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

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Date:   Thu, 6 Oct 2016 14:22:12 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Jon Hunter <jonathanh@...dia.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Kevin Hilman <khilman@...nel.org>,
        "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>
Subject: Re: [RFC PATCH 0/3] PM / Domains: Add support for devices that
 require multiple domains

On 20 September 2016 at 12:28, Jon Hunter <jonathanh@...dia.com> wrote:
> The Tegra124/210 XUSB subsystem (that consists of both host and device
> controllers) 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>;
>         };
>
> 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.

First, I don't really like extending the internal logic of genpd to
deal with multiple PM domains per device. *If* this really is needed,
I think we should try to extend the struct device to cover this, then
make genpd to use it somehow.

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?

[...]

Kind regards
Uffe