Re: [PATCH v3 1/2] PM / runtime: inform runtime PM of a device's next wakeup

["Rafael J. Wysocki" <rafael@kernel.org>]

On Thu, Oct 15, 2020 at 9:38 PM Lina Iyer <ilina@codeaurora.org> wrote:
>
> Some devices may have a predictable interrupt pattern while executing
> usecases. An example would be the VSYNC interrupt associated with
> display devices. A 60 Hz display could cause a interrupt every 16 ms. If
> the device were in a PM domain, the domain would need to be powered up
> for device to resume and handle the interrupt.
>
> Entering a domain idle state saves power, only if the residency of the
> idle state is met. Without knowing the idle duration of the domain, the
> governor would just choose the deepest idle state that matches the QoS
> requirements. The domain might be powered off just as the device is
> expecting to wake up. If devices could inform runtime PM of their next
> event, the parent PM domain's idle duration can be determined.
>
> So let's add the pm_runtime_set_next_wake() API for the device to notify
> runtime PM of the impending wakeup and document it's usage.
>
> Signed-off-by: Lina Iyer <ilina@codeaurora.org>
> ---
> Changes in v2:
>         - Update documentation
>         - Remove runtime PM enabled check
>         - Update commit text
> ---
>  Documentation/power/runtime_pm.rst | 17 +++++++++++++++++
>  drivers/base/power/runtime.c       | 24 ++++++++++++++++++++++++
>  include/linux/pm.h                 |  2 ++
>  include/linux/pm_runtime.h         |  1 +
>  4 files changed, 44 insertions(+)
>
> diff --git a/Documentation/power/runtime_pm.rst b/Documentation/power/runtime_pm.rst
> index 0553008b6279..f6aaef15a511 100644
> --- a/Documentation/power/runtime_pm.rst
> +++ b/Documentation/power/runtime_pm.rst
> @@ -515,6 +515,12 @@ drivers/base/power/runtime.c and include/linux/pm_runtime.h:
>        power.use_autosuspend isn't set, otherwise returns the expiration time
>        in jiffies
>
> +  `int pm_runtime_set_next_event(struct device *dev, ktime_t next);`
> +    - inform runtime PM of the next event on the device. Devices that are
> +      sensitive to their domain idle enter/exit latencies may provide this
> +      information for use by the PM domain governor. The domain governor would
> +      use this information to calculate it's sleep length.
> +
>  It is safe to execute the following helper functions from interrupt context:
>
>  - pm_request_idle()
> @@ -545,6 +551,7 @@ functions may also be used in interrupt context:
>  - pm_runtime_put_sync()
>  - pm_runtime_put_sync_suspend()
>  - pm_runtime_put_sync_autosuspend()
> +- pm_runtime_set_next_event()
>
>  5. Runtime PM Initialization, Device Probing and Removal
>  ========================================================
> @@ -639,6 +646,16 @@ suspend routine).  It may be necessary to resume the device and suspend it again
>  in order to do so.  The same is true if the driver uses different power levels
>  or other settings for runtime suspend and system sleep.
>
> +When a device enters idle at runtime, it may trigger the runtime PM up the
> +hierarchy and if device has a predictable interrupt pattern, we can even do a
> +better job at determining the parent's idle state. For example, a display
> +device gets a VSYNC interrupt every 16 ms when running at 60 Hz. When it's PM
> +domain is powering down and happens to be at the boundary of the VSYNC
> +interrupt, it may not be efficient to power off the domain. Knowing the next
> +wake up (when available) for devices in the domain we can determine the idle
> +duration of the domain. By comparing idle duration with the residencies of the
> +domain idle states, we can be efficient in both power and performance.
> +
>  During system resume, the simplest approach is to bring all devices back to full
>  power, even if they had been suspended before the system suspend began.  There
>  are several reasons for this, including:
> diff --git a/drivers/base/power/runtime.c b/drivers/base/power/runtime.c
> index 8143210a5c54..5d2ebacfd35e 100644
> --- a/drivers/base/power/runtime.c
> +++ b/drivers/base/power/runtime.c
> @@ -122,6 +122,27 @@ u64 pm_runtime_suspended_time(struct device *dev)
>  }
>  EXPORT_SYMBOL_GPL(pm_runtime_suspended_time);
>
> +/**
> + * pm_runtime_set_next_wakeup_event - Notify PM framework of an impending event.
> + * @dev: Device to handle
> + * @next: impending interrupt/wakeup for the device
> + */
> +int pm_runtime_set_next_event(struct device *dev, ktime_t next)
> +{
> +       unsigned long flags;
> +       int ret = -EINVAL;
> +
> +       spin_lock_irqsave(&dev->power.lock, flags);
> +       if (ktime_before(ktime_get(), next)) {
> +               dev->power.next_event = next;
> +               ret = 0;
> +       }
> +       spin_unlock_irqrestore(&dev->power.lock, flags);
> +
> +       return ret;
> +}
> +EXPORT_SYMBOL_GPL(pm_runtime_set_next_event);
> +
>  /**
>   * pm_runtime_deactivate_timer - Deactivate given device's suspend timer.
>   * @dev: Device to handle.
> @@ -1415,6 +1436,9 @@ void pm_runtime_enable(struct device *dev)
>              "Enabling runtime PM for inactive device (%s) with active children\n",
>              dev_name(dev));
>
> +       /* Reset the next wakeup for the device */
> +       dev->power.next_event = KTIME_MAX;
> +
>         spin_unlock_irqrestore(&dev->power.lock, flags);
>  }
>  EXPORT_SYMBOL_GPL(pm_runtime_enable);
> diff --git a/include/linux/pm.h b/include/linux/pm.h
> index a30a4b54df52..9051658674a4 100644
> --- a/include/linux/pm.h
> +++ b/include/linux/pm.h
> @@ -8,6 +8,7 @@
>  #ifndef _LINUX_PM_H
>  #define _LINUX_PM_H
>
> +#include <linux/ktime.h>
>  #include <linux/list.h>
>  #include <linux/workqueue.h>
>  #include <linux/spinlock.h>
> @@ -616,6 +617,7 @@ struct dev_pm_info {
>         u64                     active_time;
>         u64                     suspended_time;
>         u64                     accounting_timestamp;
> +       ktime_t                 next_event;

While there are some cosmetic changes to be made, this particular bit
is fundamentally questionable IMV, because next_event (which BTW would
better be called next_wakeup IMO) is not used by PM-runtime.

The only user of it will be genpd AFAICS, so I don't quite see a
reason to inflict this extra memory cost on everybody, even if they
don't care about genpd and may not even compile it in.

>  #endif
>         struct pm_subsys_data   *subsys_data;  /* Owned by the subsystem. */
>         void (*set_latency_tolerance)(struct device *, s32);
> diff --git a/include/linux/pm_runtime.h b/include/linux/pm_runtime.h
> index 6245caa18034..af6d35178335 100644
> --- a/include/linux/pm_runtime.h
> +++ b/include/linux/pm_runtime.h
> @@ -59,6 +59,7 @@ extern void pm_runtime_get_suppliers(struct device *dev);
>  extern void pm_runtime_put_suppliers(struct device *dev);
>  extern void pm_runtime_new_link(struct device *dev);
>  extern void pm_runtime_drop_link(struct device *dev);
> +extern int pm_runtime_set_next_event(struct device *dev, ktime_t next);
>
>  /**
>   * pm_runtime_get_if_in_use - Conditionally bump up runtime PM usage counter.
> --
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> a Linux Foundation Collaborative Project
>