Re: [RFC PATCH 0/4] PM / Domains: Add support for explicit control of PM domains

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

On Tuesday, May 02, 2017 11:10:29 AM Jon Hunter wrote:
> 
> On 25/04/17 22:17, Rafael J. Wysocki wrote:
> > On Tue, Apr 25, 2017 at 9:34 PM, Ulf Hansson <ulf.hansson@linaro.org> wrote:
> >> On 25 April 2017 at 13:13, Jon Hunter <jonathanh@nvidia.com> wrote:
> >>>
> >>> On 28/03/17 15:13, Jon Hunter wrote:
> >>>> The current generic PM domain framework (GenDP) only allows a single
> >>>> PM domain to be associated with a given device. There are several
> >>>> use-cases for various system-on-chip devices where it is necessary for
> >>>> a PM domain consumer to control more than one PM domain where the PM
> >>>> domains:
> >>>> i).  Do not conform to a parent-child relationship so are not nested
> >>>> ii). May not be powered on and off at the same time so need independent
> >>>>      control.
> >>>>
> >>>> The solution proposed in this RFC is to allow consumers to explictly
> >>>> control PM domains, by getting a handle to a PM domain and explicitly
> >>>> making calls to power on and off the PM domain. Note that referencing
> >>>> counting is used to ensure that a PM domain shared between consumers
> >>>> is not powered off incorrectly.
> >>>>
> >>>> The Tegra124/210 XUSB subsystem (that consists of both host and device
> >>>> controllers) is an example of a consumer that needs to control more than
> >>>> one PM domain because the logic is partitioned across 3 PM domains which
> >>>> are:
> >>>> - XUSBA: Superspeed logic (for USB 3.0)
> >>>> - XUSBB: Device controller
> >>>> - XUSBC: Host controller
> >>>>
> >>>> These power domains are not nested and can be powered-up and down
> >>>> independently of one another. In practice different scenarios require
> >>>> different combinations of the power domains, for example:
> >>>> - Superspeed host: XUSBA and XUSBC
> >>>> - Superspeed device: XUSBA and XUSBB
> >>>>
> >>>> Although it could be possible to logically nest both the XUSBB and XUSBC
> >>>> domains under the XUSBA, superspeed may not always be used/required and
> >>>> so this would keep it on unnecessarily.
> >>>>
> >>>> Given that Tegra uses device-tree for describing the hardware, it would
> >>>> be ideal that the device-tree 'power-domains' property for generic PM
> >>>> domains could be extended to allow more than one PM domain to be
> >>>> specified. For example, define the following the Tegra210 xHCI device ...
> >>>>
> >>>>       usb@70090000 {
> >>>>               compatible = "nvidia,tegra210-xusb";
> >>>>               ...
> >>>>               power-domains = <&pd_xusbhost>, <&pd_xusbss>;
> >>>>               power-domain-names = "host", "superspeed";
> >>>>       };
> >>>>
> >>>> This RFC extends the generic PM domain framework to allow a device to
> >>>> define more than one PM domain in the device-tree 'power-domains'
> >>>> property. If there is more than one then the assumption is that these
> >>>> PM domains will be controlled explicitly by the consumer and the device
> >>>> will not be automatically bound to any PM domain.
> >>>
> >>> Any more comments/inputs on this? I can address Rajendra's feedback, but
> >>> before I did I wanted to see if this is along the right lines or not?
> >>
> >> I discussed this with Rafael at the OSPM summit in Pisa a couple of
> >> weeks ago. Apologize for the delay in providing additional feedback.
> >>
> >> First, whether the problem is really rare, perhaps adding a new
> >> API/framework can't be justified - then it may be better to add some
> >> kind of aggregation layer on top of the current PM domain
> >> infrastructure (something along the first attempt you made for genpd).
> >> That was kind of Rafael's thoughts (Rafael, please correct me if I am
> >> wrong).
> > 
> > We were talking about the original idea behind the pm_domain pointer
> > concept, which was about adding a set of PM operations above the bus
> > type/class layer, which could be used for intercepting bus-type PM
> > operations and providing some common handling above them.  This is
> > still relevant IMO.
> > 
> > The basic observation here is that the PM core takes only one set of
> > PM operation per device into account and therefore, in every stage of
> > system suspend, for example, the callback invoked by it has to take
> > care of all actions that need to be carried out for the given device,
> > possibly by invoking callbacks from other code layers.  That
> > limitation cannot be removed easily, because it is built into the PM
> > core design quite fundamentally.
> > 
> > However, this series seems to be about controlling power resources
> > represented by power domain objects rather than about PM operations.
> > In ACPI there is a power resource concept which seems to be quite
> > similar to this, so it is not entirely new. :-)
> > 
> > Of course, question is whether or not to extend genpd this way and I'm
> > not really sure.  I actually probably wouldn't do that, because
> > poweron/poweroff operations used by genpd can be implemeted in terms
> > of lower-level power resource control and I don't see the reason for
> > mixing the two in one framework.
> 
> That seems fine to me. However, it seems that genpd itself should also
> be a client of this 'low-level power resource control' so that
> power-domains are registered once and can be used by either method.

Right.

> So unless I am misunderstanding you here, it seems that what we need to do
> is split the current genpd framework into a couple layers:
> 
> 1. Low-level power resource control which has:
> - Power resource registration (ie. pm_genpd_init/remove())
> - Power resource provider registration (ie. of_genpd_add_xxx())
> - Power resource control (on/off etc)

And reference counting.

> - Power resource lookup (what this series is adding)
> 
> 2. Generic power-domain infrastructure which is a client of the
> low-level power resource control that can automatically bind a device to
> a singular power resource entity (ie. power-domain).

Something like that, but I would not require an additional complex framework
to be present below genpd.  I would make it *possible* for genpd to use that
framework if available, but if something simpler is sufficient, it should be
fine to use that instead.

That is, I would allow genpd to use either a list of power resources or the on/off
callbacks provided by itself to cover different use cases.  That should be
flexible enough.

> Is this along the right lines?

It is, at least for the very narrow definition of "right" as being done along
the lines I would do that. :-)

Thanks,
Rafael