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Date: Thu, 29 Oct 2015 15:03:17 +0100 From: Tomeu Vizoso <tomeu.vizoso@...labora.com> To: "Rafael J. Wysocki" <rjw@...ysocki.net> Cc: Linux PM list <linux-pm@...r.kernel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Alan Stern <stern@...land.harvard.edu>, Grant Likely <grant.likely@...aro.org>, Mark Brown <broonie@...nel.og>, Rob Herring <robh@...nel.org>, Thierry Reding <treding@...dia.com>, Dmitry Torokhov <dtor@...gle.com>, Geert Uytterhoeven <geert@...ux-m68k.org>, Michael Turquette <mturquette@...libre.com> Subject: Re: [RFD] Functional dependencies between devices On 28 October 2015 at 16:54, Rafael J. Wysocki <rjw@...ysocki.net> wrote: > On Wednesday, October 28, 2015 03:26:14 PM Tomeu Vizoso wrote: >> On 28 October 2015 at 03:15, Rafael J. Wysocki <rjw@...ysocki.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@...ysocki.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. There are initcalls that assume that a given resource is available. Because of async probes, or because the resource's driver being built as a module, or because the resource's driver gained a dependency (direct or not), those initcalls break unexpectedly at times. If resource getters could probe dependencies on-demand, those initcalls would be more robust to changes in other parts of the codebase. AFAIUI, your proposal would help with a device's dependencies being there when it's probed, but initcalls could still run into unfulfilled dependencies. I would personally prefer if those initcalls were turned into drivers because that's the component model we have in the kernel, but that doesn't seem to be a popular idea. Regards, Tomeu > Thanks, > Rafael > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@...r.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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