Date:   Wed, 23 Nov 2016 09:29:31 +0000
From:   Jon Hunter <jonathanh@...dia.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Kevin Hilman <khilman@...libre.com>
CC:     Ulf Hansson <ulf.hansson@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        "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>,
        Rajendra Nayak <rnayak@...eaurora.org>
Subject: Re: [RFC PATCH 0/3] PM / Domains: Add support for devices that
 require multiple domains


On 22/11/16 21:55, Rafael J. Wysocki wrote:
> On Tue, Nov 22, 2016 at 7:26 PM, Kevin Hilman <khilman@...libre.com> wrote:
>> Jon Hunter <jonathanh@...dia.com> writes:
>>
>>> On 16/11/16 12:53, Rafael J. Wysocki wrote:
>>>> On Wed, Nov 16, 2016 at 11:48 AM, Jon Hunter <jonathanh@...dia.com> wrote:
>>>>> Hi Kevin, Ulf,
>>>>>
>>>>> On 03/11/16 14:20, Jon Hunter wrote:
>>>>>>
>>>>>> On 11/10/16 10:15, Jon Hunter wrote:
>>>>>>
>>>>>> ...
>>>>>>
>>>>>>>>>> Second, another way of seeing this is: Depending on the current
>>>>>>>>>> runtime selected configuration you need to re-configure the PM domain
>>>>>>>>>> topology - but the device would still remain in the same PM domain.
>>>>>>>>>>
>>>>>>>>>> In other words, you would need to remove/add subdomain(s) depending on
>>>>>>>>>> the selected configuration. Would that better reflect the HW?
>>>>>>>>>
>>>>>>>>> I am not 100% sure I follow what you are saying, but ultimately, I would
>>>>>>>>> like to get to ...
>>>>>>>>>
>>>>>>>>>         usb@...90000 {
>>>>>>>>>                 compatible = "nvidia,tegra210-xusb";
>>>>>>>>>                 ...
>>>>>>>>>                 power-domains = <&pd_xusbhost>, <&pd_xusbss>;
>>>>>>>>>         };
>>>>>>>>
>>>>>>>> So, is this really is a proper description of the HW? Isn't it so,
>>>>>>>> that the usb device always resides in one and the same PM domain?
>>>>>>>
>>>>>>> I guess technically, the usbhost controller resides in one partition and
>>>>>>> the super-speed logic in another. So could the usbhost domain be the
>>>>>>> primary? Possibly, but the device cannot be probed without both enabled.
>>>>>>>
>>>>>>>> Now, depending on the selected speed mode (superspeed) additional
>>>>>>>> logic may needs to be powered on and configured for the usb device to
>>>>>>>> work?
>>>>>>>> Perhaps, one could consider those additional logics as a master/parent
>>>>>>>> PM domain for the usb device's PM domain?
>>>>>>>>
>>>>>>>> Or this is not how the HW works? :-)
>>>>>>>
>>>>>>> It might be possible for this case, but to be honest, the more I think
>>>>>>> about this, I do wonder if we need to be able to make the framework a
>>>>>>> lot more flexible for devices that need multiple power-domains. In other
>>>>>>> words, for devices that use multiple domains allow them to control them
>>>>>>> similarly to what we do for regulators or clocks. So if there is more
>>>>>>> than one defined, then the genpd core will not bind the device to the
>>>>>>> pm-domain and let the driver handle it. This way if you do need more
>>>>>>> granular control of the pm-domains in the driver you can do whatever you
>>>>>>> need to.
>>>>>>>
>>>>>>> I know that Rajendra (CC'ed) was looking into whether he had a need to
>>>>>>> control multiple power-domains individually from within the context of a
>>>>>>> single device driver.
>>>>>>
>>>>>> So Rajendra commented to say that he does not see a need for individual
>>>>>> control of power-domains for now, but a need for specifying multiple.
>>>>>>
>>>>>> One simple option would be to allow users to specify multiple and have
>>>>>> the genpd core effectively ignore such devices and leave it to the
>>>>>> driver to configure manually. I have been able to do this for XUSB by
>>>>>> dynamically adding power-domains to the device.
>>>>>>
>>>>>> Let me know if you have any more thoughts on how we can do this.
>>>>>
>>>>> Any more thoughts on this? Seems that there are a few others that would
>>>>> be interested in supporting multiple domains for a device.
>>>>
>>>> There is a design limitation to that, however.
>>>>
>>>> The PM domain concept really is about intercepting the flow of PM
>>>> callbacks for a device in order to carry out additional operations,
>>>> not covered by the bus type or driver.  That's why there is only one
>>>> set of PM domain callbacks per device and I don't quite see how and
>>>> why it would be useful to add more of them in there.
>>
>> @Rafael: Re: why it would be useful...
>>
>> Many ARM SoCs have devices that have independent power rails for the
>> memory and the logic of an IP block.  For example, while powering off
>> the logic you could keep the memory at a retention voltage, so you'd
>> want to treat those power domains separately.
>>
>> Today, in order to model this, you'd have to create another (dummy)
>> device, just for the memory and put it in its own domain so the two
>> could be controlled separately.
> 
> Perhaps if you want to use genpd for that. :-)
> 
> Let me rephrase, though.  I don't see why and how it would be useful
> to intercept the flow of PM callbacks for a given device more than
> once.

In this RFC, all I was proposing is that we create a dummy pm-domain
that is a child of the actual pm-domains it uses and this new dummy
pm-domain is associated with the device. Hence, you are still only
intercepting the flow of PM callback once even with this approach. I am
just using the parent-child relationship to ensure that all require
pm-domains are turned on thats all. Sorry if I am still missing your point!

Cheers
Jon

-- 
nvpublic

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