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

From: [email-masked] (Stephen Warren)
Date: Tue, 06 May 2014 11:24:52 -0600

On 05/06/2014 11:15 AM, Tomasz Figa wrote:
> 
> 
> 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 <[email-masked]> 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.

Physical-vs-logical probably means different things to different people.
What I mean is that DT should represent the HW modules at the
granularity that the HW documentation or designers describe the HW.

> 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?

a) Linux has absolutely nothing to do with this. Again, DT is purely a
HW description that should not be influenced by OS limitations.

b) Linux does have to deal with this. HW exists which uses multiple
power domains per DT device node.

The fact that Linux forces the driver internals to create a separate
struct device for each part of the actual device that's in a different
power domain should not force us into representing the HW incorrectly in DT.

> 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.

In my opinion, for all new resource bindings, we need to *exclusively*
support named-based lookup. That's the only way to ensure a simple
forward-compatible path to extensible DT bindings in the face of
multiple instances of a certain type of resource, where some are
optional. If resource lookups start out by index, and then later
(missing information when the DT binding was written, extending the DT
binding to a new HW revision/model) we find we need to add more
instances of that resource, some of which are optional, then anything
but purely name-based lookup makes the situation very complex.

We should solve this by simply using named-based lookup from the start
in all cases. That's my point here. Hence, we need to addm and enforce
usage of, a power-domain-names property.