Re: [PATCH v2 5/9] PM / ACPI: Provide option to disable direct_complete for ACPI devices

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

On Monday, August 28, 2017 10:31:44 AM CEST Ulf Hansson wrote:
> On 28 August 2017 at 03:30, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> > On Friday, August 25, 2017 3:42:35 PM CEST Rafael J. Wysocki wrote:
> >> On Thursday, August 24, 2017 11:50:40 PM CEST Rafael J. Wysocki wrote:
> >> > On Thursday, August 24, 2017 6:35:49 PM CEST Rafael J. Wysocki wrote:
> >> > > On Thursday, August 24, 2017 11:15:26 AM CEST Ulf Hansson wrote:
> >> >
> >> > [cut]
> >> >
> >> > > [BTW, it is not entirely clear to me why it ever is necessary to runtime resume
> >> > > a device with direct_complete set after __device_suspend(), because it can only
> >> > > have direct_complete set at that point if all of the hierarchy below it has
> >> > > this flag set too and so runtime PM has to be disabled for all of those
> >> > > devices as well.]
> >> >
> >> > Which makes me realize that we should take a step back and look at what
> >> > problems there are.
> >> >
> >> > First, there are devices (I know about two examples so far and both are PCI)
> >> > that may need to be runtime resumed during system suspend for reasons other
> >> > than the ones checked by the ACPI PM domain (or the PCI bus type).  There needs
> >> > to be a way to indicate that from the driver side.
> >> >
> >> > However, it still may be valuable to check the power-related conditions for
> >> > leaving the device in runtime suspend over system suspend/resume in case
> >> > it actually doesn't need to be runtime resumed during system suspend after
> >> > all.  That's what the majority of my patch was about.
> >> >
> >> > The second problem is that the ACPI PM domain (and the PCI bus type)
> >> > runtime resumes all devices unconditionally in its ->suspend callback,
> >> > even though that may not be necessary for some devices.  Therefore there
> >> > needs to be a way to indicate that too.  That still would be good to
> >> > have *regardless* of the direct_complete mechanism, because the direct_complete
> >> > flag may not be set very often due to dependencies and then the
> >> > resume-during-suspend will take place unnecessarily.
> >> >
> >> > Accordingly, it looks like we need a "no need to resume me" flag in the first
> >> > place.  That would indicate to interested pieces of code that, from the
> >> > driver perspective, the device doesn't need to be runtime resumed before
> >> > invoking its system suspend callbacks.  This should be clear enough to everyone
> >> > IMO.
> >> >
> >> > [Note that if that flag is set for all devices, we may drop it along with
> >> > direct_complete, but before that happens both are needed.]
> >>
> >> I think we are in agreement that direct_complete will not be necessary any
> >> more when all drivers/bus types/PM domains and so on can do the "safe
> >> suspend", but we're not there yet. :-)
> >>
> >> > To address the first issue I would add something like the flag in the patches
> >> > I sent (but without the ACPI PM domain part which should be covered by the
> >> > "no need to resume me" flag above), because that allows the device's ->suspend
> >> > callback to run in principle and the driver may use that callback even to
> >> > runtime resume the device if that's what it wants to do.  So something like
> >> > "run my ->suspend callback even though I might stay in runtime suspend".
> >> >
> >> > I would probably add driver_flags to dev_pm_info for that to set at the probe
> >> > time (and I would make the core clear that on driver removal).
> >> >
> >> > The complexity concern is there, but honestly I don't see a better way at
> >> > this point.
> >>
> >> So below is a prototype patch.  It still is missing a documentation update, but
> >> other than that it should be complete unless I missed something.
> >>
> >> The way it works is that the SAFE_SUSPEND flag is not looked at by the core
> >> at all.  The ACPI PM domain looks at it and the PCI bus type can be modified
> >> to take it into account in the future.  That is what causes the "runtime resume
> >> during system suspend" to be skipped.
> >>
> >> In turn, the ALWAYS_SUSPEND flag is only looked at by the core and it causes
> >> the decision on whether or not to use direct_complete to be deferred to the
> >> __device_suspend_late() time.  If you set it for a PCI device, the effect is
> >> equivalent to "no direct_complete".  If you set it for a device in the ACPI
> >> PM domain, that depends on whether or not SAFE_SUSPEND is set.  If it isn't
> >> set, the effect is equivalent to "no direct_complete" too, but if it is set,
> >> the core may still try to use direct_complete for the device, but it will
> >> make the decision on it in __device_suspend_late() and then it will not invoke
> >> the ->suspend_late callback for the device if it is still runtime suspended.
> >> [Note that you cannot runtime resume and runtime suspend again a device during
> >> system suspend, so if it is runtime suspended in __device_suspend_late(), it
> >> has been runtime suspend all the way since device_prepare().]
> >>
> >> So say you point the ->suspend_late and ->resume_early callbacks of
> >> the designware i2c driver to pm_runtime_force_suspend() and
> >> pm_runtime_force_resume(), respectively, and set both the SAFE_SUSPEND
> >> and ALWAYS_SUSPEND flags for the device.
> >>
> >> If system suspend is started and the device is not runtime suspended,
> >> direct_complete is not set for it and everything works as usual, so say
> >> the device is runtime suspended in device_prepare().  Then, the ACPI PM
> >> domain checks the other conditions for leaving it in runtime suspend and
> >> returns either 0 or a positive number from acpi_subsys_prepare().
> >>
> >> If 0 is returned, direct_complete is not set by the core and
> >> acpi_subsys_suspend() is called.  It checks the SAFE_SUSPEND flag and sees
> >> that the device need not be runtime resumed, so it invokes the driver's
> >> ->suspend callback (which is not present, so it doesn't do anything).
> >> Next, in __device_suspend_late(), acpi_subsys_suspend_late() is invoked
> >> and it calls pm_runtime_force_suspend(), which executes the driver's
> >> ->runtime_suspend() callback, and then (if successful) calls
> >> acpi_dev_suspend_late() to put the device into a low-power state.  The
> >> resume path is a reverse of the above in this case.  So far, so good.
> >
> > Well, not really, because if the device remains runtime suspended,
> > ->runtime_suspend() will not be called by pm_runtime_force_suspend() and
> > acpi_dev_suspend_late() should not be called then.
> >
> > So more changes in the ACPI PM domain are needed after all.
> 
> Yes, that's what I thought as well.
> 
> Anyway, let me cook a new version of the series - trying to address
> the first bits you have pointed out. Then we can continue with
> fine-tuning on top, addressing further optimizations of the ACPI PM
> domain.

Actually, please hold on and let me show you what I would like to do
first.

Thanks,
Rafael