Re: [Ksummit-discuss] [TECH TOPIC] PM dependencies

[Daniel Vetter <daniel.vetter@ffwll.ch>]

On Tue, May 13, 2014 at 12:07 AM, Mark Brown <broonie@kernel.org> wrote:
> On Mon, May 12, 2014 at 11:16:57PM +0200, Tomasz Figa wrote:
>> On 12.05.2014 22:31, Mark Brown wrote:
>> > It also solves the system suspend dependencies.  Why don't the
>> > runtime PM dependencies just work with reference counting?
>
>> Runtime PM dependencies work with reference counting just fine, but
>> only for topologies matching Linux driver model, e.g. devices with
>> exactly one device they depend on, e.g. SPI controller and SPI devices
>> on the bus driven by it. Add there an IOMMU and other various strange
>> things that should be transparent to the drivers and it stops working.
>
> There's no reason why runtime PM references have to follow the topology
> - you do get a default reference count up to any parent (though we break
> that sometimes, as is the case with SPI controllers being suspended even
> though the devices below them are active) but there's nothing stopping
> references being taken outside the topology.

I guess some helpers to grab/drop runtime PM references on all parts
of a componentized device should resolve this? We propably don't want
to bake this into the driver core since often the driver will know
that it e.g. doesn't need a specific encoder block (if it's not
enabled), so doesn't to grab a reference. But for simple drivers, and
to get runtime PM off the ground when enabling new hardware this could
be rather useful.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch