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Date: Mon, 6 Nov 2017 21:05:45 -0800
From: Ramesh Thomas <ramesh.thomas@...el.com>
To: "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc: 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 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".
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
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