Re: [PATCH v2 3/3] pmdommain: add support system-wide resume latency constraints

[Ulf Hansson]

On Fri, 6 Feb 2026 at 01:30, Kevin Hilman (TI) <khilman@xxxxxxxxxxxx> wrote:
>
> In addition to checking for CPU latency constraints when checking if
> OK to power down a domain, also check for QoS latency constraints in
> all devices of a domain and use that in determining the final latency
> constraint to use for the domain.
>
> Since cpu_system_power_down_ok() is used for system-wide suspend, the
> per-device constratints are only relevant if the LATENCY_SYS QoS flag
> is set.
>
> Signed-off-by: Kevin Hilman (TI) <khilman@xxxxxxxxxxxx>
> ---
>  drivers/pmdomain/governor.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 45 insertions(+)
>
> diff --git a/drivers/pmdomain/governor.c b/drivers/pmdomain/governor.c
> index 96737abbb496..bbf59b93c8b6 100644
> --- a/drivers/pmdomain/governor.c
> +++ b/drivers/pmdomain/governor.c
> @@ -13,6 +13,8 @@
>  #include <linux/cpumask.h>
>  #include <linux/ktime.h>
>
> +#include "core.h"
> +
>  static int dev_update_qos_constraint(struct device *dev, void *data)
>  {
>         s64 *constraint_ns_p = data;
> @@ -425,17 +427,60 @@ static bool cpu_power_down_ok(struct dev_pm_domain *pd)
>         return true;
>  }
>
> +/**
> + * check_device_qos_latency - Callback to check device QoS latency constraints
> + * @dev: Device to check
> + * @data: Pointer to s32 variable holding minimum latency found so far
> + *
> + * This callback checks if the device has a system-wide resume latency QoS
> + * constraint and updates the minimum latency if this device has a stricter
> + * constraint.
> + *
> + * Returns: 0 to continue iteration.
> + */
> +static int check_device_qos_latency(struct device *dev, void *data)
> +{
> +       s32 *min_dev_latency = data;
> +       enum pm_qos_flags_status flag_status;
> +       s32 dev_latency;
> +
> +       dev_latency = dev_pm_qos_read_value(dev, DEV_PM_QOS_RESUME_LATENCY);

->cpu_system_power_down_ok() executes in atomic context on a
PREEMPT_RT configured system.

dev_pm_qos_read_value() uses spinlocks, which sleep in that configuration.

I don't know the best approach to fix this. Perhaps add a specific
dev_pm_qos interface that can be used here? Or re-work the code so it
only applies for !PREEMPT_RT?

> +       flag_status = dev_pm_qos_flags(dev, PM_QOS_FLAG_LATENCY_SYS);
> +       if ((dev_latency != PM_QOS_RESUME_LATENCY_NO_CONSTRAINT) &&
> +           (flag_status == PM_QOS_FLAGS_ALL)) {
> +               dev_dbg(dev,
> +                       "has QoS system-wide resume latency=%d\n",
> +                       dev_latency);
> +               if (dev_latency < *min_dev_latency)
> +                       *min_dev_latency = dev_latency;
> +       }
> +
> +       return 0;
> +}
> +
>  static bool cpu_system_power_down_ok(struct dev_pm_domain *pd)
>  {
>         s64 constraint_ns = cpu_wakeup_latency_qos_limit() * NSEC_PER_USEC;
>         struct generic_pm_domain *genpd = pd_to_genpd(pd);
>         int state_idx = genpd->state_count - 1;
> +       s32 min_dev_latency = PM_QOS_RESUME_LATENCY_NO_CONSTRAINT;
> +       s64 min_dev_latency_ns = PM_QOS_RESUME_LATENCY_NO_CONSTRAINT_NS;
>
>         if (!(genpd->flags & GENPD_FLAG_CPU_DOMAIN)) {
>                 genpd->state_idx = state_idx;
>                 return true;
>         }
>
> +       genpd_for_each_child(genpd, check_device_qos_latency,
> +                            &min_dev_latency);
> +
> +       /* If device latency < CPU wakeup latency, use it instead */
> +       if (min_dev_latency != PM_QOS_RESUME_LATENCY_NO_CONSTRAINT) {
> +               min_dev_latency_ns = min_dev_latency * NSEC_PER_USEC;
> +               if (min_dev_latency_ns < constraint_ns)
> +                       constraint_ns = min_dev_latency_ns;
> +       }
> +
>         /* Find the deepest state for the latency constraint. */
>         while (state_idx >= 0) {
>                 s64 latency_ns = genpd->states[state_idx].power_off_latency_ns +
>
> --
> 2.51.0
>

Kind regards
Uffe