G29: Report final position back to host when using MBL

[jbrazio]

Closes #3770, fix by @Blue-Marlin.. but too lazy to submit it !

🍻

[thinkyhead]

Very nice. Do we also have a PR (or want to extend this) to keep the nozzle where it is at the end of probing, instead of moving it back down to Z=0? On my own setup, I used MIN_Z_FOR_HOMING to prevent the nozzle from scraping the bed on the final G28 after MBL. But maybe I don't need that if all the G29 routines will leave the nozzle above the bed.

Side note: Since we have home_offset[Z_AXIS], any final move intended to be at the bed height should actually be to Z=home_offset[Z_AXIS], rather than to Z=0 anyway. (Haven't checked the code to see whether that is the case.)

[jbrazio]

Do we also have a PR (or want to extend this) to keep the nozzle where it is at the end of probing

I would say no.. it may cause us more problems than benefits.

[epatel]

Well, the reason for doing this reporting is to not having to move to [0,0,0]. So not adding that change seem confusing I think. I have a commit for this that @brainscan has agreed to test over here https://github.com/MarlinFirmware/MarlinDev/issues/162#issuecomment-219864846

[jbrazio]

@epatel but that's is only related with MBL's G29 right ?
MBL is your domain I will not fight for it despite I believe no probing or homing should leave the nozzle touching the bed for obvious security reasons.

[epatel]

@jbrazio Yes it is for MBL only. There are request for both behaviors and I do not see this that dangerous, I have actually been using it like that for over a year. But again, I don't leave my hotend hot for long periods of time if I don't print with my printers. Also, if MBL is active it shouldn't drag across the bed as MBL should be compensating the bed as intended. If it is not active, well, then we are back to the discussion about having the endstop at the bed or above (which I consider a much more important security issue, but hey that new stuff is in so what can I do?)

Also, I set the default behavior to not do this move.

Well, I guess there are a lot of printers that have no bed leveling and have the Z endstop on the bed (I guess this would be pretty normal too)