Re: [PATCH v3 1/2] PM / domains: Rework governor code to be more consistent

[Ramesh Thomas <ramesh.thomas@intel.com>]

On 2017-11-07 at 11:22:48 +0100, Rafael J. Wysocki wrote:
> On Tue, Nov 7, 2017 at 6:05 AM, Ramesh Thomas <ramesh.thomas@intel.com> wrote:
> > On 2017-11-07 at 02:23:18 +0100, Rafael J. Wysocki wrote:
> >> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> >>
> >> The genpd governor currently uses negative PM QoS values to indicate
> >> the "no suspend" condition and 0 as "no restriction", but it doesn't
> >> use them consistently.  Moreover, it tries to refresh QoS values for
> >> already suspended devices in a quite questionable way.
> >>
> >> For the above reasons, rework it to be a bit more consistent.
> >>
> >> First off, note that dev_pm_qos_read_value() in
> >> dev_update_qos_constraint() and __default_power_down_ok() is
> >> evaluated for devices in suspend.  Moreover, that only happens if the
> >> effective_constraint_ns value for them is negative (meaning "no
> >> suspend").  It is not evaluated in any other cases, so effectively
> >> the QoS values are only updated for devices in suspend that should
> >> not have been suspended in the first place.  In all of the other
> >> cases, the QoS values taken into account are the effective ones from
> >> the time before the device has been suspended, so generally devices
> >> need to be resumed and suspended again for new QoS values to take
> >> effect anyway.  Thus evaluating dev_update_qos_constraint() in
> >> those two places doesn't make sense at all, so drop it.
> >>
> >> Second, initialize effective_constraint_ns to 0 ("no constraint")
> >> rather than to (-1) ("no suspend"), which makes more sense in
> >> general and in case effective_constraint_ns is never updated
> >> (the device is in suspend all the time or it is never suspended)
> >> it doesn't affect the device's parent and so on.
> >>
> >> Finally, rework default_suspend_ok() to explicitly handle the
> >> "no restriction" and "no suspend" special cases.
> >>
> >> Also add WARN_ON() around checks that should never trigger.
> >>
> >> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> >> Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
> >> ---
> >>
> >> v2 -> v3: Take children that don't belong to genpd power domains into
> >>           account in dev_update_qos_constraint().
> >>
> >> ---
> >>  drivers/base/power/domain.c          |    2
> >>  drivers/base/power/domain_governor.c |   71 ++++++++++++++++++++++++-----------
> >>  2 files changed, 50 insertions(+), 23 deletions(-)
> >>
> >> Index: linux-pm/drivers/base/power/domain.c
> >> ===================================================================
> >> --- linux-pm.orig/drivers/base/power/domain.c
> >> +++ linux-pm/drivers/base/power/domain.c
> >> @@ -1331,7 +1331,7 @@ static struct generic_pm_domain_data *ge
> >>
> >>       gpd_data->base.dev = dev;
> >>       gpd_data->td.constraint_changed = true;
> >> -     gpd_data->td.effective_constraint_ns = -1;
> >> +     gpd_data->td.effective_constraint_ns = 0;
> >>       gpd_data->nb.notifier_call = genpd_dev_pm_qos_notifier;
> >>
> >>       spin_lock_irq(&dev->power.lock);
> >> Index: linux-pm/drivers/base/power/domain_governor.c
> >> ===================================================================
> >> --- linux-pm.orig/drivers/base/power/domain_governor.c
> >> +++ linux-pm/drivers/base/power/domain_governor.c
> >> @@ -14,22 +14,33 @@
> >>  static int dev_update_qos_constraint(struct device *dev, void *data)
> >>  {
> >>       s64 *constraint_ns_p = data;
> >> -     s32 constraint_ns = -1;
> >> +     s64 constraint_ns;
> >>
> >> -     if (dev->power.subsys_data && dev->power.subsys_data->domain_data)
> >> +     if (dev->power.subsys_data && dev->power.subsys_data->domain_data) {
> >> +             /*
> >> +              * Only take suspend-time QoS constraints of devices into
> >> +              * account, because constraints updated after the device has
> >> +              * been suspended are not guaranteed to be taken into account
> >> +              * anyway.  In order for them to take effect, the device has to
> >> +              * be resumed and suspended again.
> >> +              */
> >>               constraint_ns = dev_gpd_data(dev)->td.effective_constraint_ns;
> >> -
> >> -     if (constraint_ns < 0) {
> >> +     } else {
> >> +             /*
> >> +              * The child is not in a domain and there's no info on its
> >> +              * suspend/resume latencies, so assume them to be negligible and
> >> +              * take its current PM QoS constraint (that's the only thing
> >> +              * known at this point anyway).
> >> +              */
> >>               constraint_ns = dev_pm_qos_read_value(dev);
> >> -             constraint_ns *= NSEC_PER_USEC;
> >> +             if (constraint_ns > 0)
> >> +                     constraint_ns *= NSEC_PER_USEC;
> >>       }
> >> +
> >> +     /* 0 means "no constraint" */
> >>       if (constraint_ns == 0)
> >>               return 0;
> >>
> >> -     /*
> >> -      * constraint_ns cannot be negative here, because the device has been
> >> -      * suspended.
> >> -      */
> >>       if (constraint_ns < *constraint_ns_p || *constraint_ns_p == 0)
> >>               *constraint_ns_p = constraint_ns;
> >>
> >> @@ -76,14 +87,32 @@ static bool default_suspend_ok(struct de
> >>               device_for_each_child(dev, &constraint_ns,
> >>                                     dev_update_qos_constraint);
> >>
> >> -     if (constraint_ns > 0) {
> >> +     if (constraint_ns == 0) {
> >> +             /* "No restriction", so the device is allowed to suspend. */
> >> +             td->effective_constraint_ns = 0;
> >> +             td->cached_suspend_ok = true;
> >> +     } else if (constraint_ns < 0) {
> >> +             /*
> >> +              * This triggers if one of the children that don't belong to a
> >> +              * domain has a negative PM QoS constraint and it's better not
> >> +              * to suspend then.  effective_constraint_ns is negative already
> >> +              * and cached_suspend_ok is false, so bail out.
> >> +              */
> >> +             return false;
> >
> > This change is ok. However, would like to bring to your attention a possible
> > inconsistency in the treatment of negative value as "no suspend at all" that
> > can affect this.
> >
> > user level entry does not allow negative values. Only way to enter a negative
> > value is if the kernel API to add/update is used. In that interface, if -1
> > (PM_QOS_DEFAULT_VALUE) is passed, pm_qos_update_target will actually assign
> > the default value stored in the constraint. The default value is
> > PM_QOS_RESUME_LATENCY_DEFAULT_VALUE which is 0. 0 means "no constraint".
> 
> OK, but that only means that default_suspend_ok() will never see -1 as
> a value.  It may see other negative values, though, and treating them
> as "no suspend" is not incorrect.  So I don't think the patch needs to
> be updated.

Right. The issue with -1 is a bug at another place and the second patch fixes
that anyway. Looks good to me.

Reviewed-by: Ramesh Thomas <ramesh.thomas@intel.com>

Thanks,
Ramesh

> 
> In any case, good catch!
> 
> Thanks,
> Rafael