[PATCH v2 1/4] cpuidle: Skip governor when only one idle state is available

[Aboorva Devarajan <aboorvad@linux.ibm.com>]

On certain platforms (PowerNV systems without a power-mgt DT node),
cpuidle may register only a single idle state. In cases where that
single state is a polling state (state 0), the ladder governor may
incorrectly treat state 1 as the first usable state and pass an
out-of-bounds index. This can lead to a NULL enter callback being
invoked, ultimately resulting in a system crash.

[   13.342636] cpuidle-powernv : Only Snooze is available
[   13.351854] Faulting instruction address: 0x00000000
[   13.376489] NIP [0000000000000000] 0x0
[   13.378351] LR  [c000000001e01974] cpuidle_enter_state+0x2c4/0x668

Fix this by adding a bail-out in cpuidle_select() that returns state 0
directly when state_count <= 1, bypassing the governor and keeping the
tick running.

Fixes: dc2251bf98c6 ("cpuidle: Eliminate the CPUIDLE_DRIVER_STATE_START symbol")
Signed-off-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
---
 drivers/cpuidle/cpuidle.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
index c7876e9e024f..65fbb8e807b9 100644
--- a/drivers/cpuidle/cpuidle.c
+++ b/drivers/cpuidle/cpuidle.c
@@ -359,6 +359,16 @@ noinstr int cpuidle_enter_state(struct cpuidle_device *dev,
 int cpuidle_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 		   bool *stop_tick)
 {
+	/*
+	 * If there is only a single idle state (or none), there is nothing
+	 * meaningful for the governor to choose. Skip the governor and
+	 * always use state 0 with the tick running.
+	 */
+	if (drv->state_count <= 1) {
+		*stop_tick = false;
+		return 0;
+	}
+
 	return cpuidle_curr_governor->select(drv, dev, stop_tick);
 }
 
-- 
2.52.0