Merge pull request #8399 from thinkyhead/bf2_fix_M428

[2.0] Improve and clarify M428 code

Marlin/src/gcode/geometry/G53-G59.cpp

 #if ENABLED(CNC_COORDINATE_SYSTEMS)
 
   /**
-   * Select a coordinate system and update the current position.
+   * Select a coordinate system and update the workspace offset.
    * System index -1 is used to specify machine-native.
    */
   bool GCodeSuite::select_coordinate_system(const int8_t _new) {
     if (WITHIN(_new, 0, MAX_COORDINATE_SYSTEMS - 1))
       COPY(new_offset, coordinate_system[_new]);
     active_coordinate_system = _new;
-    bool didXYZ = false;
     LOOP_XYZ(i) {
       const float diff = new_offset[i] - old_offset[i];
       if (diff) {
         position_shift[i] += diff;
         update_software_endstops((AxisEnum)i);
-        didXYZ = true;
       }
     }
-    if (didXYZ) SYNC_PLAN_POSITION_KINEMATIC();
     return true;
   }
 

Marlin/src/gcode/geometry/G92.cpp

     #define IS_G92_0 true
   #endif
 
-  bool didXYZ = false, didE = false;
+  bool didE = false;
+  #if IS_SCARA || !HAS_POSITION_SHIFT
+    bool didXYZ = false;
+  #else
+    constexpr bool didXYZ = false;
+  #endif
 
   if (IS_G92_0) LOOP_XYZE(i) {
     if (parser.seenval(axis_codes[i])) {
                   v = i == E_AXIS ? l : LOGICAL_TO_NATIVE(l, i),
                   d = v - current_position[i];
       if (!NEAR_ZERO(d)) {
-        if (i == E_AXIS) didE = true; else didXYZ = true;
-        #if IS_SCARA
-          current_position[i] = v;        // For SCARA just set the position directly
+        #if IS_SCARA || !HAS_POSITION_SHIFT
+          if (i == E_AXIS) didE = true; else didXYZ = true;
+          current_position[i] = v;        // Without workspaces revert to Marlin 1.0 behavior
         #elif HAS_POSITION_SHIFT
-          if (i == E_AXIS)
+          if (i == E_AXIS) {
+            didE = true;
             current_position[E_AXIS] = v; // When using coordinate spaces, only E is set directly
+          }
           else {
             position_shift[i] += d;       // Other axes simply offset the coordinate space
             update_software_endstops((AxisEnum)i);
           }
-        #else
-          current_position[i] = v;        // Without workspaces revert to Marlin 1.0 behavior
         #endif
       }
     }

Marlin/src/gcode/geometry/M206_M428.cpp

     if (parser.seen('P')) set_home_offset(B_AXIS, parser.value_linear_units()); // Psi
   #endif
 
-  SYNC_PLAN_POSITION_KINEMATIC();
   report_current_position();
 }
 
  *       Use M206 to set these values directly.
  */
 void GcodeSuite::M428() {
-  bool err = false;
+  if (axis_unhomed_error()) return;
+
+  float diff[XYZ];
   LOOP_XYZ(i) {
-    if (axis_homed[i]) {
-      const float base = (current_position[i] > (soft_endstop_min[i] + soft_endstop_max[i]) * 0.5) ? base_home_pos((AxisEnum)i) : 0,
-                  diff = base - current_position[i];
-      if (WITHIN(diff, -20, 20)) {
-        set_home_offset((AxisEnum)i, diff);
-      }
-      else {
-        SERIAL_ERROR_START();
-        SERIAL_ERRORLNPGM(MSG_ERR_M428_TOO_FAR);
-        LCD_ALERTMESSAGEPGM("Err: Too far!");
-        BUZZ(200, 40);
-        err = true;
-        break;
-      }
+    diff[i] = base_home_pos((AxisEnum)i) - current_position[i];
+    if (!WITHIN(diff[i], -20, 20) && home_dir((AxisEnum)i) > 0)
+      diff[i] = -current_position[i];
+    if (!WITHIN(diff[i], -20, 20)) {
+      SERIAL_ERROR_START();
+      SERIAL_ERRORLNPGM(MSG_ERR_M428_TOO_FAR);
+      LCD_ALERTMESSAGEPGM("Err: Too far!");
+      BUZZ(200, 40);
+      return;
     }
   }
 
-  if (!err) {
-    SYNC_PLAN_POSITION_KINEMATIC();
-    report_current_position();
-    LCD_MESSAGEPGM(MSG_HOME_OFFSETS_APPLIED);
-    BUZZ(100, 659);
-    BUZZ(100, 698);
-  }
+  LOOP_XYZ(i) set_home_offset((AxisEnum)i, diff[i]);
+  report_current_position();
+  LCD_MESSAGEPGM(MSG_HOME_OFFSETS_APPLIED);
+  BUZZ(100, 659);
+  BUZZ(100, 698);
 }
 
 #endif // HAS_M206_COMMAND

Marlin/src/module/motion.h

       || ENABLED(NOZZLE_CLEAN_FEATURE)                                             \
       || ENABLED(NOZZLE_PARK_FEATURE)                                              \
       || (ENABLED(ADVANCED_PAUSE_FEATURE) && ENABLED(HOME_BEFORE_FILAMENT_CHANGE)) \
+      || HAS_M206_COMMAND                                                          \
     ) || ENABLED(NO_MOTION_BEFORE_HOMING)
 
 #if HAS_AXIS_UNHOMED_ERR