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

[Ulf Hansson]

On 23 May 2018 at 11:45, Jon Hunter <jonathanh@xxxxxxxxxx> wrote:
>
> On 23/05/18 10:33, Ulf Hansson wrote:
>>
>> On 23 May 2018 at 11:27, Rajendra Nayak <rnayak@xxxxxxxxxxxxxx> 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!
>
>
> Yes this is technically correct, however, in reality I think we are always
> going to enable the superspeed domain if either the host or device domain is
> enabled. So we would probably always link the superspeed with the host and
> device devices.

Why? Wouldn't that waste power if the superspeed mode isn't used?

Kind regards
Uffe