add get_current_line_number

Author: ellensp

Marlin/src/gcode/gcode.h

@@ -143,7 +143,7 @@
  *        R<temp> Wait for extruder current temp to reach target temp. ** Wait for heating or cooling. **
  *        If AUTOTEMP is enabled, S<mintemp> B<maxtemp> F<factor>. Exit autotemp by any M109 without F
  *
- * M110 - Set the current line number. (Used by host printing)
+ * M110 - Get or set the current line number. (Used by host printing)
  * M111 - Set debug flags: "M111 S<flagbits>". See flag bits defined in enum.h.
  * M112 - Full Shutdown.
  *

Marlin/src/gcode/host/M110.cpp

@@ -24,11 +24,12 @@
 #include "../queue.h" // for last_N
 
 /**
- * M110: Set Current Line Number
+ * M110: Get or set Current Line Number
  */
 void GcodeSuite::M110() {
 
   if (parser.seenval('N'))
     queue.set_current_line_number(parser.value_long());
-
+  else
+    SERIAL_ECHO_MSG("M110 N", queue.get_current_line_number());
 }

Marlin/src/gcode/queue.h

@@ -212,6 +212,11 @@ class GCodeQueue {
    */
   static void set_current_line_number(long n) { serial_state[ring_buffer.command_port().index].last_N = n; }
 
+  /**
+   * Get the current line number for the last received command
+   */
+  static long get_current_line_number() { return serial_state[ring_buffer.command_port().index].last_N; }
+
   #if ENABLED(BUFFER_MONITORING)
 
     private: