MAG_ENABLE fails to disable compass

[budebulai]

Issue details

Using old version ardupilot firmware, we can use MAG_ENABLE to select if enabling compass, now it fails in 0a18f5e and others.
With new firmware, when setting MAG_ENABLE to 0, firmware skip compass init, but it will still check compass status when arming motors.
We can see there are three _state[i].use_for_yaw in parameters, if I choose not to use compass for vehicle heading, I have to set:
MAG_ENABLE = 0
_state[0].use_for_yaw = 0
_state[1].use_for_yaw = 0
_state[2].use_for_yaw = 0
ARMING_CHECK = -5
boring
So MAG_ENABLE means nothing now.

Version

0a18f5e

Platform

[ ] All

[OXINARF]

What firmware is this with? Plane, Copter, Rover?

[rmackay9]

Yeah, I've been thinking about this MAG_ENABLE parameter on and off. I'm not surprised if we find out it's meaningless. When we added the COMPASS_USE parameters we probably didn't take the extra step to remove MAG_ENABLE.

[rmackay9]

By the way, we have some related issues in our to-do list:
#5454
#5451

I guess the point is that it may or may not be possible to fly without a compass in AC3.4. I think you could try by setting all the COMPASS_USE parameters to zero, then disable the compass part of the ARMING_CHECK parameter and then see what happens :-). Certainly loiter won't work of course, but Stabilize, AltHold and ACRO should work.

[budebulai]

Sorry, I made a mistake.
I forget what I've done to the code, now MP report "PreArm: Compass not healthy" even I set all compass params and arming_check to 0.
I have to check now
Sorry again

[budebulai]

Thank you, @rmackay9
Copter 3.5 will stop pixhawk from being armed if compass disabled.
And I comment compass check, pixhawk armed

[rmackay9]

My main sponsor brought this up today actually. They found it confusing that to arm and takeoff in stabilize mode (without a compass) they need to leave MAG_ENABLE = 1 and then set the COMPASS_USE, COMPASS_USE2 = 0.
I think we should remove the MAG_ENABLE parameter or move it into the COMPASS library.

[OXINARF]

@rmackay9 I completely agree that it is very confusing. In my opinion we should just remove it. If users don't want to use compass they disable it with the COMPASS_USE parameters. I think that adding a general compass enable parameter would also be confusing.

[rmackay9]

This parameter MAG_ENABLE has been moved to COMPASS_ENABLE so I think I'll close this issue. Hopefully compass-enable does disable all compasses - I think it must.