G28 with features

[greatEndian]

G-code machine homing: G28.2 / G28.3 and optional GCODE_HOMING for plain G28

What this is

Adds the ability to reference (home) a machine from G-code, instead of
only from the GUI, in three small, opt-in pieces:

• G28.2 — run the homing cycle on all joints (in HOME_SEQUENCE order)
• G28.3 — unhome all joints
• [RS274NGC]GCODE_HOMING=1 (INI opt-in) — a plain G28 references the
  machine first, before its normal return move, only when the machine is
  not already fully homed

Plus two small fixes that make a queued (in-program / MDI) home actually
reach motion (see below).

G28.2 / G28.3 are non-modal (modal group 0) codes, following the
existing G28.1 / G30.1 pattern. Bare form — axis words are ignored; they
act on all joints.

What it's for

• Unattended / scripted power-up — a program or MDI line can reference
  the machine, so an operator (or an automation layer) doesn't have to open
  the GUI and click Home All.
• Operators who prefer a typed reference command over the GUI.
• GCODE_HOMING=1 lets you put a plain G28 at the top of a program: on a
  cold machine it homes first, then returns; on an already-homed machine it
  is a pure return move and costs nothing.

Relation to industrial-standard G28

• Plain G28 return-to-reference behavior is unchanged and still matches
  the Fanuc/Haas convention: go to the machine reference point through the
  intermediate point given by the axis words.
• GCODE_HOMING=1 makes the first G28 after power-up also establish
  the reference (run the homing cycle), which mirrors how a Fanuc-style
  control without absolute encoders performs its zero-return/reference on a
  G28. Once referenced, subsequent G28s are pure returns, exactly as on
  those controls.
• G28.2 / G28.3 (explicit home / unhome) are LinuxCNC extensions — there
  is no standard Fanuc equivalent — kept in the spirit of the existing
  G28.1 / G30.1 reference-point codes.

[RS274NGC]GCODE_HOMING=1 — behavior

Situation	Plain G28 does
Flag absent / =0 (default)	Stock LinuxCNC G28 (return only)
=1, machine not fully homed	Home all joints, then the G28 return
=1, machine fully homed	Pure return (no homing)
=1, G30 / G28.2 / G28.3	Unchanged (flag is G28-only)

Default-off is bit-identical to stock: the homing call is only emitted under
FEATURE(GCODE_HOMING) for G_28.

How it works (single channel)

1. interp convert_home: under FEATURE_GCODE_HOMING and move == G_28,
   emit a new canon op HOME_CYCLE_IF_UNHOMED() before the waypoint /
   return moves (so it completes while joint positions are still unknown).
2. canon queues EMC_JOINT_HOME carrying an EMC_HOME_ALL_IF_UNHOMED
   (-3) sentinel in its existing joint field.
3. task resolves the sentinel at execution time: if all_homed(), drop the
   home (queued return alone = legacy G28); otherwise issue the normal
   home-all (emcJointHome(-1)) — the same path G28.2 and the GUI Home
   All already use.

The sentinel mechanism itself adds no new message field and no NML-layout
change, and touches neither homing.c nor the motion command path — it
resolves entirely in task by reusing the existing joint field with a
reserved value. (The small command.c idle-homing fix below is a separate
prerequisite, shared with G28.2.)

Supporting fixes (also needed for G28.2)

• task: EMC_JOINT_HOME_TYPE / EMC_JOINT_UNHOME_TYPE were missing from
  emcTaskCheckPreconditions(), so a queued home/unhome hit the default
  case and was silently dropped before reaching motion. They now return
  WAITING_FOR_MOTION (drain prior motion, then home).
• motion: single-channel homing required motion_state == FREE; now it is
  also permitted when motion is otherwise idle (in position, queue empty), so
  a home issued from MDI or a program is honored. Refusing mid-motion still
  holds.

