rs274ngc: add G28.2 to home the machine from G-code

G28.2 lets a program or MDI line reference the machine instead of
requiring the GUI Home All button. The bare form homes every joint in
HOME_SEQUENCE order; an optional Pn homes joint n only (0-based
[JOINT_n] numbering), reusing the existing EMC_JOINT_HOME 'joint' field
so no NML change is needed and it works on any kinematics. Axis-letter
forms are deliberately not supported - resolving a letter to a joint
needs the kinematics map, and homing is a joint concept. There is no
G-code unhome (dropped in review of PR LinuxCNC#4172).

New canon calls HOME_CYCLE() / HOME_CYCLE_JOINT(n). The milltask backend
flushes the segment buffer before queuing the home, or a move buffered
for arc-blend lookahead would reorder after it.

When the cycle finishes the interpreter resyncs its current position
from the machine, the same way it does after probing or a tool change
(home_flag -> INTERP_EXECUTE_FINISH -> refresh_actual_position() in
read_inputs). An immediate home rewrites the joint coordinate to
HOME_OFFSET even with no physical motion, so without the resync a
following G91 move or an I/J/K arc centre is computed from the stale
pre-home point - on a wrapped rotary head re-homed mid-program with
G28.2 Pn, the axis would then sweep the whole error. (Raised in
PR LinuxCNC#4172 review.)

Author: greatEndian

src/emc/nml_intf/canon.hh

@@ -243,6 +243,13 @@ extern void SET_G92_OFFSET(double x, double y, double z,
 
 extern void SET_XY_ROTATION(double t);
 
+/* G28.2: trigger the machine homing cycle from G-code (bare form = all
+ * joints, in HOME_SEQUENCE order). Maps to EMC_JOINT_HOME(-1). */
+extern void HOME_CYCLE(void);
+/* G28.2 Pn: home a single joint by its 0-based joint number (matching
+ * [JOINT_n] INI section numbering). Maps to EMC_JOINT_HOME(joint). */
+extern void HOME_CYCLE_JOINT(int joint);
+
 /* Offset the origin to the point with absolute coordinates x, y, z,
 a, b, c, u, v, and w. Values of x, y, z, a, b, c, u, v, and w are real 
 numbers. The units are whatever length units are being used at the time 

src/emc/rs274ngc/gcodemodule.cc

@@ -705,6 +705,9 @@ void SELECT_PLANE(CANON_PLANE pl) {
     Py_XDECREF(result);
 }
 
+void HOME_CYCLE(void) {}
+void HOME_CYCLE_JOINT(int) {}
+
 void SET_TRAVERSE_RATE(double rate) {
     maybe_new_line();   
     if(interp_error) return;

src/emc/rs274ngc/interp_array.cc

@@ -84,7 +84,7 @@ const int Interp::gees[] = {
 /* 220 */  -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
 /* 240 */  -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
 /* 260 */  -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
-/* 280 */   0, 0,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
+/* 280 */   0, 0, 0,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,	// 282=G28.2
 /* 300 */   0, 0,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
 /* 320 */  -1,-1,-1,-1,-1,-1,-1,-1,-1,-1, 1, 1,-1,-1,-1,-1,-1,-1,-1,-1,
 /* 340 */  -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,

src/emc/rs274ngc/interp_check.cc

@@ -100,6 +100,7 @@ int Interp::check_g_codes(block_pointer block,   //!< pointer to a block to be c
   } else if (mode1 == G_5_2){
   } else if (mode1 == G_6_2){
   } else if (mode0 == G_28_1 || mode0 == G_30_1) {
+  } else if (mode0 == G_28_2) {   // G-code homing
   } else if (mode0 == G_52) {
   } else if (mode0 == G_53) {
     CHKS(((block->motion_to_be != G_0) && (block->motion_to_be != G_1)),
@@ -326,12 +327,14 @@ int Interp::check_other_codes(block_pointer block)       //!< pointer to a block
 					(motion != G_6) && (motion != G_6_2) &&
           (motion != G_2) && (motion != G_3) &&
 	  (motion != G_74) && (motion != G_84) &&
+	  (block->g_modes[GM_MODAL_0] != G_28_2) &&
           (block->m_modes[9] != 50) && (block->m_modes[9] != 51) && (block->m_modes[9] != 52) &&
           (block->m_modes[9] != 53) && (block->m_modes[5] != 62) && (block->m_modes[5] != 63) &&
           (block->m_modes[5] != 64) && (block->m_modes[5] != 65) && (block->m_modes[5] != 66) &&
           (block->m_modes[7] != 19) && (block->user_m != 1) &&
           (block->o_type != M_98)),
           _("P word with no G2 G3 G4 G10 G64 G5 G5.2 G6, G6.2, G76 G82 G86 G88 G89"
+            " G28.2"
             " or M50 M51 M52 M53 M62 M63 M64 M65 M66 M98 "
             "or user M code to use it"));
       int p_value = round_to_int(block->p_number);

