Date:   Mon, 14 Nov 2016 16:48:05 +0100
From:   Lukas Wunner <lukas@...ner.de>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM list <linux-pm@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Mark Brown <broonie@...nel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Kevin Hilman <khilman@...nel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Grant Likely <grant.likely@...retlab.ca>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Andrzej Hajda <a.hajda@...sung.com>
Subject: Re: [PATCH v5 2/5] driver core: Functional dependencies tracking
 support

On Mon, Nov 14, 2016 at 02:48:32PM +0100, Luis R. Rodriguez wrote:
> 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.

It's a typo, I meant to say "provably correct". Sorry.

Quite a difference in meaning. :-)


> > 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 ?

The device links turn the device tree into a directed acyclic graph.
For the dpm_list and devices_kset, that graph is flattened into a
one-dimensional form such that all ancestors and suppliers of a
device appear in front of that device in the lists.  I'm not a
graph theorist and can't provide a formal proof.  I think Rafael
is a Dr., maybe he can do it. :-)  I merely looked at this from a
practical point of view, i.e. I tried to come up with corner cases
where dependencies are added that would result in incorrect ordering,
and concluded that I couldn't find any.

Thanks,

Lukas

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