linux-kernel - Re: [PATCH v1] PM: runtime: Take active children into account in pm_runtime_get_if_in

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Message-ID: <CAJZ5v0isuV-HDyGCrLeFp0WFcCQoQpi9dMEbJnWrsOdvk4Bf1A@mail.gmail.com>
Date: Wed, 9 Jul 2025 14:48:00 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Ulf Hansson <ulf.hansson@...aro.org>
Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>, Linux PM <linux-pm@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, Sakari Ailus <sakari.ailus@...ux.intel.com>, 
	Alex Elder <elder@...aro.org>, Laurent Pinchart <laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH v1] PM: runtime: Take active children into account in pm_runtime_get_if_in_use()

On Wed, Jul 9, 2025 at 2:06 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Wed, Jul 9, 2025 at 1:47 PM Ulf Hansson <ulf.hansson@...aro.org> wrote:
> >
> > On Wed, 9 Jul 2025 at 12:41, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> > >
> > > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > >
> > > For all practical purposes, there is no difference between the situation
> > > in which a given device is not ignoring children and its active child
> > > count is nonzero and the situation in which its runtime PM usage counter
> > > is nonzero.  However, pm_runtime_get_if_in_use() will only increment the
> > > device's usage counter and return 1 in the latter case.
> > >
> > > For consistency, make it do so in the former case either by adjusting
> > > pm_runtime_get_conditional() and update the related kerneldoc comments
> > > accordingly.
> > >
> > > Fixes: c0ef3df8dbae ("PM: runtime: Simplify pm_runtime_get_if_active() usage")
> > > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > > ---
> > >  drivers/base/power/runtime.c |   27 ++++++++++++++++++---------
> > >  1 file changed, 18 insertions(+), 9 deletions(-)
> > >
> > > --- a/drivers/base/power/runtime.c
> > > +++ b/drivers/base/power/runtime.c
> > > @@ -1203,10 +1203,12 @@
> > >   *
> > >   * Return -EINVAL if runtime PM is disabled for @dev.
> > >   *
> > > - * Otherwise, if the runtime PM status of @dev is %RPM_ACTIVE and either
> > > - * @ign_usage_count is %true or the runtime PM usage counter of @dev is not
> > > - * zero, increment the usage counter of @dev and return 1. Otherwise, return 0
> > > - * without changing the usage counter.
> > > + * Otherwise, if its runtime PM status is %RPM_ACTIVE and (1) @ign_usage_count
> > > + * is set, or (2) @dev is not ignoring children and its active child count is
> > > + * nonero, or (3) the runtime PM usage counter of @dev is not zero, increment
> > > + * the usage counter of @dev and return 1.
> > > + *
> > > + * Otherwise, return 0 without changing the usage counter.
> > >   *
> > >   * If @ign_usage_count is %true, this function can be used to prevent suspending
> > >   * the device when its runtime PM status is %RPM_ACTIVE.
> > > @@ -1228,7 +1230,8 @@
> > >                 retval = -EINVAL;
> > >         } else if (dev->power.runtime_status != RPM_ACTIVE) {
> > >                 retval = 0;
> > > -       } else if (ign_usage_count) {
> > > +       } else if (ign_usage_count || (!dev->power.ignore_children &&
> > > +                  atomic_read(&dev->power.child_count) > 0)) {
> >
> > I am not sure I understand why this is needed, sorry.
> >
> > If someone and somehow we have "dev->power.runtime_status ==
> > RPM_ACTIVE", then the dev's parents/childrens and suppliers/consumers
> > should have been reference counted correctly already.
>
> Sure.
>
> > Otherwise it should not have been possible to set the runtime_status to RPM_ACTIVE
> > in the first place, right?
>
> Right.
>
> runtime_status must be RPM_ACTIVE, but pm_runtime_get_if_in_use() only
> wants to bump it up if the device is in use in addition to that.

I mean pm_runtime_get_if_in_use() only wants to bump up the device's
usage counter if it is in use already.

> So far it's been checking the usage counter only though.

And the above is correct.

> >
> > >                 retval = 1;
> > >                 atomic_inc(&dev->power.usage_count);
> > >         } else {
> > > @@ -1261,10 +1264,16 @@
> > >   * @dev: Target device.
> > >   *
> > >   * Increment the runtime PM usage counter of @dev if its runtime PM status is
> > > - * %RPM_ACTIVE and its runtime PM usage counter is greater than 0, in which case
> > > - * it returns 1. If the device is in a different state or its usage_count is 0,
> > > - * 0 is returned. -EINVAL is returned if runtime PM is disabled for the device,
> > > - * in which case also the usage_count will remain unmodified.
> > > + * %RPM_ACTIVE and its runtime PM usage counter is greater than 0 or it is not
> > > + * ignoring children and its active child count is nonzero.  1 is returned in
> > > + * this case.
> > > + *
> > > + * If @dev is in a different state or it is not in use (that is, its usage
> > > + * counter is 0, or it is ignoring children, or its active child count is 0),
> > > + * 0 is returned.
> > > + *
> > > + * -EINVAL is returned if runtime PM is disabled for the device, in which case
> > > + * also the usage counter of @dev is not updated.
> > >   */
> > >  int pm_runtime_get_if_in_use(struct device *dev)
> > >  {
> > >
> > >
> > >
> >
> > Kind regards
> > Uffe
> >