273). The reason for the failure is that it takes too long to recover from deep sleep mode. Here is a simplified version of a Greentea LowPowerTimeout test. When running the test on a STM32U5 target, it will print "error try_acquire #2" in the console. To pass the test I have to increase the TEST_LATENCY from 2ms to 10ms. ```C++ #include "mbed.h" #include <atomic> EventQueue queue; const chrono::milliseconds TEST_DELAY = 10ms; const chrono::milliseconds TEST_LATENCY = 2ms; uint32_t start_time, end_time; std:: atomic_bool called = false; void sem_callback(Semaphore *sem) { sem->release(); called = true; } template<typename T> void test_single_call(void) { Semaphore sem(0, 1); T timeout; start_time = osKernelGetTickCount(); timeout.attach(mbed::callback(sem_callback, &sem), TEST_DELAY); bool acquired = sem.try_acquire(); if(acquired) printf("error try_acquire #1\n"); acquired = sem.try_acquire_for(TEST_DELAY + TEST_LATENCY); end_time = osKernelGetTickCount(); if(!acquired) printf("error try_acquire #2\n"); acquired = sem.try_acquire_for(TEST_DELAY + TEST_LATENCY); if(acquired) printf("error try_acquire #3\n"); timeout.detach(); if(called) { printf("Interval: %lu ms.\n", end_time - start_time); } else { printf("sem_callback not called!\n"); } } int main() { printf("Application started!\n"); test_single_call<LowPowerTimeout>(); queue.dispatch_forever(); return 0; } ``` But if I comment out the calls to ForceOscOutofDeepSleep() and ForcePeriphOutofDeepSleep() here, the recovery time can be significantly reduced. After this modification, the above test can be passed with TEST_LATENCY = 2ms, and the previously failed 3 Greentea tests are all passable. https://github.com/mbed-ce/mbed-os/blob/9ce9f386c0049f2838cf85c49e614cf51dc54fdf/targets/TARGET_STM/sleep.c#L265-L273 The question now is whether it is safe to comment out these two function calls, or if there is a better way to solve this problem.