If module `core_mutex_priority_inheritance` is enabled, the scheduling has to be active to lock/unlock the mutex/rmutex used by FreeRTOS semaphores. If scheduling is not active FreeRTOS semaphore function always succeed.
For ESP32x, the operations on recursive locking variables have to be guarded by disabling interrupts to prevent unintended context switches. For ESP8266, interrupts must not be disabled, otherwise the intended context switch doesn't work when trying to lock a rmutex that is already locked by another thread.
Dynamic allocation and initialization of the mutex used by a newlib locking variable must not be interrupted. Since a thread context switch can occur on exit from an ISR, the allocation and initialization of the mutex must be guarded by disabling interrupts. The same must be done for the release of such a locking variable.
With the improvements of the locking mechanism, thread safety of malloc/realloc/calloc/free is guaranteed. Module malloc_thread_safe is not needed any longer.
When FreeRTOS semaphores, as required by ESP-IDF, are used together with `gnrc_netif`, RIOT may crash if `STATUS_RECEIVE_BLOCKED` is used as a blocking mechanism in the FreeRTOS adaptation layer. The reason for this is that `gnrc_netif` uses thread flags since PR #16748. If the `gnrc_netif` thread is blocked because of a FreeRTOS semaphore, and is thus in `STATUS_RECEIVE_BLOCKED` state, the `_msg_send` function will cause a crash because it then assumes that `target->wait_data` contains a pointer to a message of type `msg_t`, but by using thread flags it contains the flag mask. This situation can happen if the ESP hardware is used while another thread is sending something timer controlled to the `gnrc_netif` thread.
To solve this problem `STATUS_MUTEX_LOCKED` is used instead of `STATUS_RECEIVE_BLOCKED` and `STATUS_SEND_BLOCKED`
To reduce the required RAM in default configuration, the BLE interface is used as netdev_default instead of ESP-NOW. Further network interfaces can be enabled with the modules `esp_now`, `esp_wifi` or `esp_eth`.
When using Bluetooth LE, the former UART interrupt number 5 is occupied by the ESP32 Bluetooth Controller. Therefore, another interrupt number has to be used for UART.
The package uses the nRFx SDK package `nrfx`. In addition, the `mynewt-nimble` repository contains some files (`porting/nimble/src/hal_timer.c` and `porting/npl/riot/src/nrf5x_isr.c`) that are compilable only for nRF MCUs. To allow the compilation for other platforms, the use of the `nrfx` package and the compilation of these files are now dependent on the use of any nRF5x MCU.
