Date:   Wed, 3 May 2017 09:12:34 +0100
From:   Jon Hunter <jonathanh@...dia.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
CC:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        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 02/05/17 22:51, Rafael J. Wysocki wrote:
> 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@...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.
> 
> 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.

Yes, absolutely.

>> - 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.

By a 'list', do you mean the pm_domain member of 'struct device' should
become a list instead? That would be a bigger change and I would need to
see how that would work.

>> 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
Jon

-- 
nvpublic

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