FTM: Add mixing extruder

Marlin/src/inc/SanityCheck.h

@@ -4724,9 +4724,6 @@ static_assert(_PLUS_TEST(3), "DEFAULT_MAX_ACCELERATION values must be positive."
  */
 #if ENABLED(FT_MOTION)
   static_assert(FTM_BUFFER_SIZE >= 4 && (FTM_BUFFER_SIZE & (FTM_BUFFER_SIZE - 1u)) == 0, "FTM_BUFFER_SIZE must be a power of two (128, 256, 512, ...).");
-  #if ENABLED(MIXING_EXTRUDER)
-    #error "FT_MOTION does not currently support MIXING_EXTRUDER."
-  #endif
   #if !HAS_X_AXIS
     static_assert(FTM_DEFAULT_SHAPER_X == ftMotionShaper_NONE, "Without any linear axes FTM_DEFAULT_SHAPER_X must be ftMotionShaper_NONE.");
   #endif
@@ -4748,8 +4745,6 @@ static_assert(_PLUS_TEST(3), "DEFAULT_MAX_ACCELERATION values must be positive."
   #if !HAS_STANDARD_MOTION
     #if ENABLED(SMOOTH_LIN_ADVANCE)
       #error "SMOOTH_LIN_ADVANCE is not yet available in FT_MOTION. Disable NO_STANDARD_MOTION if you require it."
-    #elif ENABLED(MIXING_EXTRUDER)
-      #error "MIXING_EXTRUDER is not yet available in FT_MOTION. Disable NO_STANDARD_MOTION if you require it."
     #elif ENABLED(SOFT_FEED_HOLD)
       #error "SOFT_FEED_HOLD is not yet available in FT_MOTION. Disable NO_STANDARD_MOTION if you require it."
     #elif ENABLED(DIRECT_STEPPING)

Marlin/src/module/ft_motion.cpp

@@ -401,6 +401,7 @@ bool FTMotion::plan_next_block() {
 
     // Cache the extruder index / axis for this block
     #if ANY(HAS_MULTI_EXTRUDER, MIXING_EXTRUDER)
+      TERN_(MIXING_EXTRUDER, mixer.stepper_setup(current_block->b_color));
       stepper.stepper_extruder = current_block->extruder;
     #endif
     #if ENABLED(DISTINCT_E_FACTORS)

Marlin/src/module/stepper.cpp

@@ -3687,7 +3687,12 @@ void Stepper::report_positions() {
     // Start step pulses on all axes including the active Extruder.
     // Edge stepping will simply toggle the STEP pin.
     #define _FTM_STEP_START(A) A##_APPLY_STEP(step_bits.A, false);
-    LOGICAL_AXIS_MAP(_FTM_STEP_START);
+    #if ENABLED(MIXING_EXTRUDER)
+      E_STEP_WRITE(mixer.get_next_stepper(), step_bits.E);
+      MAIN_AXIS_MAP(_FTM_STEP_START);
+    #else
+      LOGICAL_AXIS_MAP(_FTM_STEP_START);
+    #endif
 
     // Apply steps via I2S
     TERN_(I2S_STEPPER_STREAM, i2s_push_sample());
@@ -3723,7 +3728,12 @@ void Stepper::report_positions() {
 
     // Stop pulses. Axes with DEDGE will do nothing, assuming STEP_STATE_* is HIGH
     #define _FTM_STEP_STOP(AXIS) AXIS##_APPLY_STEP(!STEP_STATE_##AXIS, false);
-    LOGICAL_AXIS_MAP(_FTM_STEP_STOP);
+    #if ENABLED(MIXING_EXTRUDER)
+      E_STEP_WRITE(mixer.get_stepper(), !STEP_STATE_E);
+      MAIN_AXIS_MAP(_FTM_STEP_STOP);
+    #else
+      LOGICAL_AXIS_MAP(_FTM_STEP_STOP);
+    #endif
 
   } // Stepper::ftMotion_stepper