linux-kernel - Re: [PATCH] ACPI: PM: postpone bringing devices to D0 unless we need them

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Message-ID: <YNIoWrR8W5Uow7kk@google.com>
Date:   Tue, 22 Jun 2021 11:13:46 -0700
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ACPI: PM: postpone bringing devices to D0 unless we need
 them

Hi Rafael,

On Tue, Jun 22, 2021 at 03:40:05PM +0200, Rafael J. Wysocki wrote:
> On Tue, Jun 22, 2021 at 12:20 AM Dmitry Torokhov
> <dmitry.torokhov@...il.com> wrote:
> >
> > Currently ACPI power domain brings devices into D0 state in the "resume
> > early" phase. Normally this does not cause any issues, as powering up
> > happens quickly. However there are peripherals that have certain timing
> > requirements for powering on, for example some models of Elan
> > touchscreens need 300msec after powering up/releasing reset line before
> > they can accept commands from the host. Such devices will dominate
> > the time spent in early resume phase and cause increase in overall
> > resume time as we wait for early resume to complete before we can
> > proceed to the normal resume stage.
> >
> > There are ways for a driver to indicate that it can tolerate device
> > being in the low power mode and that it knows how to power the device
> > back up when resuming, bit that requires changes to individual drivers
> > that may not really care about details of ACPI controlled power
> > management.
> >
> > This change attempts to solve this issue at ACPI power domain level, by
> > postponing powering up device until we get to the normal resume stage,
> > unless there is early resume handler defined for the device, or device
> > does not declare any resume handlers, in which case we continue powering
> > up such devices early. This allows us to shave off several hundred
> > milliseconds of resume time on affected systems.
> >
> > Signed-off-by: Dmitry Torokhov <dmitry.torokhov@...il.com>
> > ---
> >  drivers/acpi/device_pm.c | 46 +++++++++++++++++++++++++++++++++++-----
> >  1 file changed, 41 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/acpi/device_pm.c b/drivers/acpi/device_pm.c
> > index 096153761ebc..00b412ccb2e0 100644
> > --- a/drivers/acpi/device_pm.c
> > +++ b/drivers/acpi/device_pm.c
> > @@ -1131,17 +1131,52 @@ static int acpi_subsys_resume_noirq(struct device *dev)
> >   *
> >   * Use ACPI to put the given device into the full-power state and carry out the
> >   * generic early resume procedure for it during system transition into the
> > - * working state.
> > + * working state, but only do that if device either defines early resume
> > + * handler, or does not define power operations at all. Otherwise powering up
> > + * of the device is postponed to the normal resume phase.
> >   */
> >  static int acpi_subsys_resume_early(struct device *dev)
> >  {
> > +       const struct dev_pm_ops *pm = dev->driver ? dev->driver->pm : NULL;
> > +       struct acpi_device *adev = ACPI_COMPANION(dev);
> >         int ret;
> >
> > -       if (dev_pm_skip_resume(dev))
> > -               return 0;
> 
> The above doesn't need to be changed AFAICS.

I was trying to have if string if/else if/else, but I can keep it as it
was.

> 
> > +       if (dev_pm_skip_resume(dev)) {
> > +               ret = 0;
> > +       } else if (!pm || pm->resume_early) {
> 
> This is rather tricky, but I don't see a particular reason why it wouldn't work.
> 
> > +               ret = acpi_dev_resume(dev);
> > +               if (!ret)
> > +                       ret = pm_generic_resume_early(dev);
> > +       } else {
> > +               if (adev)
> > +                       acpi_device_wakeup_disable(adev);
> 
> This isn't necessary here.

Just to confirm - you are saying that disabling the device as a wakeup
source can be safely postponed till the normal resume stage? I was
trying to keep as much of the original behavior as possible and this is
a part of acpi_dev_resume() that we are now postponing.

Thanks.

-- 
Dmitry