On Mon, Nov 6, 2017 at 1:10 PM, Ulf Hansson <ulf.hans...@linaro.org> wrote:
> On 3 November 2017 at 12:47, Rafael J. Wysocki <r...@rjwysocki.net> wrote:
>> From: Rafael J. Wysocki <rafael.j.wyso...@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" special case.
>>
>> Signed-off-by: Rafael J. Wysocki <rafael.j.wyso...@intel.com>
>> ---
>> drivers/base/power/domain.c | 2 -
>> drivers/base/power/domain_governor.c | 61
>> +++++++++++++++++++++--------------
>> 2 files changed, 38 insertions(+), 25 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 +14,22 @@
>> static int dev_update_qos_constraint(struct device *dev, void *data)
>> {
>> s64 *constraint_ns_p = data;
>> - s32 constraint_ns = -1;
>> -
>> - if (dev->power.subsys_data && dev->power.subsys_data->domain_data)
>> - constraint_ns =
>> dev_gpd_data(dev)->td.effective_constraint_ns;
>> + s64 constraint_ns;
>>
>> - if (constraint_ns < 0) {
>> - constraint_ns = dev_pm_qos_read_value(dev);
>> - constraint_ns *= NSEC_PER_USEC;
>> - }
>> - if (constraint_ns == 0)
>> + if (!dev->power.subsys_data || !dev->power.subsys_data->domain_data)
>> return 0;
>>
>> /*
>> - * constraint_ns cannot be negative here, because the device has been
>> - * suspended.
>> + * 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.
>> */
>
> This means a change in behavior, because earlier we took into account
> QoS values for child devices that were not attached to a genpd.
OK
I have overlooked it or rather have forgotten about that.
> Is there any reason you think we should change this, or is it just
> something you overlooked here?
>
> I understand, that if the QoS constraint has been updated after such
> child device has been suspended, it's tricky to take a correct
> decision.
Right, but if they are not in a domain, the best we can do is to look
at the current value.
> To really solve this, we would either have to register a QoS notifier
> for all children devices that has its parent attached to a genpd, or
> always runtime resume devices before updating QoS constraints.
>
> Non of these options is perfect, so perhaps we should consider a "best
> effort" approach instead? Whatever that may be.
I think best effort makes most sense.
So I guess I'll simply evaluate dev_pm_qos_read_value(dev) if
subsys_data or subsys_data->domain_data is not there.
Of course, that doesn't apply to the code in __default_power_down_ok()
as that only takes device in the domain into account anyway.
Thanks,
Rafael
