linux-kernel - Re: [PATCH 8/9] PM / Domains: Add support for multi PM domains per device to genpd

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Message-ID: <CAPDyKFoqLPRwEJ9b+XBpB4Zy79SGEC-a8V8shNhMkYOCQa0=oA@mail.gmail.com>
Date:   Wed, 23 May 2018 11:33:50 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Rajendra Nayak <rnayak@...eaurora.org>,
        Jon Hunter <jonathanh@...dia.com>
Cc:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Linux PM <linux-pm@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kevin Hilman <khilman@...nel.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Todor Tomov <todor.tomov@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        linux-tegra@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 8/9] PM / Domains: Add support for multi PM domains per
 device to genpd

On 23 May 2018 at 11:27, Rajendra Nayak <rnayak@...eaurora.org> wrote:
>
>
> On 05/23/2018 02:37 PM, Jon Hunter wrote:
>>
>> On 23/05/18 07:12, Ulf Hansson wrote:
>>
>> ...
>>
>>>>>>> Thanks for sending this. Believe it or not this has still been on my to-do list
>>>>>>> and so we definitely need a solution for Tegra.
>>>>>>>
>>>>>>> Looking at the above it appears that additional power-domains exposed as devices
>>>>>>> to the client device. So I assume that this means that the drivers for devices
>>>>>>> with multiple power-domains will need to call RPM APIs for each of these
>>>>>>> additional power-domains. Is that correct?
>>>>>>
>>>>>> They can, but should not!
>>>>>>
>>>>>> Instead, the driver shall use device_link_add() and device_link_del(),
>>>>>> dynamically, depending on what PM domain that their original device
>>>>>> needs for the current running use case.
>>>>>>
>>>>>> In that way, they keep existing runtime PM deployment, operating on
>>>>>> its original device.
>>>>>
>>>>> OK, sounds good. Any reason why the linking cannot be handled by the above API? Is there a use-case where you would not want it linked?
>>>>
>>>> I am guessing the linking is what would give the driver the ability to decide which subset of powerdomains it actually wants to control
>>>> at any point using runtime PM. If we have cases wherein the driver would want to turn on/off _all_ its associated powerdomains _always_
>>>> then a default linking of all would help.
>>>
>>> First, I think we need to decide on *where* the linking should be
>>> done, not at both places, as that would just mess up synchronization
>>> of who is responsible for calling the device_link_del() at detach.
>>>
>>> Second, It would in principle be fine to call device_link_add() and
>>> device_link_del() as a part of the attach/detach APIs. However, there
>>> is a downside to such solution, which would be that the driver then
>>> needs call the detach API, just to do device_link_del(). Of course
>>> then it would also needs to call the attach API later if/when needed.
>>> Doing this adds unnecessary overhead - comparing to just let the
>>> driver call device_link_add|del() when needed. On the upside, yes, it
>>> would put less burden on the drivers as it then only needs to care
>>> about using one set of functions.
>>>
>>> Which solution do you prefer?
>>
>> Any reason why we could not add a 'boolean' argument to the API to indicate whether the new device should be linked? I think that I prefer the API handles it, but I can see there could be instances where drivers may wish to handle it themselves.
>>
>> Rajendra, do you have a use-case right now where the driver would want to handle the linking?
>
> So if I understand this right, any driver which does want to control individual powerdomain state would
> need to do the linking itself right?
>
> What I am saying is, if I have device A, with powerdomains X and Y, and if I want to turn on only X,
> then I would want only X to be linked with A, and at a later point if I want both X and Y to be turned on,
> I would then go ahead and link both X and Y to A? Is that correct or did I get it all wrong?

Correct!

>
> I know atleast Camera on msm8996 would need to do this since it has 2 vfe powerdoamins, which can be
> turned on one at a time (depending on what resolution needs to be supported) or both together if we
> really need very high resolution using both vfe modules.

I think this is also the case for the Tegra XUSB subsystem.

The usb device is always attached to one PM domain, but depending on
if super-speed mode is used, another PM domain for that logic needs to
be powered on as well.

Jon, please correct me if I am wrong!

Kind regards
Uffe