[RFC][PATCH 0/5] Functional dependencies between devices

[Rafael J. Wysocki]

On Tuesday, October 27, 2015 04:24:14 PM Rafael J. Wysocki 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.
> 

What follows is my prototype implementation of this.  It took some time
to develop (much more than I was hoping for), but here it goes at last.

The first patch rearranges the code around __device_release_driver() a bit
to prepare it for the next one.

The second patch introduces the actual device links mechanics, but without
system suspend/resume and runtime PM support which are added by the subsequent
patches.

This hasn't been really tested yet (apart from checking that it doesn't break
things when device links are not in used, which would be rather embarrassing),
but at this time I'd really like you to have a look and tell me what you think
(especially if you see a reason why this is not going to work).

Thanks,
Rafael