motion: fix for trivkins duplicate coordinate jogs

tldr: no *joint* jog if negative home_sequence

Problem Statement:
------------------
Using the trivkins kinematics module with duplicated coordinates
is useful for a gantry using two motors to move a single axis
because it allows synchronization of the final homing move for
the joints involved.

However, use of duplicated coordinate letters creates problems
that are dependent on the kinematics type (as defined by the
trivkins module kinstype= parameter) and gui conventions:

  1) Using kinstype=1 (KINEMATICS_IDENTITY the default) creates
     numerous problems with the axis gui because it always hides
     the distinctions of joints and axes for IDENTITY kinematics.
     Restrictions include:
       a) An axis with duplicated coordinate letters cannot
          be homed individually (Home-all must be used)
       b) Prior to homing, jogging a duplicated coordinate
          will move only one of the joints used for the
          duplicated coordinate letter.

  2) Using kinstype=B (KINEMATICS_BOTH) clearly distinguishes
     joints and axes and clarifies the need to HOME the machine
     before operating in world coordinates for conventional
     cartesian jogging and execution of mdi commands and gcode
     programs.

     But -- unwary operators may jog one of the joints used for
     a duplicated coordinate letter excessively and rack the
     machine prior to homing.

Solution:
---------
  a) For any kinematics type, disable joint jogging for a joint
     when it uses a negative [JOINT_n]HOME_SEQUENCE to
     synchronize the final homing moves on related joints.

  b) For KINEMATICS_BOTH, a request to jog a joint with a negative
     HOME_SEQUENCE causes motion to report an error.

  b) For KINEMATICS_IDENTITY, a request to jog a joint with a
     negative HOME_SEQUENCE causes  motion to reports an error
     and suggests the requirement to Home the machine

With this change, gantry users can be encouraged to use
KINEMATICS_BOTH since the problem of racking is eliminated with
the use of negative HOME_SEQUENCE on synchronized joints.

Author: dngarrett

docs/src/config/ini-homing.txt

@@ -191,6 +191,11 @@ with a synchronized final move*.  If the HOME_SEQUENCE value is
 zero or positive, a command to home the joint will home only the
 specified joint.
 
+Joint mode jogging of joints having a negative HOME_SEQUENCE is
+disallowed.  In common gantry applications, such jogging can lead
+to misalignment (racking).  Note that conventional jogging in
+world coordinates is always available once a machine is homed.
+
 Examples for a 3 joint system
 
 Two sequences (0,1), no synchronization

docs/src/getting-started/updating-linuxcnc.txt

@@ -1074,4 +1074,11 @@ Support connection of 'halui.mode.is-teleop' to 'pendant_util.amux-enable' via
 the signal 'pendant:amux-enable'.  This change allows homing and joint mode
 jogging to occur at normal (not reduced) acceleration settings.
 
+=== Negative [JOINT_n]HOME_SEQUENCE
+
+Joints using a negative HOME_SEQUENCE are not allowed to jog in joint
+mode in order to prevent misalignment (racking) in common gantry
+configurations.  As always, machines with any kinematics type must be
+homed prior to enabling conventional world mode jogging.
+
 // vim: set syntax=asciidoc:

src/emc/motion/command.c

@@ -480,6 +480,25 @@ void emcmotCommandHandler(void *arg, long period)
 
         if (joint_num >= 0 && joint_num < emcmotConfig->numJoints) {
             joint = &joints[joint_num];
+            if (   (   emcmotCommand->command == EMCMOT_JOG_CONT
+                    || emcmotCommand->command == EMCMOT_JOG_INCR
+                    || emcmotCommand->command == EMCMOT_JOG_ABS
+                   )
+                && !(GET_MOTION_TELEOP_FLAG())
+                && (joint->home_sequence < 0)
+               ) {
+                  if (emcmotConfig->kinType == KINEMATICS_IDENTITY) {
+                      rtapi_print_msg(RTAPI_MSG_ERR,
+                      "Homing is REQUIRED to jog requested coordinate\n"
+                      "because joint (%d) in home_sequence is negative (%d)\n"
+                      ,joint_num,joint->home_sequence);
+                  } else {
+                      rtapi_print_msg(RTAPI_MSG_ERR,
+                      "Cannot jog joint %d because home_sequence is negative (%d)\n"
+                      ,joint_num,joint->home_sequence);
+                  }
+                  return;
+            }
         }
         if (axis_num >= 0 && axis_num < EMCMOT_MAX_AXIS) {
             axis = &axes[axis_num];

src/emc/motion/control.c

@@ -862,6 +862,8 @@ static void handle_jjogwheels(void)
 	    /* if joint is not active, skip it */
 	    continue;
 	}
+
+
 	/* get counts from jogwheel */
 	new_jjog_counts = *(joint_data->jjog_counts);
 	delta = new_jjog_counts - joint->old_jjog_counts;
@@ -907,6 +909,19 @@ static void handle_jjogwheels(void)
             reportError("Can't wheel jog locking joint_num=%d",joint_num);
             continue;
         }
+        if (joint->home_sequence < 0) {
+            if (emcmotConfig->kinType == KINEMATICS_IDENTITY) {
+                rtapi_print_msg(RTAPI_MSG_ERR,
+                "Homing is REQUIRED to wheel jog requested coordinate\n"
+                "because joint (%d) in home_sequence is negative (%d)\n"
+                ,joint_num,joint->home_sequence);
+            } else {
+                rtapi_print_msg(RTAPI_MSG_ERR,
+                "Cannot wheel jog joint %d because home_sequence is negative (%d)\n"
+                ,joint_num,joint->home_sequence);
+            }
+            continue;
+        }
 	/* calculate distance to jog */
 	distance = delta * *(joint_data->jjog_scale);
 	/* check for joint already on hard limit */