Messages in this thread | | | From | "Rafael J. Wysocki" <> | Subject | Re: [RFD] Functional dependencies between devices | Date | Wed, 28 Oct 2015 16:54:04 +0100 |
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On Wednesday, October 28, 2015 03:26:14 PM Tomeu Vizoso wrote: > On 28 October 2015 at 03:15, Rafael J. Wysocki <rjw@rjwysocki.net> wrote: > > 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. > > Hi Rafael, > > could you extend on why do you prefer inferring the permanent links > before probe as opposed to collecting them during probe from the > lookup functions?
Information that is already available at the device registration time should be used at that time or it makes things harder to follow.
But that really is a tradeoff. If collecting that information requires too much effort, it may not be worth it.
> Also, have you considered that not only drivers request resources? For > example, the on-demand probing series would probe a device that is > needed by an initcall, simplifying synchronization.
You really need to explain what you mean here or maybe give an example.
Thanks, Rafael
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