src/emc/rs274ngc/interp_convert.cc

@@ -3166,6 +3166,70 @@ Called by: convert_modal_0.
 
 */
 
+/*! convert_home_cycle
+
+Handles G28.2 (run the homing cycle) from a G-code line, so machines can
+reference themselves from MDI or a program instead of only from the GUI's
+*Home All* button. The bare form homes all joints, in HOME_SEQUENCE order.
+
+An optional Pn word homes a single joint by its 0-based joint number
+(matching [JOINT_n] INI section numbering, e.g. P1 -> JOINT_1). This is the
+primitive raised in the PR #4172 discussion for re-homing a joint that is
+switched between rotary-axis and spindle use mid-program
+(https://github.com/LinuxCNC/linuxcnc/pull/4172) -- it reuses the existing
+EMC_JOINT_HOME 'joint' field, so it needs no NML change and works
+identically on any kinematics. Axis-letter forms (G28.2 X) are deliberately
+NOT supported: resolving an axis letter to a joint needs the kinematics
+coordinate map and isn't trivial even on trivkins (duplicate letters on
+gantries), and homing is a joint concept, not an axis one.
+
+There is deliberately no G-code unhome. A G28.3 was part of the original
+proposal and was dropped during review of PR #4172: no use was found for it
+that a numbered parameter would not serve better under NO_FORCE_HOMING=1,
+and it was the one operation able to leave a running program on an
+unreferenced machine. The GUI, halui and linuxcncrsh keep their existing
+unhome.
+
+On a synchronized (negative HOME_SEQUENCE) joint pair, Pn on either joint
+homes both (motion's existing gantry-homing behavior); on a positive shared
+sequence Pn homes only the named joint -- use the bare form to home both.
+
+Motion still enforces its own safety (idle / not on limits). The joint
+number is range-checked against the machine's configured joint count in
+task (emcJointHome(), taskintf.cc), which is where that count is known --
+the interpreter has no joint count in its state.
+*/
+int Interp::convert_home_cycle(block_pointer block,
+                               setup_pointer settings)
+{
+    CHKS((settings->cutter_comp_side != CUTTER_COMP::OFF),
+         "Cannot home (G28.2) with cutter radius compensation on");
+
+    int joint = -1;
+    if (block->p_flag) {
+        CHKS(((block->p_number < 0.0) ||
+              (block->p_number != round_to_int(block->p_number))),
+             "P value for G28.2 must be a non-negative whole joint number"
+             " (omit P to home every joint)");
+        joint = round_to_int(block->p_number);
+    }
+
+    if (joint < 0) {
+        HOME_CYCLE();
+    } else {
+        HOME_CYCLE_JOINT(joint);
+    }
+
+    // Homing re-establishes machine zero and, for an immediate home,
+    // rewrites the joint coordinate with no physical motion. Stop reading
+    // ahead here (INTERP_EXECUTE_FINISH, via execute_block) and, once the
+    // cycle has run, resync current_* from the machine in read_inputs() so
+    // a following G91 move or I/J/K arc centre is right. Same pattern as
+    // probing and tool change.
+    settings->home_flag = true;
+    return INTERP_OK;
+}
+
 int Interp::convert_home(int move,       //!< G-code, must be G_28 or G_30
                         block_pointer block,    //!< pointer to a block of RS274 instructions
                         setup_pointer settings) //!< pointer to machine settings
@@ -4349,6 +4413,8 @@ int Interp::convert_modal_0(int code,    						//!< G-code, must be from group 0
     CHP(convert_home(code, block, settings));
   } else if ((code == G_28_1) || (code == G_30_1)) {
     CHP(convert_savehome(code, block, settings));
+  } else if (code == G_28_2) {
+    CHP(convert_home_cycle(block, settings));
   } else if ((code == G_52) || (code == G_92)) {
     CHP(convert_axis_offsets(code, block, settings));
   } else if ((code == G_5_3)||(code == G_6_3)) { // jjf

src/emc/rs274ngc/interp_execute.cc

@@ -199,8 +199,8 @@ Returned Value: int
      convert_speed
      convert_stop
      convert_tool_select
-   Otherwise, if the probe_flag in the settings is true, 
-   or the input_flag is set to true this returns
+   Otherwise, if the probe_flag, input_flag, toolchange_flag or home_flag
+   in the settings is true, this returns
       INTERP_EXECUTE_FINISH.
    Otherwise, it returns INTERP_OK.
 
