[PATCH v2] cpuidle: teo: Do not return a disabled idle state

[Xueqin Luo]

If idle state 0 is disabled and none of the enabled idle states satisfies
the PM QoS latency constraint, constraint_idx in teo_select() stays at its
initial value of 0, because the loop that updates it starts at state 1,
and so the candidate state index is capped at state 0, which is disabled,
and that state is returned to the caller.

The cpuidle core does not validate the returned state index against the
per-state disable flags, so the CPU ends up entering the disabled state 0.

To address this, ensure that constraint_idx is at least equal to the index
of the first enabled idle state (idx0), so that the candidate state is
never capped to a disabled one.

Fixes: c410a9a142f1 ("cpuidle: teo: Change the main idle state selection logic")
Signed-off-by: Xueqin Luo <luoxueqin@xxxxxxxxxx>
---
v1 -> v2:
    - Instead of falling back to idx0 after the candidate index has been
      capped to a disabled state, ensure that constraint_idx is at least
      equal to idx0 before the capping check (suggested by Rafael J. Wysocki).

 drivers/cpuidle/governors/teo.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/cpuidle/governors/teo.c b/drivers/cpuidle/governors/teo.c
index ac43b9b013b3..906f4f89e365 100644
--- a/drivers/cpuidle/governors/teo.c
+++ b/drivers/cpuidle/governors/teo.c
@@ -426,6 +426,17 @@ static int teo_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 		}
 	}
 
+	/*
+	 * If the latency constraint does not allow any of the enabled idle
+	 * states to be used, the candidate state index will be capped to 0
+	 * by the check below, but state 0 may be disabled.  To prevent the
+	 * selection of a disabled state in that case, ensure that
+	 * constraint_idx is at least equal to the index of the first
+	 * enabled idle state.
+	 */
+	if (constraint_idx < idx0)
+		constraint_idx = idx0;
+
 	/*
 	 * If there is a latency constraint, it may be necessary to select an
 	 * idle state shallower than the current candidate one.
-- 
2.43.0