Re: [RFD] Functional dependencies between devices

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

On Tuesday, October 27, 2015 04:20:51 PM Tomeu Vizoso wrote:
> On 27 October 2015 at 16:24, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> > Hi All,
> >
> > 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).
> >
> > So I want to be able to represent those functional dependencies between devices
> > and I'd like the driver core to track them and act on them in certain cases
> > where they matter.  The argument for doing that in the driver core is that
> > there are quite a few distinct use cases related to that, they are relatively
> > hard to get right in a driver (if one wants to address all of them properly)
> > and it only gets worse if multiplied by the number of drivers potentially
> > needing to do it.  Morever, at least one case (asynchronous system suspend/resume)
> > cannot be handled in a single driver at all, because it requires the driver of A
> > to wait for B to suspend (during system suspend) and the driver of B to wait for
> > A to resume (during system resume).
> >
> > My idea is to represent a supplier-consumer dependency between devices (or
> > more precisely between device+driver combos) as a "link" object containing
> > pointers to the devices in question, a list node for each of them and some
> > additional information related to the management of those objects, ie.
> > something like:
> >
> > struct device_link {
> >         struct device *supplier;
> >         struct list_head supplier_node;
> >         struct device *consumer;
> >         struct list_head consumer_node;
> >         <flags, status etc>
> > };
> >
> > In general, there will be two lists of those things per device, one list
> > of links to consumers and one list of links to suppliers.
> >
> > In that picture, links will be created by calling, say:
> >
> > int device_add_link(struct device *me, struct device *my_supplier, unsigned int flags);
> >
> > and they will be deleted by the driver core when not needed any more.  The
> > creation of a link should also cause dpm_list and the list used during shutdown
> > to be reordered if needed.
> >
> > In principle, it seems usefult to consider two types of links, one created
> > at device registration time (when registering the second device from the linked
> > pair, whichever it is) and one created at probe time (of the consumer device).
> > I'll refer to them as "permanent" and "probe-time" links, respectively.
> >
> > The permanent links (created at device registration time) will stay around
> > until one of the linked devices is unregistered (at which time the driver
> > core will drop the link along with the device going away).  The probe-time
> > ones will be dropped (automatically) at the consumer device driver unbind time.
> >
> > There's a question about what if the supplier device is being unbound before
> > the consumer one (for example, as a result of a hotplug event).  My current
> > view on that is that the consumer needs to be force-unbound in that case too,
> > but I guess I may be persuaded otherwise given sufficiently convincing
> > arguments.  Anyway, there are reasons to do that, like for example it may
> > help with the synchronization.  Namely, if there's a rule that suppliers
> > cannot be unbound before any consumers linked to them, than the list of links
> > to suppliers for a consumer can only change at its registration/probe or
> > unbind/remove times (which simplifies things quite a bit).
> >
> > With that, the permanent links existing at the probe time for a consumer
> > device can be used to check whether or not to defer the probing of it
> > even before executing its probe callback.  In turn, system suspend
> > synchronization should be a matter of calling device_pm_wait_for_dev()
> > for all consumers of a supplier device, in analogy with dpm_wait_for_children(),
> > and so on.
> >
> > Of course, the new lists have to be stable during those operations and ensuring
> > that is going to be somewhat tricky (AFAICS right now at least), but apart from
> > that the whole concept looks reasonably straightforward to me.
> >
> > So, the question to everybody is whether or not this sounds reasonable or there
> > are concerns about it and if so what they are.  At this point I mostly need to
> > know if I'm not overlooking anything fundamental at the general level.
> 
> Sounds really great to me at the conceptual level, but wonder if you
> have already thought of how the permanent links will be inferred.

In ACPI there is a mechanism for that already.  In DT it would require walking
the phandle dependency graph I suppose.

The point is, though, that it doesn't have to be mandatory to have any
permanent links created.  If you can find a dependency at device registration
time, great.  Create a permanent link for it and use it.  If you can't,
it's fine too.  You'll find it at probe time and create a link for it then.

> When I looked at computing dependencies of a device before it's
> probed, the concern was that the code that finds the dependencies
> duplicated part of the logic when looking resources up. Because each
> subsystem has its own code for looking up dependencies for potentially
> each of DT, ACPI and board files, it will be a bit of a big task to
> refactor things to avoid that duplication. Fwnode could help with
> this, but it doesn't as of yet and I'm not sure if that's still the
> plan.

That almost certainly is going to be a fair amount of work, but that
doesn't mean we should avoid doing it.  If it leads to better code
eventually, it's worth doing.

> Also wonder if you have considered setting the permanent links also
> during probe, as the on-demand probe series did (device_add_link would
> be "sprinkled" around as of_device_probe was). That would avoid the
> problem with code duplication because the links would be established
> from the functions that do resource lookup.

I have considered that, but at this point I have some concerns about
lifecycle management related to that.

In any case, if the given link is really permanent, there should be enough
information available to find it at device registration time, although that
may require some additional computations to be carried out.

Thanks,
Rafael