@@ -324,6 +324,9 @@ int Interp::execute_block(block_pointer block,   //!< pointer to a block of RS27
   if (settings->toolchange_flag)
       return (INTERP_EXECUTE_FINISH);
 
+  if (settings->home_flag)
+      return (INTERP_EXECUTE_FINISH);
+
   // All changes to settings are complete
   write_canon_state_tag(block, settings);
   return INTERP_OK;

src/emc/rs274ngc/interp_internal.hh

@@ -220,6 +220,7 @@ enum GCodes
     G_21 = 210,
     G_28 = 280,
     G_28_1 = 281,
+    G_28_2 = 282,   /* G-code homing cycle (home one/all joints) */
     G_30 = 300,
     G_30_1 = 301,
     G_33 = 330,
@@ -747,6 +748,7 @@ struct setup
   bool probe_flag;            // flag indicating probing done
   bool input_flag;            // flag indicating waiting for input done
   bool toolchange_flag;       // flag indicating we just had a tool change
+  bool home_flag;             // flag indicating a G28.2 homing cycle just ran
   int input_index;		// channel queried
   bool input_digital;		// input queried was digital (false=analog)
   bool cutter_comp_firstmove; // this is the first comp move

src/emc/rs274ngc/interp_setup.cc

@@ -117,6 +117,7 @@ setup::setup() :
     probe_flag(0),
     input_flag(0),
     toolchange_flag(0),
+    home_flag(0),
     input_index(0),
     input_digital(0),
     cutter_comp_firstmove(0),

src/emc/rs274ngc/rs274ngc_interp.hh

@@ -324,6 +324,8 @@ public:
                         setup_pointer settings);
  int convert_savehome(int move, block_pointer block,
                         setup_pointer settings);
+ int convert_home_cycle(block_pointer block,              // G28.2
+                        setup_pointer settings);
  int convert_length_units(int g_code, setup_pointer settings);
     int convert_m(block_pointer block, setup_pointer settings);
  int convert_modal_0(int code, block_pointer block,

src/emc/rs274ngc/rs274ngc_pre.cc

@@ -1193,6 +1193,7 @@ int Interp::init()
 //_setup.plane set in Interp::synch
   _setup.probe_flag = false;
   _setup.toolchange_flag = false;
+  _setup.home_flag = false;
   _setup.input_flag = false;
   _setup.input_index = -1;
   _setup.input_digital = false;
@@ -1458,6 +1459,19 @@ int Interp::read_inputs(setup_pointer settings)
 	load_tool_table();
 	settings->toolchange_flag = false;
     }
+    if (settings->home_flag) {
+	// A G28.2 homing cycle re-establishes machine zero and, for an
+	// immediate (index/switchless) home, rewrites the joint coordinate
+	// even when nothing physically moved. Pull the interpreter's model
+	// of the current position back in line with the machine so a
+	// following G91 move or an I/J/K arc centre is computed from where
+	// the tool actually is, not from the pre-home point. Same mechanism
+	// as the tool-change resync above, without the tool-table reload.
+	CHKS((GET_EXTERNAL_QUEUE_EMPTY() == 0),
+	     _("Queue is not empty after homing"));
+	refresh_actual_position(&_setup);
+	settings->home_flag = false;
+    }
     // always track toolchanger-fault and toolchanger-reason codes
     settings->parameters[5600] = GET_EXTERNAL_TC_FAULT();
     settings->parameters[5601] = GET_EXTERNAL_TC_REASON();
@@ -2674,6 +2688,7 @@ int Interp::on_abort(int reason, const char *message)
     // if set, may cause a "Queue is not empty after tool change" error
     _setup.toolchange_flag = false;
     _setup.probe_flag = false;
+    _setup.home_flag = false;
     _setup.input_flag = false;
 
     if (_setup.on_abort_command == NULL) {