Re: [RFC/RFT][PATCH v2 0/7] Functional dependencies between devices

[Lukas Wunner <lukas@wunner.de>]

On Sun, Sep 11, 2016 at 12:04:36AM +0200, Rafael J. Wysocki wrote:
> On Sat, Sep 10, 2016 at 1:39 PM, Lukas Wunner <lukas@wunner.de> wrote:
> > On Thu, Sep 08, 2016 at 11:25:44PM +0200, Rafael J. Wysocki wrote:
> >> As discussed in the recent "On-demand device probing" thread and in a
> >> Kernel Summit session earlier today, there is a problem with handling
> >> cases where functional dependencies between devices are involved.
> >>
> >> What I mean by a "functional dependency" is when the driver of device B
> >> needs both device A and its driver to be present and functional to be
> >> able to work.  This implies that the driver of A needs to be working
> >> for B to be probed successfully and it cannot be unbound from the
> >> device before the B's driver.  This also has certain consequences for
> >> power management of these devices (suspend/resume and runtime PM
> >> ordering).
> >
> > As a general observation, this series seems to conflate two
> > separate issues:
> >
> > (a) non-hierarchical device dependencies
> >     (a device depends on another device which is not its parent
> >     with regards to (runtime) suspend/resume ordering)
> 
> You need to say what it means that one device depends on another one.
> Without that you don't get anything useful.

As stated above, what it means is that "a device depends on another device
with regards to (runtime) suspend/resume ordering".


> > (b) driver-presence dependencies
> >     (a device depends on another device to be bound to a driver
> >     before it can probe / must be unbound before the other device
> >     can unbind)
> >
> > Those two issues are orthogonal.
> 
> No, they aren't.
> 
> At least for a number of devices (quite likely it would be safe to say
> that for the majority of them I think) the supplier can only provide

You got numbers to back up the majority claim?


> any useful service to its consumers when it has a driver bound to it
> and running and the consumers cannot operate correctly without that
> service.  That's why the "unbind consumers before unbinding the
> supplier" logic is in there.
> 
> And in the context of this series a "dependency" boils down to the
> ordering of execution of callbacks of certain type.  Like, for
> example, "can I execute ->runtime_suspend() for device A before
> executing it for device B?"  If there's a link between A and B where
> the former is the supplier, the answer is "no".  The reason why is the
> assumption that after ->runtime_suspend() A will stop responding and
> will not be able to provide the service in question to B any more.
> 
> Likewise, if the driver of A goes away, this device will not be able
> to provide the service in question to B any more, so it doesn't make
> sense for the driver of B to still be running then.
> 
> This is the case I wanted to cover with this series if it was not clear
> before.

That much was clear to me. I maintain that dependency with regards to
suspend/resume ordering and dependency with regards to driver presence
are two different things. The case you wanted to cover just happens to
be a combination of the two. The series would be more useful if either
type of dependency could be achieved without gratuitously also getting
the other type.

You could make the case that providing two separate APIs seems
wasteful since it would increase the amount of code and sometimes
both types of dependency are required at the same time anyway.
That would be a valid argument. But you should design the API thus
that either dependency type is optional.

Providing just dependency with regards to suspend/resume ordering
should be as simple as adding a "DEVICE_LINKS_DRIVER_DONT_CARE" flag.
Then the various calls that you're adding in dd.c would only have to
be called if the flag isn't set, same for the various places where
you check the link state.


> Of course, you don't like what device_links_unbind_consumers() does
> because of your "Thunderbolt on Macs" use case, but guess what,

To name a different use case: On hybrid graphics laptops, the discrete
GPU usually includes an HDA controller for external HDMI displays.
The GPU and the HDA controllers are siblings (functions 0 and 1 of a
PCI slot), yet in many laptops power is cut to both devices when _PS3
is executed for the GPU function. Currently we have a kludge where
the HDA controller is suspended before the GPU is powered down
(see vga_switcheroo_init_domain_pm_optimus_hdmi_audio()).

I envision the HDA controller to be a consumer of the GPU in those
cases, thus ensuring that it's suspended before power is cut.

The way your series works right now, the HDA controller can't be
bound unless the GPU is bound. That restriction is unnecessary.


I've also heard from the i915 folks that on some Intel chipsets, the
integrated HDA controller depends on power wells that are controlled
by the integrated GPU. I'm not sure if a driver for the GPU needs
to be present in this use case.


> you can add a link flag to keep the consumer around when the supplier
> driver goes away.  I just don't want that to be the default behavior.

I assume you're referring to the DEVICE_LINK_PERSISTENT flag and that
a separate patch is needed to make this work, as you've pointed out
in your e-mail of Sep 7.

According to the commit message, such links need to be added before
the consumer probes, which I fear will require addition of PCI quirks.
That's not very pretty, a DEVICE_LINKS_DRIVER_DONT_CARE flag would
seem preferrable.


> > E.g. a driver-presence dependency may exist between a child and a
> > parent, or between siblings, whereas a non-hierarchical device
> > dependency by definition cannot exist between child/parent.
> >
> > Let's say I need a driver-presence dependency between parent/child.
> > The PM core already guarantees correct (runtime) suspend/resume
> > ordering between child. Device links duplicate that functionality.
> > Oops?
> 
> No, they don't and that should be clear.
> 
> They are for out-of-the-tree dependencies only and the parent-child
> one is *in* the tree.

I'm sure there are situations where a driver presence dependency
is needed between parent/child and you should fully expect that
developers will try to employ device links for these use cases.
Which means that the code for suspend/resume device ordering is
executed twice.

Best regards,

Lukas