Re: [PATCH] PM / QoS: Fix device resume latency PM QoS

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

On Tue, Oct 24, 2017 at 10:49 AM, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> On Tuesday, October 24, 2017 7:54:09 AM CEST Ramesh Thomas wrote:
>> On 2017-10-20 at 13:27:34 +0200, Rafael J. Wysocki wrote:
>> > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>> >

[cut]

>> > @@ -63,10 +60,14 @@ static bool default_suspend_ok(struct de
>> >
>> >     spin_unlock_irqrestore(&dev->power.lock, flags);
>> >
>> > -   if (constraint_ns < 0)
>> > +   if (constraint_ns == 0)
>> >             return false;
>> >
>> > -   constraint_ns *= NSEC_PER_USEC;
>> > +   if (constraint_ns == PM_QOS_RESUME_LATENCY_NO_CONSTRAINT)
>> > +           constraint_ns = -1;
>> > +   else
>> > +           constraint_ns *= NSEC_PER_USEC;
>> > +
>> >     /*
>> >      * We can walk the children without any additional locking, because
>> >      * they all have been suspended at this point and their
>> > @@ -76,14 +77,19 @@ static bool default_suspend_ok(struct de
>> >             device_for_each_child(dev, &constraint_ns,
>> >                                   dev_update_qos_constraint);
>> >
>> > -   if (constraint_ns > 0) {
>> > -           constraint_ns -= td->suspend_latency_ns +
>> > -                           td->resume_latency_ns;
>> > -           if (constraint_ns == 0)
>> > -                   return false;
>> > +   if (constraint_ns < 0) {
>> > +           /* The children have no constraints. */
>> > +           td->effective_constraint_ns = PM_QOS_RESUME_LATENCY_NO_CONSTRAINT;
>> > +           td->cached_suspend_ok = true;
>> > +   } else {
>> > +           constraint_ns -= td->suspend_latency_ns + td->resume_latency_ns;
>> > +           if (constraint_ns > 0) {
>> > +                   td->effective_constraint_ns = constraint_ns;
>> > +                   td->cached_suspend_ok = true;
>> > +           } else {
>> > +                   td->effective_constraint_ns = 0;
>>
>> Previously effective_constraint_ns was left as -1 if constraint_ns becomes 0
>> Not sure if this change is intentional.
>
> Yes, it is.
>
>> I think at dev_update_qos_constraint, this can cause to skip call to
>> dev_pm_qos_read_value.
>
> I need to double check that.

If constraint_ns becomes 0 (or less) here, power cannot be removed
from the device, because it would add an unacceptable latency.

Thus effective_constraint_ns has to be 0 for it to indicate that
situation.  If it was left at -1, it would mean "no requirement", but
that wouldn't be correct.

Thanks,
Rafael