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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Fri, 26 Nov 2021 14:30:00 +0100
Message-ID: <CAJZ5v0hrTDsCUn4vgmFrTTgd6H=orh-Kb5b3+_H9St4n8fTxBw@mail.gmail.com>
Subject: Re: [PATCH] PM: runtime: Allow rpm_resume() to succeed when runtime
 PM is disabled
To:     Ulf Hansson <ulf.hansson@linaro.org>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Alan Stern <stern@rowland.harvard.edu>,
        "Rafael J . Wysocki" <rjw@rjwysocki.net>,
        Linux PM <linux-pm@vger.kernel.org>,
        Kevin Hilman <khilman@kernel.org>,
        Maulik Shah <mkshah@codeaurora.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Fri, Nov 26, 2021 at 1:20 PM Ulf Hansson <ulf.hansson@linaro.org> wrote:
>
> On Mon, 1 Nov 2021 at 10:27, Ulf Hansson <ulf.hansson@linaro.org> wrote:
> >
> > On Fri, 29 Oct 2021 at 20:27, Rafael J. Wysocki <rafael@kernel.org> wrote:
> > >
> > > On Fri, Oct 29, 2021 at 12:20 AM Ulf Hansson <ulf.hansson@linaro.org> wrote:
> > > >
> > > > On Wed, 27 Oct 2021 at 16:33, Alan Stern <stern@rowland.harvard.edu> wrote:
> > > > >
> > > > > On Wed, Oct 27, 2021 at 12:55:43PM +0200, Ulf Hansson wrote:
> > > > > > On Wed, 27 Oct 2021 at 04:02, Alan Stern <stern@rowland.harvard.edu> wrote:
> > > > > > >
> > > > > > > On Wed, Oct 27, 2021 at 12:26:26AM +0200, Ulf Hansson wrote:
> > > > > > > > During system suspend, the PM core sets dev->power.is_suspended for the
> > > > > > > > device that is being suspended. This flag is also being used in
> > > > > > > > rpm_resume(), to allow it to succeed by returning 1, assuming that runtime
> > > > > > > > PM has been disabled and the runtime PM status is RPM_ACTIVE, for the
> > > > > > > > device.
> > > > > > > >
> > > > > > > > To make this behaviour a bit more useful, let's drop the check for the
> > > > > > > > dev->power.is_suspended flag in rpm_resume(), as it doesn't really need to
> > > > > > > > be limited to this anyway.
> > > > > > > >
> > > > > > > > Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
> > > > > > > > ---
> > > > > > > >  drivers/base/power/runtime.c | 4 ++--
> > > > > > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > > > > >
> > > > > > > > diff --git a/drivers/base/power/runtime.c b/drivers/base/power/runtime.c
> > > > > > > > index ec94049442b9..fadc278e3a66 100644
> > > > > > > > --- a/drivers/base/power/runtime.c
> > > > > > > > +++ b/drivers/base/power/runtime.c
> > > > > > > > @@ -742,8 +742,8 @@ static int rpm_resume(struct device *dev, int rpmflags)
> > > > > > > >   repeat:
> > > > > > > >       if (dev->power.runtime_error)
> > > > > > > >               retval = -EINVAL;
> > > > > > > > -     else if (dev->power.disable_depth == 1 && dev->power.is_suspended
> > > > > > > > -         && dev->power.runtime_status == RPM_ACTIVE)
> > > > > > > > +     else if (dev->power.disable_depth > 0 &&
> > > > > > > > +             dev->power.runtime_status == RPM_ACTIVE)
> > > > > > >
> > > > > > > IIRC there was a good reason why the original code checked for
> > > > > > > disable_depth == 1 rather than > 0.  But I don't remember exactly what
> > > > > > > the reason was.  Maybe it had something to do with the fact that during
> > > > > > > a system sleep __device_suspend_late calls __pm_runtime_disable, and the
> > > > > > > code was checking that there were no other disables in effect.
> > > > > >
> > > > > > The check was introduced in the below commit:
> > > > > >
> > > > > > Commit 6f3c77b040fc
> > > > > > Author: Kevin Hilman <khilman@ti.com>
> > > > > > Date:   Fri Sep 21 22:47:34 2012 +0000
> > > > > > PM / Runtime: let rpm_resume() succeed if RPM_ACTIVE, even when disabled, v2
> > > > > >
> > > > > > By reading the commit message it's pretty clear to me that the check
> > > > > > was added to cover only one specific use case, during system suspend.
