fix(positioner): validate deadband against tolerance

ophyd_devices/interfaces/base_classes/psi_positioner_base.py

@@ -142,7 +142,7 @@ class PSISimplePositionerBase(ABC, PSIDeviceBase, PositionerBase):
         Args:
             name (str): (required) the name of the device
             limits (list | tuple | None): If given, a length-2 sequence within the range of which movement is allowed.
-            deadband (float | None): If given, set a soft deadband of this absolute value, within which positioner moves will return immediately. If the positioner has no motor_done_move signal, you must provide this.
+            deadband (float | None): If given, set a soft deadband of this absolute value, within which positioner moves will return immediately. If the positioner has no motor_done_move signal, you must provide this. It must not exceed the configured tolerance when moving.
             use_put_completion (bool | None): If given, use put completion on the setpoint signal to resolve the move status.
             override_suffixes (dict[str, str]): a dictionary of signal_name: pv_suffix which will replace the values in the signal classvar.
         """
@@ -294,13 +294,17 @@ class PSISimplePositionerBase(ABC, PSIDeviceBase, PositionerBase):
         TimeoutError
             When motion takes longer than `timeout`
         ValueError
-            On invalid positions
+            On invalid positions or if the deadband exceeds the configured tolerance
         RuntimeError
             If motion fails other than timing out
         """
 
-        if self._deadband is not None and abs(position - self._position) < self._deadband:
-            return MoveStatus(self, position, done=True, success=True)
+        if self._deadband is not None:
+            tolerance = self.tolerance.get()
+            if self._deadband > tolerance:
+                raise ValueError(f"Deadband {self._deadband} must not exceed tolerance {tolerance}")
+            if abs(position - self._position) < self._deadband:
+                return MoveStatus(self, position, done=True, success=True)
 
         if timeout is None:
             timeout = self._timeout

tests/test_psi_positioner.py

@@ -178,7 +178,7 @@ def mock_done_signal_positioner_with_tolerance():
     with patch.object(ophyd, "cl") as mock_cl:
         mock_cl.get_pv = MockPV
         mock_cl.thread_class = threading.Thread
-        dev = DoneSignalPositionerWithTolerance(name=name, prefix=prefix, deadband=0.01)
+        dev = DoneSignalPositionerWithTolerance(name=name, prefix=prefix, deadband=0.001)
         patch_dual_pvs(dev)
         dev.wait_for_connection()
         dev._set_position(0)
@@ -287,3 +287,29 @@ def test_tolerance_checks_requested_target_even_if_setpoint_changes(
     assert st.target == 5
     with pytest.raises(RuntimeError, match="outside of tolerance"):
         st.wait(timeout=1)
+
+
+def test_move_rejects_tolerance_tighter_than_deadband_before_setpoint_write(
+    mock_done_signal_positioner_with_tolerance,
+):
+    dev = mock_done_signal_positioner_with_tolerance
+    dev.tolerance.put(0.0005)
+
+    with patch.object(dev.user_setpoint, "put", wraps=dev.user_setpoint.put) as put:
+        with pytest.raises(ValueError, match="tolerance"):
+            dev.move(5, wait=False)
+        put.assert_not_called()
+
+
+def test_equal_deadband_and_tolerance_allow_instant_success(
+    mock_done_signal_positioner_with_tolerance,
+):
+    dev = mock_done_signal_positioner_with_tolerance
+    assert dev.tolerance.get() == dev._deadband
+
+    with patch.object(dev.user_setpoint, "put", wraps=dev.user_setpoint.put) as put:
+        st = dev.move(0.0005, wait=False)
+        put.assert_not_called()
+
+    assert st.done
+    assert st.success