linux-kernel - Re: [RFC PATCH 0/3] PM / Domains: Add support for devices that require multiple domains

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Message-ID: <CAJZ5v0gUO8TqxbyLh37mfvoTLY1y1Uj91i9T6zcx5A9SmmcReA@mail.gmail.com>
Date:   Tue, 22 Nov 2016 22:55:33 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Kevin Hilman <khilman@...libre.com>
Cc:     Jon Hunter <jonathanh@...dia.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        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 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.

Thanks,
Rafael