Allow buffer clean without release command

Marlin/stepper.cpp

 // private:
 
 uint8_t Stepper::last_direction_bits = 0;        // The next stepping-bits to be output
-uint16_t Stepper::cleaning_buffer_counter = 0;
+int16_t Stepper::cleaning_buffer_counter = 0;
 
 #if ENABLED(X_DUAL_ENDSTOPS)
   bool Stepper::locked_x_motor = false, Stepper::locked_x2_motor = false;
 
   uint16_t ocr_val;
 
-  #define ENDSTOP_NOMINAL_OCR_VAL 3000    // check endstops every 1.5ms to guarantee two stepper ISRs within 5ms for BLTouch
-  #define OCR_VAL_TOLERANCE 1000          // First max delay is 2.0ms, last min delay is 0.5ms, all others 1.5ms
+  #define ENDSTOP_NOMINAL_OCR_VAL 3000 // Check endstops every 1.5ms to guarantee two stepper ISRs within 5ms for BLTouch
+  #define OCR_VAL_TOLERANCE       1000 // First max delay is 2.0ms, last min delay is 0.5ms, all others 1.5ms
 
   #if DISABLED(LIN_ADVANCE)
     // Disable Timer0 ISRs and enable global ISR again to capture UART events (incoming chars)
 
   #define _SPLIT(L) (ocr_val = (uint16_t)L)
   #if ENABLED(ENDSTOP_INTERRUPTS_FEATURE)
+
     #define SPLIT(L) _SPLIT(L)
-  #else                 // sample endstops in between step pulses
+
+  #else // !ENDSTOP_INTERRUPTS_FEATURE : Sample endstops between stepping ISRs
+
     static uint32_t step_remaining = 0;
+
     #define SPLIT(L) do { \
       _SPLIT(L); \
       if (ENDSTOPS_ENABLED && L > ENDSTOP_NOMINAL_OCR_VAL) { \
 
     if (step_remaining && ENDSTOPS_ENABLED) {   // Just check endstops - not yet time for a step
       endstops.update();
-      if (step_remaining > ENDSTOP_NOMINAL_OCR_VAL) {
-        step_remaining -= ENDSTOP_NOMINAL_OCR_VAL;
-        ocr_val = ENDSTOP_NOMINAL_OCR_VAL;
-      }
-      else {
-        ocr_val = step_remaining;
-        step_remaining = 0;  //  last one before the ISR that does the step
-      }
 
+      // Next ISR either for endstops or stepping
+      ocr_val = step_remaining <= ENDSTOP_NOMINAL_OCR_VAL ? step_remaining : ENDSTOP_NOMINAL_OCR_VAL;
+      step_remaining -= ocr_val;
       _NEXT_ISR(ocr_val);
-
       NOLESS(OCR1A, TCNT1 + 16);
-
       _ENABLE_ISRs(); // re-enable ISRs
       return;
     }
-  #endif
 
+  #endif // !ENDSTOP_INTERRUPTS_FEATURE
+
+  //
+  // When cleaning, discard the current block and run fast
+  //
   if (cleaning_buffer_counter) {
-    --cleaning_buffer_counter;
     current_block = NULL;
     planner.discard_current_block();
-    #ifdef SD_FINISHED_RELEASECOMMAND
-      if (!cleaning_buffer_counter && (SD_FINISHED_STEPPERRELEASE)) enqueue_and_echo_commands_P(PSTR(SD_FINISHED_RELEASECOMMAND));
-    #endif
-    _NEXT_ISR(200); // Run at max speed - 10 KHz
-    _ENABLE_ISRs(); // re-enable ISRs
+    if (cleaning_buffer_counter < 0)
+      ++cleaning_buffer_counter;            // Count up for endstop hit
+    else {
+      --cleaning_buffer_counter;            // Count down for abort print
+      #ifdef SD_FINISHED_RELEASECOMMAND
+        if (!cleaning_buffer_counter && (SD_FINISHED_STEPPERRELEASE)) enqueue_and_echo_commands_P(PSTR(SD_FINISHED_RELEASECOMMAND));
+      #endif
+    }
+    _NEXT_ISR(200);                             // Run at max speed - 10 KHz
+    _ENABLE_ISRs();
     return;
   }
 

Marlin/stepper.h

   private:
 
     static uint8_t last_direction_bits;        // The next stepping-bits to be output
-    static uint16_t cleaning_buffer_counter;
+    static int16_t cleaning_buffer_counter;
 
     #if ENABLED(X_DUAL_ENDSTOPS)
       static bool locked_x_motor, locked_x2_motor;