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Message-ID: <20171107232449.GB26314@intel.com>
Date: Tue, 7 Nov 2017 15:24:49 -0800
From: Ramesh Thomas <ramesh.thomas@...el.com>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>,
Linux PM <linux-pm@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>,
Ulf Hansson <ulf.hansson@...aro.org>,
Geert Uytterhoeven <geert@...ux-m68k.org>,
Tero Kristo <t-kristo@...com>,
Reinette Chatre <reinette.chatre@...el.com>,
Alex Shi <alex.shi@...aro.org>
Subject: Re: [PATCH v3 1/2] PM / domains: Rework governor code to be more
consistent
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@...el.com> wrote:
> > On 2017-11-07 at 02:23:18 +0100, Rafael J. Wysocki wrote:
> >> From: Rafael J. Wysocki <rafael.j.wysocki@...el.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@...el.com>
> >> Tested-by: Geert Uytterhoeven <geert+renesas@...der.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@...el.com>
Thanks,
Ramesh
>
> In any case, good catch!
>
> Thanks,
> Rafael
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