linux-kernel - Re: [RFC PATCH 0/4] PM / Domains: Add support for explicit control of PM domains

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Date:   Tue, 25 Apr 2017 12:13:27 +0100
From:   Jon Hunter <jonathanh@...dia.com>
To:     "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Kevin Hilman <khilman@...nel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>, <geert@...ux-m68k.org>,
        <rnayak@...eaurora.org>
CC:     <stanimir.varbanov@...aro.org>, <sboyd@...eaurora.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        <linux-pm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-tegra@...r.kernel.org>
Subject: Re: [RFC PATCH 0/4] PM / Domains: Add support for explicit control of
 PM domains


On 28/03/17 15:13, Jon Hunter wrote:
> The current generic PM domain framework (GenDP) only allows a single
> PM domain to be associated with a given device. There are several
> use-cases for various system-on-chip devices where it is necessary for
> a PM domain consumer to control more than one PM domain where the PM
> domains:
> i).  Do not conform to a parent-child relationship so are not nested
> ii). May not be powered on and off at the same time so need independent
>      control.
> 
> The solution proposed in this RFC is to allow consumers to explictly
> control PM domains, by getting a handle to a PM domain and explicitly
> making calls to power on and off the PM domain. Note that referencing
> counting is used to ensure that a PM domain shared between consumers
> is not powered off incorrectly.
> 
> The Tegra124/210 XUSB subsystem (that consists of both host and device
> controllers) is an example of a consumer that needs to control more than
> one PM domain because the logic is partitioned across 3 PM domains which
> are:
> - XUSBA: Superspeed logic (for USB 3.0)
> - XUSBB: Device controller
> - XUSBC: Host controller
> 
> These power domains are not nested and can be powered-up and down
> independently of one another. In practice different scenarios require
> different combinations of the power domains, for example:
> - Superspeed host: XUSBA and XUSBC
> - Superspeed device: XUSBA and XUSBB
> 
> Although it could be possible to logically nest both the XUSBB and XUSBC
> domains under the XUSBA, superspeed may not always be used/required and
> so this would keep it on unnecessarily.
> 
> Given that Tegra uses device-tree for describing the hardware, it would
> be ideal that the device-tree 'power-domains' property for generic PM
> domains could be extended to allow more than one PM domain to be
> specified. For example, define the following the Tegra210 xHCI device ...
> 
> 	usb@...90000 {
> 		compatible = "nvidia,tegra210-xusb";
> 		...
> 		power-domains = <&pd_xusbhost>, <&pd_xusbss>;
> 		power-domain-names = "host", "superspeed";
> 	};
> 
> This RFC extends the generic PM domain framework to allow a device to
> define more than one PM domain in the device-tree 'power-domains'
> property. If there is more than one then the assumption is that these
> PM domains will be controlled explicitly by the consumer and the device
> will not be automatically bound to any PM domain.

Any more comments/inputs on this? I can address Rajendra's feedback, but
before I did I wanted to see if this is along the right lines or not?

Cheers
Jon

-- 
nvpublic