🐛 Fix steps-to-mm with backlash (#23814)

Co-authored-by: Scott Lahteine <thinkyhead@users.noreply.github.com>

Marlin/src/feature/backlash.cpp

 #include "../module/motion.h"
 #include "../module/planner.h"
 
+axis_bits_t Backlash::last_direction_bits;
+#ifdef BACKLASH_SMOOTHING_MM
+  xyz_long_t Backlash::residual_error{0};
+#endif
+
 #ifdef BACKLASH_DISTANCE_MM
   #if ENABLED(BACKLASH_GCODE)
     xyz_float_t Backlash::distance_mm = BACKLASH_DISTANCE_MM;
  */
 
 void Backlash::add_correction_steps(const int32_t &da, const int32_t &db, const int32_t &dc, const axis_bits_t dm, block_t * const block) {
-  static axis_bits_t last_direction_bits;
   axis_bits_t changed_dir = last_direction_bits ^ dm;
   // Ignore direction change unless steps are taken in that direction
   #if DISABLED(CORE_BACKLASH) || EITHER(MARKFORGED_XY, MARKFORGED_YX)
     // smoothing distance. Since the computation of this proportion involves a floating point
     // division, defer computation until needed.
     float segment_proportion = 0;
-
-    // Residual error carried forward across multiple segments, so correction can be applied
-    // to segments where there is no direction change.
-    static xyz_long_t residual_error{0};
   #else
     // No direction change, no correction.
     if (!changed_dir) return;
   }
 }
 
+int32_t Backlash::applied_steps(const AxisEnum axis) {
+  if (axis >= LINEAR_AXES) return 0;
+
+  const bool reversing = TEST(last_direction_bits, axis);
+
+  #ifdef BACKLASH_SMOOTHING_MM
+    const int32_t residual_error_axis = residual_error[axis];
+  #else
+    constexpr int32_t residual_error_axis = 0;
+  #endif
+
+  // At startup it is assumed the last move was forwards. So the applied
+  // steps will always be a non-positive number.
+
+  if (!reversing) return -residual_error_axis;
+
+  const float f_corr = float(correction) / 255.0f;
+  const int32_t full_error_axis = -f_corr * distance_mm[axis] * planner.settings.axis_steps_per_mm[axis];
+  return full_error_axis - residual_error_axis;
+}
+
 #if ENABLED(MEASURE_BACKLASH_WHEN_PROBING)
 
   #include "../module/probe.h"

Marlin/src/feature/backlash.h

 constexpr uint8_t all_on = 0xFF, all_off = 0x00;
 
 class Backlash {
+private:
+  static axis_bits_t last_direction_bits;
+  #ifdef BACKLASH_SMOOTHING_MM
+    static xyz_long_t residual_error;
+  #endif
+
 public:
   #if ENABLED(BACKLASH_GCODE)
     static xyz_float_t distance_mm;
     return has_measurement(X_AXIS) || has_measurement(Y_AXIS) || has_measurement(Z_AXIS);
   }
 
-  void add_correction_steps(const int32_t &da, const int32_t &db, const int32_t &dc, const axis_bits_t dm, block_t * const block);
+  static void add_correction_steps(const int32_t &da, const int32_t &db, const int32_t &dc, const axis_bits_t dm, block_t * const block);
+  static int32_t applied_steps(const AxisEnum axis);
 };
 
 extern Backlash backlash;

Marlin/src/module/planner.cpp

 }
 
 float Planner::triggered_position_mm(const AxisEnum axis) {
-  return stepper.triggered_position(axis) * mm_per_step[axis];
+  const float result = DIFF_TERN(BACKLASH_COMPENSATION, stepper.triggered_position(axis), backlash.applied_steps(axis));
+  return result * mm_per_step[axis];
 }
 
 void Planner::finish_and_disable() {
       // Protect the access to the position.
       const bool was_enabled = stepper.suspend();
 
-      const int32_t p1 = stepper.position(CORE_AXIS_1),
-                    p2 = stepper.position(CORE_AXIS_2);
+      const int32_t p1 = DIFF_TERN(BACKLASH_COMPENSATION, stepper.position(CORE_AXIS_1), backlash.applied_steps(CORE_AXIS_1)),
+                    p2 = DIFF_TERN(BACKLASH_COMPENSATION, stepper.position(CORE_AXIS_2), backlash.applied_steps(CORE_AXIS_2));
 
       if (was_enabled) stepper.wake_up();
 
       axis_steps = (axis == CORE_AXIS_2 ? CORESIGN(p1 - p2) : p1 + p2) * 0.5f;
     }
     else
-      axis_steps = stepper.position(axis);
+      axis_steps = DIFF_TERN(BACKLASH_COMPENSATION, stepper.position(axis), backlash.applied_steps(axis));
 
   #elif EITHER(MARKFORGED_XY, MARKFORGED_YX)
 
       axis_steps = ((axis == CORE_AXIS_1) ? p1 - p2 : p2);
     }
     else
-      axis_steps = stepper.position(axis);
+      axis_steps = DIFF_TERN(BACKLASH_COMPENSATION, stepper.position(axis), backlash.applied_steps(axis));
 
   #else
 
     axis_steps = stepper.position(axis);
+    TERN_(BACKLASH_COMPENSATION, axis_steps -= backlash.applied_steps(axis));
 
   #endif
 
   block->flag = sync_flag;
 
   block->position = position;
+  #if ENABLED(BACKLASH_COMPENSATION)
+    LOOP_LINEAR_AXES(axis) block->position[axis] += backlash.applied_steps((AxisEnum)axis);
+  #endif
 
   #if BOTH(HAS_FAN, LASER_SYNCHRONOUS_M106_M107)
     FANS_LOOP(i) block->fan_speed[i] = thermalManager.fan_speed[i];
       LROUND(abce.k * settings.axis_steps_per_mm[K_AXIS])
     )
   );
+
   if (has_blocks_queued()) {
     //previous_nominal_speed_sqr = 0.0; // Reset planner junction speeds. Assume start from rest.
     //previous_speed.reset();
     buffer_sync_block();
   }
-  else
-    stepper.set_position(position);
+  else {
+    #if ENABLED(BACKLASH_COMPENSATION)
+      abce_long_t stepper_pos = position;
+      LOOP_LINEAR_AXES(axis) stepper_pos[axis] += backlash.applied_steps((AxisEnum)axis);
+      stepper.set_position(stepper_pos);
+    #else
+      stepper.set_position(position);
+    #endif
+  }
 }
 
 void Planner::set_position_mm(const xyze_pos_t &xyze) {