DO_FENCE_ENABLE when sent as a command

[hamishwillee]

The MAV_CMD_DO_FENCE_ENABLE states that it is a mission item, but it can, and is sent as a command. This attempts to clarify the expected behaviour in this case.

The points of interest being that:

• mission settings persist in the mission context, even on flight stacks that pause by switching mode to "Hold": on return to the mission the mission setting would be used.
• As a command the value only persists until mode change or reboot.
• A command can affect a mission

Originates from discussion with @andyp1per in ArduPilot#349 (comment)

@sfuhrer @peterbarker Does this makes sense? Is it what is done? That's my assumption on what you'd want to happen but I haven't tested.

[sfuhrer]

Looks correct to me. The same way as we have implemented the SET_SPEED from what I see.

[hamishwillee]

Thanks @sfuhrer !

@peterbarker Does this work for you too?

[peterbarker]

On Wed, 20 Mar 2024, Hamish Willee wrote: > + <description> + Enable the geofence. + This can be used in a mission or via the command protocol. + The persistence/lifetime of the setting is undefined. + Depending on flight stack implementation it may persist until superseded (even across reboots), or it may revert to a system default at the end of a mission or on reboot. + </description> @peterbarker Work for you now?

About as good as it's going to get ATM :-/ Note that ArduPilot has had some behaviour where non-parameter mavlink commands have changed persistent state on the vehicle where they shouldn't. We are cleaning those up as time goes on. I don't think ArduPilot persists any fence state based on these mavlink commands, 'though.... we initialise state from parameters but don't store the state back into those parameters.

[hamishwillee]

I don't think ArduPilot persists any fence state based on these mavlink commands, 'though.... we initialise state from parameters but don't store the state back into those parameters.

Oh, I understood from your comment that ArduPilot wrote to a parameter. So you reset on boot too?

[peterbarker]

On Thu, 21 Mar 2024, Hamish Willee wrote: I don't think ArduPilot persists any fence state based on these mavlink commands, 'though.... we initialise state from parameters but don't store the state back into those parameters. Oh, I understood from your comment that ArduPilot wrote to a parameter. So you reset on boot too?

Net outcome, yes. Not really a matter of "reset on boot" so much as "at boot the value is initialised from parameter" and that parameter's not changed by these commands.

[hamishwillee]

Thanks @peterbarker - that's what I understood. I've updated to make it clear that "most flight stacks update on reboot". Going to merge now, as this is hand wavey, but more correct than before.