fire_interrupt function should be used for events in the past. As we have now
64bit timestamp, we can figure out what is in the past, and ask a target to invoke
an interrupt immediately. The previous attemps in the target HAL tickers were not ideal, as it can wrap around easily (16 or 32 bit counters). This new
functionality should solve this problem.
set_interrupt for tickers in HAL code should not handle anything but the next match interrupt. If it was in the past is handled by the upper layer.
It is possible that we are setting next event to the close future, so once it is set it is already in the past. Therefore we add a check after set interrupt to verify it is in future.
If it is not, we fire interrupt immediately. This results in
two events - first one immediate, correct one. The second one might be scheduled in far future (almost entire ticker range),
that should be discarded.
The specification for the fire_interrupts are:
- should set pending bit for the ticker interrupt (as soon as possible),
the event we are scheduling is already in the past, and we do not want to skip
any events
- no arguments are provided, neither return value, not needed
- ticker should be initialized prior calling this function (no need to check if it is already initialized)
All our targets provide this new functionality, removing old misleading if (timestamp is in the past) checks.
For STM32 targets using a 32-bit timer for the microsecond ticker, the
driver did not properly handle timestamps that are in the past. It
would just blindly set the compare register to the requested timestamp,
resulting in the interrupt being serviced up to 4295 seconds late
(i.e. after the 32-bit timer counts all the way around to hit the
timestamp again).
This problem can easily be reproduced by creating a Timeout object
then calling the timeout's attach_us() member function to attach a
callback with a timeout of 0 us. The callback will not get called for
over 2147 seconds, and possibly up to 4295 seconds late if no other
microsecond ticker events are getting scheduled in the meantime.
Now, after the compare register has been set, the timestamp is checked
against the current time to see if the timestamp is in the past, and
if so, the compare event is manually set.
NOTE: By checking if the timestamp is in the past after configuring the
capture register, we ensure proper handling in the case where the timer
updates past the timestamp while setting the capture register.
Wolfgang Betz
Martin Kojtal
Bradley Scott
bcostm