Re: [PATCH] cpuidle: warn and fixup on sanity check instead of rejecting the driver

[Rafael J. Wysocki]

On Fri, Nov 21, 2025 at 2:08 AM Val Packett <val@xxxxxxxxxxxx> wrote:
>
> On Device Tree platforms, the latency and target residency values come
> directly from device trees, which are numerous and weren't all written
> with cpuidle invariants in mind. For example, qcom/hamoa.dtsi currently
> trips this check: exit latency 680000 > residency 600000.

So this breaks cpuidle expectations and it doesn't work correctly on
the affected platforms.

> Instead of harshly rejecting the entire cpuidle driver with a mysterious
> error message, print a warning and set the target residency value to be
> equal to the exit latency.

This generally doesn't work because the new target residency may be
greater than the target residency of the next state.

> Fixes: 76934e495cdc ("cpuidle: Add sanity check for exit latency and target residency")
> Signed-off-by: Val Packett <val@xxxxxxxxxxxx>
> ---
>  drivers/cpuidle/driver.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/cpuidle/driver.c b/drivers/cpuidle/driver.c
> index 1c295a93d582..06aeb59c1017 100644
> --- a/drivers/cpuidle/driver.c
> +++ b/drivers/cpuidle/driver.c
> @@ -199,8 +199,11 @@ static int __cpuidle_driver_init(struct cpuidle_driver *drv)
>                  * exceed its target residency which is assumed in cpuidle in
>                  * multiple places.
>                  */
> -               if (s->exit_latency_ns > s->target_residency_ns)
> -                       return -EINVAL;
> +               if (s->exit_latency_ns > s->target_residency_ns) {
> +                       pr_warn("cpuidle: state %d: exit latency %lld > residency %lld (fixing)\n",
> +                               i, s->exit_latency_ns, s->target_residency_ns);
> +                       s->target_residency_ns = s->exit_latency_ns;

And you also need to update s->target_residency.

Moreover, that needs to be done when all of the target residency and
exit latency values have been computed and full sanitization of all
the states would need to be done (including the ordering checks), but
the kernel has insufficient information to do that (for instance, if
the ordering is not as expected, it is not clear how to fix it up).
Even the above sanitization is unlikely to result in the intended
behavior.

So if returning the error code doesn't work, printing a warning is as
much as can be done, like in the attached patch.

If this works for you, I'll submit it properly later.

> +               }
>         }
>
>         return 0;
> --
---
 drivers/cpuidle/driver.c |   18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

--- a/drivers/cpuidle/driver.c
+++ b/drivers/cpuidle/driver.c
@@ -8,6 +8,8 @@
  * This code is licenced under the GPL.
  */
 
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
 #include <linux/mutex.h>
 #include <linux/module.h>
 #include <linux/sched.h>
@@ -152,7 +154,7 @@ static void cpuidle_setup_broadcast_time
  * __cpuidle_driver_init - initialize the driver's internal data
  * @drv: a valid pointer to a struct cpuidle_driver
  */
-static int __cpuidle_driver_init(struct cpuidle_driver *drv)
+static void __cpuidle_driver_init(struct cpuidle_driver *drv)
 {
 	int i;
 
@@ -195,15 +197,13 @@ static int __cpuidle_driver_init(struct
 			s->exit_latency = div_u64(s->exit_latency_ns, NSEC_PER_USEC);
 
 		/*
-		 * Ensure that the exit latency of a CPU idle state does not
-		 * exceed its target residency which is assumed in cpuidle in
-		 * multiple places.
+		 * Warn if the exit latency of a CPU idle state exceeds its
+		 * target residency which is assumed to never happen in cpuidle
+		 * in multiple places.
 		 */
 		if (s->exit_latency_ns > s->target_residency_ns)
-			return -EINVAL;
+			pr_warn("Idle state %d target residency too low\n", i);
 	}
-
-	return 0;
 }
 
 /**
@@ -233,9 +233,7 @@ static int __cpuidle_register_driver(str
 	if (cpuidle_disabled())
 		return -ENODEV;
 
-	ret = __cpuidle_driver_init(drv);
-	if (ret)
-		return ret;
+	__cpuidle_driver_init(drv);
 
 	ret = __cpuidle_set_driver(drv);
 	if (ret)