[BUG/DOCS] G28Z with MULTIPLE_PROBING doesn't behave as expected without HOMING_BUMP_MM

[slowbro]

Did you test the latest bugfix-2.0.x code?

Yes, and the problem still exists.

Bug Description

Beginning a while back, I noticed that my Z probe no longer double-checked the bed, but was not able to relate it to any changes made at the time (since I had been making a lot of changes/updated/etc). I rolled with it for a while, but got curious, and believe I found the answer. Around that time, I switched to SENSORLESS_HOMING for X and Y. In the comment above it, it says:

  /**
   * Use StallGuard to home / probe X, Y, Z.
   *
   ...
   * It is recommended to set HOMING_BUMP_MM to { 0, 0, 0 }.

I did as instructed, even though I am not using sensorless homing for Z. As it turns out, this seems to be what caused the MULTIPLE_PROBING breakdown on Z; in motion.cpp:

Marlin/Marlin/src/module/motion.cpp

Lines 1796 to 1800 in 8e06e9c

	// If a second homing move is configured...
	if (bump) {
	// Move away from the endstop by the axis HOMING_BUMP_MM
	if (DEBUGGING(LEVELING)) DEBUG_ECHOLNPAIR("Move Away: ", -bump, "mm");
	do_homing_move(axis, -bump, TERN(HOMING_Z_WITH_PROBE, (axis == Z_AXIS ? z_probe_fast_mm_s : 0), 0), false);

So, the second homing probe is buried under the bump conditional, which is defined as needing a HOMING_BUMP_MM for the axis..

Marlin/Marlin/src/module/motion.cpp

Lines 1780 to 1783 in 8e06e9c

	const bool use_probe_bump = TERN0(HOMING_Z_WITH_PROBE, axis == Z_AXIS && home_bump_mm(axis));
	const float bump = axis_home_dir * (
	use_probe_bump ? _MAX(TERN0(HOMING_Z_WITH_PROBE, Z_CLEARANCE_BETWEEN_PROBES), home_bump_mm(axis)) : home_bump_mm(axis)
	);

Marlin/Marlin/src/module/motion.h

Lines 136 to 139 in 8e06e9c

	inline float home_bump_mm(const AxisEnum axis) {
	static const xyz_pos_t home_bump_mm_P DEFS_PROGMEM = HOMING_BUMP_MM;
	return pgm_read_any(&home_bump_mm_P[axis]);
	}

Bug Timeline

After enabling sensorless homing, which recommends setting all homing bumps to 0 (and recommendations were followed).

Expected behavior

I expected the second Z probe to be based on the MULTIPLE_PROBING and/or EXTRA_PROBES settings, thus I expected Z to double probe on homing.

Actual behavior

With HOMING_BUMP_MM set to 0 for Z, MULTIPLE_PROBING is not respected for homing Z. There does not seem to be any documentation/comments that allude to this being the case.

Steps to Reproduce

1. Have a Z probe, set MULTIPLE_PROBING to 2 (or more), set HOMING_BUMP_MM to {0, 0, 0}
2. G28
3. Probe only probes once.

Version of Marlin Firmware

bugfix-2.0.x latest

Printer model

"ender 3" abomination

Electronics

SKR 1.4, TMC2209 drivers

Add-ons

BLTouch

Your Slicer

No response

Host Software

No response

Additional information & file uploads

I'm unsure what would be the best solution to this, but I have a few ideas:

1. Change homeaxis(...) to use MULTIPLE_PROBING value to detemine how many probes should be executed for Z, if HOMING_Z_WITH_PROBE is enabled.
2. Update comments/docs for MULTIPLE_PROBING to note it only applies to bed leveling/G38/etc.
3. Update comments/docs for HOMING_BUMP_MM to note that it controls multiple-probe on Z homing.

Either way, the comment SENSORLESS_HOMING should reflect only disabling HOMING_BUMP_MM for axes that you're enabling sensorless homing on.

[slowbro]

Well, I just found #9802, 3+ years old! It seems to also talk about this issue, though from my perspective multi-probing for Z always "worked" since I have had HOMING_BUMP_MM enabled for as long as I can remember.

Not sure if this should be closed in favor of that, or not.

[thisiskeithb]

Well, I just found #9802, 3+ years old! It seems to also talk about this issue

Yes, please consolidate discussions into that issue.

[thisiskeithb]

Duplicate of #9802

[github-actions]

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