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 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".

All this gets fixed in the next patch though. 

> +	} else {
>  		constraint_ns -= td->suspend_latency_ns +
>  				td->resume_latency_ns;
> -		if (constraint_ns == 0)
> +		/*
> +		 * effective_constraint_ns is negative already and
> +		 * cached_suspend_ok is false, so if the computed value is not
> +		 * positive, return right away.
> +		 */
> +		if (constraint_ns <= 0)
>  			return false;
> +
> +		td->effective_constraint_ns = constraint_ns;
> +		td->cached_suspend_ok = true;
>  	}
> -	td->effective_constraint_ns = constraint_ns;
> -	td->cached_suspend_ok = constraint_ns >= 0;
>  
>  	/*
>  	 * The children have been suspended already, so we don't need to take
> @@ -144,18 +173,16 @@ static bool __default_power_down_ok(stru
>  		 */
>  		td = &to_gpd_data(pdd)->td;
>  		constraint_ns = td->effective_constraint_ns;
> -		/* default_suspend_ok() need not be called before us. */
> -		if (constraint_ns < 0) {
> -			constraint_ns = dev_pm_qos_read_value(pdd->dev);
> -			constraint_ns *= NSEC_PER_USEC;
> -		}
> +		/*
> +		 * Negative values mean "no suspend at all" and this runs only
> +		 * when all devices in the domain are suspended, so it must be
> +		 * 0 at least.
> +		 *
> +		 * 0 means "no constraint"
> +		 */
>  		if (constraint_ns == 0)
>  			continue;
>  
> -		/*
> -		 * constraint_ns cannot be negative here, because the device has
> -		 * been suspended.
> -		 */
>  		if (constraint_ns <= off_on_time_ns)
>  			return false;
>  
> 

Regards,
Ramesh