[1/3] PM / core: Add SAFE_SUSPEND driver flag

From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Add a driver_flags field to struct dev_pm_info for flags that can be
set by device drivers at the probe time to inform the PM core and/or
bus types, PM domains and so on on the capabilities and/or
preferences of device drivers.  It is anticipated that more than one
flag of this kind will be necessary going forward.

Define and document a SAFE_SUSPEND flag to instruct bus types and PM
domains that the system suspend callbacks provided by the driver can
cope with runtime suspended devices, so from the driver's perspective
it should be safe to leave devices in runtime suspend during system
suspend.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---
 Documentation/driver-api/pm/devices.rst |    7 +++++++
 drivers/base/dd.c                       |    2 ++
 include/linux/pm.h                      |   16 ++++++++++++++++
 3 files changed, 25 insertions(+)

On 29 August 2017 at 02:20, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>
> Add a driver_flags field to struct dev_pm_info for flags that can be
> set by device drivers at the probe time to inform the PM core and/or
> bus types, PM domains and so on on the capabilities and/or
> preferences of device drivers.  It is anticipated that more than one
> flag of this kind will be necessary going forward.
>
> Define and document a SAFE_SUSPEND flag to instruct bus types and PM
> domains that the system suspend callbacks provided by the driver can
> cope with runtime suspended devices, so from the driver's perspective
> it should be safe to leave devices in runtime suspend during system
> suspend.

This changelog is a bit too vague to me. Wouldn't it be more clear if
also adding something along the lines that this also means that
runtime resuming a device isn't needed by the subsystem/PM domain
during system sleep? Because ideally that is what you want to avoid,
right?

Moreover I am also not convinced that this solution really is the
right path. It seems like we might end up adding more bits for the
"driver_flag" field and it gets complicated. Do we really need to
distinguish between all different cases in such detail?

I will continue to review this tomorrow, however in the meantime I
have finalized a re-spin of my v3 series so I decided to post it
anyway. I am adding only one new flag to the PM core, perhaps I am
over-simplifying things, but please have look once more. I think I
have addressed all your concerns you have raised so far.

Kind regards
Uffe

>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> ---
>  Documentation/driver-api/pm/devices.rst |    7 +++++++
>  drivers/base/dd.c                       |    2 ++
>  include/linux/pm.h                      |   16 ++++++++++++++++
>  3 files changed, 25 insertions(+)
>
> Index: linux-pm/include/linux/pm.h
> ===================================================================
> --- linux-pm.orig/include/linux/pm.h
> +++ linux-pm/include/linux/pm.h
> @@ -550,6 +550,21 @@ struct pm_subsys_data {
>  #endif
>  };
>
> +/*
> + * Driver flags to control system suspend/resume behavior.
> + *
> + * These flags can be set by device drivers at the probe time.  They need not be
> + * cleared by the drivers as the driver core will take care of that.
> + *
> + * SAFE_SUSPEND: No need to runtime resume the device during system suspend.
> + *
> + * Setting SAFE_SUSPEND instructs bus types and PM domains which may want to
> + * runtime resume the device upfront during system suspend that doing so is not
> + * necessary from the driver's perspective, because the system suspend callbacks
> + * provided by it can cope with a runtime suspended device.
> + */
> +#define DPM_FLAG_SAFE_SUSPEND  BIT(0)
> +
>  struct dev_pm_info {
>         pm_message_t            power_state;
>         unsigned int            can_wakeup:1;
> @@ -561,6 +576,7 @@ struct dev_pm_info {
>         bool                    is_late_suspended:1;
>         bool                    early_init:1;   /* Owned by the PM core */
>         bool                    direct_complete:1;      /* Owned by the PM core */
> +       unsigned int            driver_flags;
>         spinlock_t              lock;
>  #ifdef CONFIG_PM_SLEEP
>         struct list_head        entry;
> Index: linux-pm/drivers/base/dd.c
> ===================================================================
> --- linux-pm.orig/drivers/base/dd.c
> +++ linux-pm/drivers/base/dd.c
> @@ -436,6 +436,7 @@ pinctrl_bind_failed:
>         if (dev->pm_domain && dev->pm_domain->dismiss)
>                 dev->pm_domain->dismiss(dev);
>         pm_runtime_reinit(dev);
> +       dev->power.driver_flags = 0;
>
>         switch (ret) {
>         case -EPROBE_DEFER:
> @@ -841,6 +842,7 @@ static void __device_release_driver(stru
>                 if (dev->pm_domain && dev->pm_domain->dismiss)
>                         dev->pm_domain->dismiss(dev);
>                 pm_runtime_reinit(dev);
> +               dev->power.driver_flags = 0;
>
>                 klist_remove(&dev->p->knode_driver);
>                 device_pm_check_callbacks(dev);
> Index: linux-pm/Documentation/driver-api/pm/devices.rst
> ===================================================================
> --- linux-pm.orig/Documentation/driver-api/pm/devices.rst
> +++ linux-pm/Documentation/driver-api/pm/devices.rst
> @@ -729,6 +729,13 @@ state temporarily, for example so that i
>  disabled.  This all depends on the hardware and the design of the subsystem and
>  device driver in question.
>
> +Some bus types and PM domains have a policy to runtime resume all
> +devices upfront in their ``->suspend`` callbacks, but that may not be really
> +necessary if the system suspend-resume callbacks provided by the device's
> +driver can cope with a runtime-suspended device.  The driver can indicate that
> +by setting ``DPM_FLAG_SAFE_SUSPEND`` in :c:member:`power.driver_flags` at the
> +probe time.
> +
>  During system-wide resume from a sleep state it's easiest to put devices into
>  the full-power state, as explained in :file:`Documentation/power/runtime_pm.txt`.
>  Refer to that document for more information regarding this particular issue as
>
>

On Tuesday, August 29, 2017 4:57:28 PM CEST Ulf Hansson wrote:
> On 29 August 2017 at 02:20, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> >
> > Add a driver_flags field to struct dev_pm_info for flags that can be
> > set by device drivers at the probe time to inform the PM core and/or
> > bus types, PM domains and so on on the capabilities and/or
> > preferences of device drivers.  It is anticipated that more than one
> > flag of this kind will be necessary going forward.
> >
> > Define and document a SAFE_SUSPEND flag to instruct bus types and PM
> > domains that the system suspend callbacks provided by the driver can
> > cope with runtime suspended devices, so from the driver's perspective
> > it should be safe to leave devices in runtime suspend during system
> > suspend.
> 
> This changelog is a bit too vague to me. Wouldn't it be more clear if
> also adding something along the lines that this also means that
> runtime resuming a device isn't needed by the subsystem/PM domain
> during system sleep?

No.

> Because ideally that is what you want to avoid, right?

Not really.  The driver doesn't know what the needs of the higher level
are.  It may only say what it can do and the bus type can use this
information to make a decision.

> Moreover I am also not convinced that this solution really is the
> right path. It seems like we might end up adding more bits for the
> "driver_flag" field and it gets complicated. Do we really need to
> distinguish between all different cases in such detail?

Yes, we do.

Every time we try to address two different problems with one mechanism,
it backfires later.

> I will continue to review this tomorrow, however in the meantime I
> have finalized a re-spin of my v3 series so I decided to post it
> anyway. I am adding only one new flag to the PM core, perhaps I am
> over-simplifying things, but please have look once more. I think I
> have addressed all your concerns you have raised so far.

I'll have a look, but I really don't want to conflate the "I'm fine
with not resuming the device" case with the "I don't want to use
direct_complete with it" one.  To me, they are fundamentally different
and I'm not going to apply any patches conflating them.

Thanks,
Rafael