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

[Christian Loehle <christian.loehle@arm.com>]

On 2/16/26 18:50, Aboorva Devarajan wrote:
> 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>

Reviewed-by: Christian Loehle <christian.loehle@arm.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);
>  }
>