Re: [RFC/RFT][PATCH v2 2/7] driver core: Functional dependencies tracking support

[Ulf Hansson]

+ Mark

On 8 September 2016 at 23:27, Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
>
> 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 hasnât 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