Re: [PATCH v5 2/5] driver core: Functional dependencies tracking support

[Luis R. Rodriguez]

On Sun, Nov 13, 2016 at 06:34:13PM +0100, Lukas Wunner wrote:
> On Mon, Nov 07, 2016 at 10:39:54PM +0100, Luis R. Rodriguez wrote:
> > On Mon, Oct 10, 2016 at 02:51:04PM +0200, Rafael J. Wysocki wrote:
> > > 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 lists
> > > in order to ensure the right ordering between all of the supplier
> > > and consumer devices.
> > 
> > There's no explanation as to why this order is ensured to be
> > correct, I think its important to document this. From our discussions
> > at Plumbers it seems the order is ensured due to the fact that order
> > was already implicitly provided through platform firmware (ACPI
> > enumeration is one), adjusting order on the dpm list is just shuffling
> > order between consumer / provider, but nothing else.
> 
> ACPI specifies a hierarchy and the order on the dpm_list and
> devices_kset is such that children are behind their parent.
> 
> A device link specifies a dependency that exists in addition
> to the hierarchy, hence consumers need to be moved behind
> their supplier.  And not only the consumers themselves but
> also recursively their children and consumers. Essentially
> the entire subtree is moved to the back.  That happens in
> device_reorder_to_tail() in patch 2.

Ah neat, I failed to notice this full subtree tree move, its
rather important.

> If another device is enumerated which acts as a supplier to
> an existing other supplier, that other supplier and all its
> dependents are moved behind the newly enumerated device,
> and so on.
> 
> That is probably correct so long as no loops are introduced
> in the dependency graph.

"Probably" is what concerns me, there is no formality about
the correctness of this.

> That is checked by device_is_dependent(),
> which is called from device_link_add(), and the addition of the
> link is aborted if a loop is detected.

And that is sufficient ?

 Luis