Re: [PATCH v3 1/3] base: power: Add generic OF-based power domain look-up

[Tomasz Figa <tomasz.figa@gmail.com>]

On 06.05.2014 18:26, Stephen Warren wrote:
> On 05/06/2014 02:22 AM, Ulf Hansson wrote:
>> On 28 April 2014 19:35, Stephen Warren <swarren@wwwdotorg.org> wrote:
>>> On 04/23/2014 10:46 AM, Tomasz Figa wrote:
>>>> This patch introduces generic code to perform power domain look-up using
>>>> device tree and automatically bind devices to their power domains.
>>>> Generic device tree binding is introduced to specify power domains of
>>>> devices in their device tree nodes.
>>>>
>>>> Backwards compatibility with legacy Samsung-specific power domain
>>>> bindings is provided, but for now the new code is not compiled when
>>>> CONFIG_ARCH_EXYNOS is selected to avoid collision with legacy code. This
>>>> will change as soon as Exynos power domain code gets converted to use
>>>> the generic framework in further patch.
>>>
>>>> diff --git a/Documentation/devicetree/bindings/power/power_domain.txt b/Documentation/devicetree/bindings/power/power_domain.txt
>>>
>>>> +==Power domain consumers==
>>>> +
>>>> +Required properties:
>>>> + - power-domain : A phandle and power domain specifier as defined by bindings
>>>> +                  of power controller specified by phandle.
>>>
>>> It seems quite likely that a single logical device could have components
>>> in multiple power domains. Consider an HDMI controller with different
>>> power domains for the HDMI core, CEC communication, DDC/I2C
>>> communication, and the I/O pads, with no clear separation between those
>>> two components of the module (no separate register spaces, but the
>>> bits/registers are interleaved all together).
>>>
>>> As such, I think that rather than a "power-domain" property, we need a
>>> pair of "power-domains", and "power-domain-names" properties, and
>>> preferably with mandatory usage of name-based lookups, rather than
>>> allowing a random mix of name-based and index-based lookups like we have
>>> with some existing resource bindings.
>>
>> Each struct device have only one dev_pm_domain pointer, thus a device
>> are not able to reside in more than one power domain.
>>
>> Therefore I doubt this will be very useful, unless I missed your point. :-)
>
> A struct device is a Linux-internal concept. DT is supposed to represent
> the HW in an OS-agnostic fashion, not according to the limitations of
> any one OS's driver model.
>
> It's certainly true that a single logical HW module (represented by a
> single DT node) can have parts in multiple power domains.
>

AFAIK a single DT node is supposed to represent a single _physical_ HW 
module. It all depends on description granularity, though. A physical 
chip (or IP block) might be further divided into multiple blocks, so I 
can imagine some of them being in different power domains than another. 
Still, based on the fact that in Linux we never needed to specify 
multiple power domains for a device, is this really a real use case?

Anyway, the binding is defined in a way that lets you simply turn the 
single phandle+specifier into a list of them, if such need ever shows up.

Best regards,
Tomasz