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

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Date:   Wed, 23 May 2018 10:07:43 +0100
From:   Jon Hunter <jonathanh@...dia.com>
To:     Ulf Hansson <ulf.hansson@...aro.org>,
        Rajendra Nayak <rnayak@...eaurora.org>
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 23/05/18 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?

Cheers
Jon

-- 
nvpublic