+ Mark On 8 September 2016 at 23:27, Rafael J. Wysocki <r...@rjwysocki.net> wrote: > From: Rafael J. Wysocki <rafael.j.wyso...@intel.com> > > Currently, 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). > > Add support for representing those functional dependencies between > devices to allow 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). > > To that end, represent links between devices (or more precisely > between device+driver combos) as a struct device_link object > containing pointers to the devices in question, a list node for > each of them, status information, flags, a lock and an RCU head > for synchronization. > > Also add two new list heads, links_to_consumers and links_to_suppliers, > to struct device to represent the lists of links to the devices that > depend on the given one (consumers) and to the devices depended on > by it (suppliers), respectively. > > The entire data structure consisting of all of the lists of link > objects for all devices is protected by SRCU (for list walking) > and a by mutex (for link object addition/removal). In addition > to that, each link object has an internal status field whose > value reflects what's happening to the devices pointed to by > the link. That status field is protected by an internal spinlock. > > New links are added by calling device_link_add() which may happen > either before the consumer device is probed or when probing it, in > which case the caller must ensure that the driver of the supplier > device is present and functional and the DEVICE_LINK_SUPPLIER_READY > flag must be passed to device_link_add() to reflect that. > > Link objects are deleted either explicitly, by calling > device_link_del() on the link object in question, or automatically, > when the consumer device is unbound from its driver or when one > of the target devices is deleted, depending on the link type. > > There are two types of link objects, persistent and non-persistent. > The persistent ones stay around until one of the target devices is > deleted, while the non-persistent ones are deleted when the consumer > driver is unbound from its device (ie. they are assumed to be valid > only as long as the consumer device has a driver bound to it). The > DEVICE_LINK_PERSISTENT flag is passed to device_link_add() to create > a persistent link and it cannot be used for links created at the > consumer probe time (that is, persistent links must be created before > probing the consumer devices). > > One of the actions carried out by device_link_add() is to reorder > the lists used for device shutdown and system suspend/resume to > put the consumer device along with all of its children and all of > its consumers (and so on, recursively) to the ends of those list > in order to ensure the right ordering between the all of the supplier > and consumer devices.
Rafael, thanks for working on this and re-spinning this series. It's indeed very interesting! I am hoping "device links" should be able to solve some of those device ordering issues I have observed for several SoCs, particularly during system PM and in combination with runtime PM. I intend to test the series as soon as I can and try to deploy it to see if it solves some of the issues I have seen. I will also try to review in more detail. No promises short term though. :-) BTW, as I am mostly working on DT based platforms, I guess we would later on need to discuss with the DT maintainers how to describe device links. A minor comment to the change-log. I would appreciate some information about "error" handling. Especially, what happens in the driver core when it's about to probe a device with a configured device link, but the link hasn’t been established yet (the other device isn't successfully probed). In the ideal scenario this shouldn't happen, but of course it will. So I assume the driver core relies on the existing deferred probe mechanism for this. [...] Kind regards Uffe
