sanity check for Z_CLEARANCE_FOR_HOMING

[DerAndere1]

Description

Certain configs require Z_CLEARANCE_FOR_HOMING <= 0 to prevent a crash into Z max during homing. This adds another sanity check

Requirements

#define Z_HOME_DIR 1
#define HOME_Z_FIRST

Benefits

prevent a crash into Z max during homing

Configurations

None

Related Issues

None

[DerAndere1]

MarlinFirmware/Configurations#1094 must be merged first to pass CI tests

[thinkyhead]

Especially when homing Z first, there should probably never be any instance where Z_CLEARANCE_FOR_HOMING is less than 0 because this would inevitably cause the toolhead to dig into the bed or workpiece. So the only change I would suggest is to assert that it must be == 0 instead of <= 0.

[thinkyhead]

While we're ruminating on homing and raises, I believe we need a z_min_trusted flag to avoid unnecessary raises. Here is a common scenario:

• Start a 3D printer and before using Home All use Home X.
• The machine raises Z before homing X because Z is "not homed" and "not trusted."
• Click on Home X again.
• The machine raises Z before homing X because Z is "not homed" and "not trusted."
• …and on it goes…

With the new flag we can get the desired effect…

• Start a 3D printer and before using Home All use Home X.
• The machine raises Z before homing X because z_min_trusted is false.
• As soon as the Z motor is powered set z_min_trusted.
• Click on Home X again.
• The machine doesn't raises Z because z_min_trusted Is true.

The z_min_trusted flag would be cleared only when the Z motors go to sleep on a delta or other machine where the consequence would be a smaller (possibly zero) Z coordinate. If the steppers were powered off and back on in such a scenario, the Z position would be meaningless, so we would want to keep z_min_trusted set to false until the machine was re-homed. In any case, the situation with z_min_trusted would need to be evaluated on a case-by-case basis, but basically it will either track with axes_trusted or it won't depending on the mechanics.