Fix mixing extruder feedrate (#15905)

Marlin/src/module/planner.cpp

@@ -2007,27 +2007,37 @@ bool Planner::_populate_block(block_t * const block, bool split_move,
       filwidth.advance_e(delta_mm.e);
   #endif
 
-  // Calculate and limit speed in mm/sec for each axis
+  // Calculate and limit speed in mm/sec
+
   xyze_float_t current_speed;
   float speed_factor = 1.0f; // factor <1 decreases speed
-  LOOP_XYZE(i) {
+
+  // Linear axes first with less logic
+  LOOP_XYZ(i) {
+    current_speed[i] = delta_mm[i] * inverse_secs;
+    const feedRate_t cs = ABS(current_speed[i]),
+                 max_fr = settings.max_feedrate_mm_s[i];
+    if (cs > max_fr) NOMORE(speed_factor, max_fr / cs);
+  }
+
+  // Limit speed on extruders, if any
+  #if EXTRUDERS
+    {
+      current_speed.e = delta_mm.e * inverse_secs;
       #if BOTH(MIXING_EXTRUDER, RETRACT_SYNC_MIXING)
-      // In worst case, only one extruder running, no change is needed.
-      // In best case, all extruders run the same amount, we can divide by MIXING_STEPPERS
-      float delta_mm_i = 0;
-      if (i == E_AXIS && mixer.get_current_vtool() == MIXER_AUTORETRACT_TOOL)
-        delta_mm_i = delta_mm.e / MIXING_STEPPERS;
-      else
-        delta_mm_i = delta_mm.e;
-    #else
-      const float delta_mm_i = delta_mm[i];
+        // Move all mixing extruders at the specified rate
+        if (mixer.get_current_vtool() == MIXER_AUTORETRACT_TOOL)
+          current_speed.e *= MIXING_STEPPERS;
       #endif
-    const feedRate_t cs = ABS(current_speed[i] = delta_mm_i * inverse_secs);
-    #if ENABLED(DISTINCT_E_FACTORS)
-      if (i == E_AXIS) i += extruder;
+      const feedRate_t cs = ABS(current_speed.e),
+                   max_fr = (settings.max_feedrate_mm_s[E_AXIS_N(extruder)]
+                              #if BOTH(MIXING_EXTRUDER, RETRACT_SYNC_MIXING)
+                                * MIXING_STEPPERS
                               #endif
-    if (cs > settings.max_feedrate_mm_s[i]) NOMORE(speed_factor, settings.max_feedrate_mm_s[i] / cs);
+                            );
+      if (cs > max_fr) NOMORE(speed_factor, max_fr / cs);
     }
+  #endif
 
   // Max segment time in µs.
   #ifdef XY_FREQUENCY_LIMIT