> > > > > >
> > > > > > That is, that a driver may want to call pm_runtime_get_sync() from a
> > > > > > late/noirq callback (when the PM core has disabled runtime PM), to
> > > > > > understand whether the device is still powered on and accessible.
> > > > > >
> > > > > > > This is
> > > > > > > related to the documented behavior of rpm_resume (it's supposed to fail
> > > > > > > with -EACCES if the device is disabled for runtime PM, no matter what
> > > > > > > power state the device is in).
> > > > > > >
> > > > > > > That probably is also the explanation for why dev->power.is_suspended
> > > > > > > gets checked: It's how the code tells whether a system sleep is in
> > > > > > > progress.
> > > > > >
> > > > > > Yes, you are certainly correct about the current behaviour. It's there
> > > > > > for a reason.
> > > > > >
> > > > > > On the other hand I would be greatly surprised if this change would
> > > > > > cause any issues. Of course, I can't make guarantees, but I am, of
> > > > > > course, willing to help to fix problems if those happen.
> > > > > >
> > > > > > As a matter of fact, I think the current behaviour looks quite
> > > > > > inconsistent, as it depends on whether the device is being system
> > > > > > suspended.
> > > > > >
> > > > > > Moreover, for syscore devices (dev->power.syscore is set for them),
> > > > > > the PM core doesn't set the "is_suspended" flag. Those can benefit
> > > > > > from a common behaviour.
> > > > > >
> > > > > > Finally, I think the "is_suspended" flag actually needs to be
> > > > > > protected by a lock when set by the PM core, as it's being used in two
> > > > > > separate execution paths. Although, rather than adding a lock for
> > > > > > protection, we can just rely on the "disable_depth" in rpm_resume().
> > > > > > It would be easier and makes the behaviour consistent too.
> > > > >
> > > > > As long as is_suspended isn't _written_ in two separate execution paths,
> > > > > we're probably okay without a lock -- provided the code doesn't mind
> > > > > getting an indefinite result when a read races with a write.
> > > >
> > > > Well, indefinite doesn't sound very good to me for these cases, even
> > > > if it most likely never will happen.
> > > >
> > > > >
> > > > > > > So overall, I suspect this change should not be made.  But some other
> > > > > > > improvement (like a nice comment) might be in order.
> > > > > > >
> > > > > > > Alan Stern
> > > > > >
> > > > > > Thanks for reviewing!
> > > > >
> > > > > You're welcome.  Whatever you eventually decide to do should be okay
> > > > > with me.  I just wanted to make sure that you understood the deeper
> > > > > issue here and had given it some thought.  For example, it may turn out
> > > > > that you can resolve matters simply by updating the documentation.
> > > >
> > > > I observed the issue on cpuidle-psci. The devices it operates upon are
> > > > assigned as syscore devices and these are hooked up to a genpd.
> > > >
> > > > A call to pm_runtime_get_sync() can happen even after the PM core has
> > > > disabled runtime PM in the "late" phase. So the error code is received
> > > > for these real use-cases.
> > > >
> > > > Now, as we currently don't check the return value of
> > > > pm_runtime_get_sync() in cpuidle-psci, it's not a big deal. But it
> > > > certainly seems worth fixing in my opinion.
> > > >
> > > > Let's see if Rafael has some thoughts around this.
> > >
> > > Am I thinking correctly that this is mostly about working around the
> > > limitations of pm_runtime_force_suspend()?
> >
> > No, this isn't related at all.
> >
> > The cpuidle-psci driver doesn't have PM callbacks, thus using
> > pm_runtime_force_suspend() would not work here.
>
> Just wanted to send a ping on this to see if we can come to a
> conclusion. Or maybe we did? :-)
>
> I think in the end, what slightly bothers me, is that the behavior is
> a bit inconsistent. Although, maybe it's the best we can do.

I've been thinking about this and it looks like we can do better, but
instead of talking about this I'd rather send a patch.

Stay tuned.