Testing

• Interpreter (rs274 -i): GCODE_HOMING=1 plain G28 emits
  HOME_CYCLE_IF_UNHOMED() ahead of the return traverse; G30 does not home;
  G28.2 homes unconditionally; default-off G28 emits no homing.
• Builds clean (interp / task / motion).
• On-machine homing sign-off (a real reference run triggered from MDI / a
  program) is pending and will be added.

Credits

Developed in collaboration with @grandixximo, with whom I discussed the requirements that
shaped this feature. Thanks for the design discussion and for driving the
real-world G28 / homing-from-G-code use case.

[Sigma1912]

Would it be possible to add a parameter for homing individual joints?

Example:

{G28.2, G28.3} P1 to home/ unhome joint 1

This would be very useful for configurations with joints that switch between rotary and spindle use. It would also circumvent #3556 that complicates the current way of 'rehoming'

[greatEndian]

Home individual axis-

G28.2XC will force home axis X and C ...
G28.3YZ will force unhome axis YZ ...

for non trivial kinematics we need to start discussion how to handle it per joint..

[Sigma1912]

What happens if we try to run a program in unhomed state ( will that trigger an interpreter error?):

Example 1 :

G28.3
G1 x2

How are these commands handled when used inside a gcode program? (I presume it is like a queue buster command that halts the read ahead until all queued command have been executed) :

Example 2 :

G1 x1
G28.3
G28.2
G1 x2

[greatEndian]

Example 1 :
force unhome
if NO_FORCE_HOMING==1 program run
else message to home again

Example 2 :
program run
force unhome
force home
program run

[Sigma1912]

for non trivial kinematics we need to start discussion how to handle it per joint..

Using axes is probably enough for the majority of use cases and most machine operators would likely not even know about the joint-axis mappings on their particular machines.
However, it might be useful for more exotic configurations (eg for homing/unhoming extra joints that do not have axis letters assigned).
It seems an easy enough addition to introduce an optional parameter word with the joint number (but I might be wrong about that) ;)

[grandixximo]

I think the cleanest direction is to keep the G-code surface dead simple and put the real effort into execution. For per-joint homing, Sigma's Pn is the right primitive: the joint index rides the field that EMC_JOINT_HOME already carries, so G28.2 P1 needs no NML or motion change and behaves the same on any kinematics. Bare G28.2 stays home-all. Gantries need no extra word since Pn reuses the existing single-joint homing behavior, but that's worth a doc note: on a synchronized (negative HOME_SEQUENCE) pair, homing either joint brings its partner, while on a positive shared sequence Pn homes only the named joint and you'd use bare G28.2 to do both. I'd leave the axis-letter form (G28.2 X) out for now, since resolving axis letters to joints needs the kinematics coordinates map and isn't actually trivial even on trivkins (duplicate letters for gantries), so it's better as its own later piece.

The part that really needs rethinking isn't the syntax, it's how a queued home executes. Homing only runs in free mode, and nothing currently flips the machine into free for a G-code-triggered home, so just relaxing the motion guard means the command can silently stall in teleop. It needs to be sequenced in task: drain motion, switch to free, home, wait for it to actually finish, then restore the prior mode. And if homing fails, the program has to abort rather than carry on unreferenced. A nice consequence is the trivkins question answers itself: home-all works everywhere, but a partial home can only resume coordinated motion on identity kinematics, because non-trivkins won't re-enter teleop until everything's homed.

There's also the unhome-then-keep-running hole Sigma raised. The existing force-homing check only fires at program start, so G28.3 mid-program followed by a move slips through. Rather than checking every move, we re-apply that same NO_FORCE_HOMING gate at the home/unhome sync point the command already forces, so it's governed by exactly the same policy as starting a program, with no cost on the motion path.

Given all that, I'd split it: land the explicit G28.2/G28.3 (with Pn) first, and hold GCODE_HOMING plain-G28, which adds the home-then-return move on top, until it's been signed off on real hardware.