Clock sync and unrealistic extruder temps after restart

[chuckwagoncomputing]

I have been Mildly Annoyed, and decided to do something about it.

I have been having the same issue as some other folks:
@viesturz
Cosmin
Not sure if the same issue, but seems likely

In short, when using a SB2209 CAN bus toolhead board, when restarting Klipper with RESTART, the extruder temperature is reported as a very large number, either negative or positive, before returning to normal over the next ~10 seconds. This doesn't happen all the time; it's pretty random.

Here is the cause as well as I can understand it:

1. The toolhead keeps sending thermocouple_result commands, because it wasn't restarted.
2. When a thermocouple_result command is sent before clock sync finishes, SecondarySync's clock_adj still has 1 for the mcu frequency, which results in spi_temperature.py's _handle_temperature_response getting an undivided value from its call to clock_to_print_time().
3. If the temperature reading has changed on the first thermocouple_result command after clock sync has finished, temperature_callback in heaters.py will multiply the difference in temperature by the difference between the undivided value and the new correct value, resulting in a very large "smoothed" temperature, either negative or positive depending on the direction of the temperature change.

On my machine, I implemented the following fix:

In _handle_spi_response in spi_temperature.py:

        last_read_time  = self.mcu.clock_to_print_time(last_read_clock)
+        if last_read_time == last_read_clock:
+            logging.warning("Ignoring untrustworthy clock sync")
+            return
        self._callback(last_read_time, temp)

I'm not sure if this is the appropriate fix, as this is the first time I've touched the Klipper codebase, but I rather doubt it. It feels like something more fundamental should change, like maybe clock_to_print_time should throw an error if the clock isn't synced.

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[Sineos]

We do not use GH to discuss such topics. Please refer to above link.

[chuckwagoncomputing]

Did you read it? This is not a request for help, a feature request, a question, or a request for diagnosis.
I found a firmware bug, and presented a solution for comment. Would you rather I open a pull request with my half-baked fix?

[Sineos]

https://www.klipper3d.org/Contact.html#i-found-a-bug-in-the-klipper-software