homing: Log a warning if probe alters stepper kinematic positions

After a probe attempt the toolhead position needs to be recalculated
to the position that the toolhead ultimately halted at.  Check that
the position setting wouldn't actually change the internal view of the
stepper motor and log a warning if any skew is detected.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/extras/homing.py

@@ -1,6 +1,6 @@
 # Helper code for implementing homing operations
 #
-# Copyright (C) 2016-2021  Kevin O'Connor <kevin@koconnor.net>
+# Copyright (C) 2016-2024  Kevin O'Connor <kevin@koconnor.net>
 #
 # This file may be distributed under the terms of the GNU GPLv3 license.
 import logging, math
@@ -29,10 +29,17 @@ def __init__(self, stepper, endstop_name):
         self.endstop_name = endstop_name
         self.stepper_name = stepper.get_name()
         self.start_pos = stepper.get_mcu_position()
+        self.start_cmd_pos = stepper.mcu_to_commanded_position(self.start_pos)
         self.halt_pos = self.trig_pos = None
     def note_home_end(self, trigger_time):
         self.halt_pos = self.stepper.get_mcu_position()
         self.trig_pos = self.stepper.get_past_mcu_position(trigger_time)
+    def verify_no_probe_skew(self, haltpos):
+        new_start_pos = self.stepper.get_mcu_position(self.start_cmd_pos)
+        if new_start_pos != self.start_pos:
+            logging.warning(
+                "Stepper '%s' position skew after probe: pos %d now %d",
+                self.stepper.get_name(), self.start_pos, new_start_pos)
 
 # Implementation of homing/probing moves
 class HomingMove:
@@ -121,6 +128,9 @@ def homing_move(self, movepos, speed, probe_pos=False,
             haltpos = trigpos = self.calc_toolhead_pos(kin_spos, trig_steps)
             if trig_steps != halt_steps:
                 haltpos = self.calc_toolhead_pos(kin_spos, halt_steps)
+            self.toolhead.set_position(haltpos)
+            for sp in self.stepper_positions:
+                sp.verify_no_probe_skew(haltpos)
         else:
             haltpos = trigpos = movepos
             over_steps = {sp.stepper_name: sp.halt_pos - sp.trig_pos
@@ -130,7 +140,7 @@ def homing_move(self, movepos, speed, probe_pos=False,
                 halt_kin_spos = {s.get_name(): s.get_commanded_position()
                                  for s in kin.get_steppers()}
                 haltpos = self.calc_toolhead_pos(halt_kin_spos, over_steps)
-        self.toolhead.set_position(haltpos)
+            self.toolhead.set_position(haltpos)
         # Signal homing/probing move complete
         try:
             self.printer.send_event("homing:homing_move_end", self)

klippy/stepper.py

@@ -138,8 +138,10 @@ def set_position(self, coord):
     def get_commanded_position(self):
         ffi_main, ffi_lib = chelper.get_ffi()
         return ffi_lib.itersolve_get_commanded_pos(self._stepper_kinematics)
-    def get_mcu_position(self):
-        mcu_pos_dist = self.get_commanded_position() + self._mcu_position_offset
+    def get_mcu_position(self, cmd_pos=None):
+        if cmd_pos is None:
+            cmd_pos = self.get_commanded_position()
+        mcu_pos_dist = cmd_pos + self._mcu_position_offset
         mcu_pos = mcu_pos_dist / self._step_dist
         if mcu_pos >= 0.:
             return int(mcu_pos + 0.5)