1. call esp_timer_get_time and ref_clock_get in the same order on
start and in the loop
2. disable interrupts when calculating delta between ref_clock
and esp_timer
3. ensure both functions are in cache before calculating the delta
Timer callback can delete the timer. If CONFIG_ESP_TIMER_PROFILING was
enabled, this caused an access to invalid (freed) memory.
This fix adds a pointer to track the timer while executing the callback.
This is needed so that we can check if callback deletes the timer,
in which case we won't try updating profiling counters for this timer
after the callback is done.
Since timestamps are 64-bit, loosing one bit of range due to sign does
not present an issue, however for applications doing calculations on
timestamps, signed return type is more convenient.
The implementation of esp_timer_get_time used a critical section, which
resulted in a call time of ~1.8us. To make esp_timer_get_time more
useable as a high-resolution time source, this change replaces the lock
with polling. Call time is reduced to ~0.7us.
