Date:   Tue, 2 May 2017 11:10:29 +0100
From:   Jon Hunter <jonathanh@...dia.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>
CC:     "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Kevin Hilman <khilman@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        "Rajendra Nayak" <rnayak@...eaurora.org>,
        Stanimir Varbanov <stanimir.varbanov@...aro.org>,
        Stephen Boyd <sboyd@...eaurora.org>,
        "Marek Szyprowski" <m.szyprowski@...sung.com>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [RFC PATCH 0/4] PM / Domains: Add support for explicit control of
 PM domains


On 25/04/17 22:17, Rafael J. Wysocki wrote:
> On Tue, Apr 25, 2017 at 9:34 PM, Ulf Hansson <ulf.hansson@...aro.org> wrote:
>> On 25 April 2017 at 13:13, Jon Hunter <jonathanh@...dia.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@...90000 {
>>>>               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. 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)
- 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).

Is this along the right lines?

Cheers
Jon

-- 
nvpublic

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