Fix LTM Scheduler not being set if port is shared.

[giacomo892]

Thanks @fiam to pointing this out during tests to try to share SBUS port with LTM.

Seems like there are only a few useful usages related to sharing LTM with SBUS due to the nature of SBUS protocol.

[stronnag]

Doesn't this break the speed sanity check, allowing a configured value to override a speed limitation?

[giacomo892]

When LTM is shared with a Serial RX protocol the baud rate will be high enough. Like 100000b/s for SBUS and 115200b/s for IBUS for istance. Am I wrong?

[stronnag]

It may be shared with a MSP at 1200 baud. Does your change cover that case?

[stronnag]

Currently the code handles two cases:

• Baud rate is adequate, but the user wants to set a slower rate (e.g. 9600 and slow LTM)
• User has set a fast rate, but the baud rate is too low (1200 baud)

In both cases, the LTM rate 'does the right thing', in the latter case avoiding data overrun / corruption.

It would be helpful if this behaviour could be preserved for all use cases.

[giacomo892]

MSP and LTM on the same port is not really a "shared" port. Since LTM isn't working at the same time as MSP. LTM will be emitted only when armed. So everything should work as it was working before in my opinion.

[stronnag]

I don''t think so, in the latter case, the scheduler is set unconditionally from config in configureLtmScheduler, overriding the baud checks in configureLtmTelemetryPort(). Moving the baud based checks to after the configureLtmScheduler call would restore the expected behaviour in this case.

[giacomo892]

Fact is that port setup is not done when the port is shared with another protocol since the port has being already initialized by the other protocol.

[stronnag]

That is not the point, consider the case where the user forgets that the default for ltm_update_rate is NORMAL (300 bytes/sec) and starts with:

serial 2 17 9600 38400 9600 115200

Everything works, 300 bytes/sec LTM is supported by 9600 baud.
Then the user decidest o see if she can get much better range at a lower speed and sets:

serial 2 17 1200 38400 1200 115200

The current code will automatically switch the ltm_scheduler to SLOW based on the baud rate; 1200 can only support 100 bytes/sec. And everything magically still works.

With this PR, this case is broken, the ltm_schduler is always selected from config and not sanitised by the baud rate. This is an (unintended) regression. The 1200 baud pipe cannot support 300 bytes/sec. Telemetry is corrupted.

It is also easily fixed by changing:

  configureLtmTelemetryPort();
  configureLtmScheduler();

to

  configureLtmScheduler();
  configureLtmTelemetryPort();

then the previous, intended behaviour is maintained.

[stronnag]

as configureLtmScheduler() is only used by ltm.c, it can be declared before use as static and does not need to be forward declared in ltm.h

Please change the order of

configureLtmTelemetryPort();
configureLtmScheduler();

to

configureLtmScheduler();
configureLtmTelemetryPort();

in order to maintain the current baud rate sanitisation of ltm_schedule

[giacomo892]

@stronnag Followed your advices. Thanks.

[digitalentity]

Looks ok to merge to me