[Ksummit-discuss] [TECH TOPIC] PM dependencies

[Geert Uytterhoeven]

On Mon, May 19, 2014 at 3:00 AM, Shuah Khan <shuahkhan at gmail.com> wrote:
> On Thu, May 15, 2014 at 5:43 PM, Laurent Pinchart
> <laurent.pinchart at ideasonboard.com> wrote:
>> On Tuesday 13 May 2014 08:26:09 Shuah Khan wrote:
>>>
>>> Can we use pm domains concept to solve the problem? As it exists today, it
>>> probably can't support loosely associated devices, however, could it be
>>> extended to support handling device groups that don't necessarily share
>>> power source and/or clock.
>>
>> If I understand them properly, power domains model groups of devices that
>> share common power handling. I'm not sure how they could be used to model PM
>> dependencies between devices. Feel free to prove me wrong though :-)
>>
>>> I started looking at this as a way to solve media device PM issues,
>>> however I haven't had a chance to experiment with this idea.
>
> Right now, PM domains currently support grouping of devices that share
> power source. What if we extended PM domains to support a concept of

Physical power domain...

> logical grouping of devices that have PM dependencies with a way to
> specify dependency ordering?
>
> For example, a media usb device example:
> The main driver could create a logical power domain and rest of the
> drivers add their devices to the same domain specifying their
> dependencies. logical power domain pm handlers can handle ordering. In

... and logical power domain...

> some cases, there is a need to save and restore state of tuner i2c and
> the existing save_state and restore_state could be used to within the
> scope of this logical domain.
>
> I think PM domains can definitely be used for media device case, but I
> am not sure about the specific case you are looking at where there is
> a clearly identified master driver/device.

Sure, it can be done.

> These are at the moment ideas and I need to do some work to test these
> theories. :)

.. and we also have clock power domains (cfr. pm_clk_notify()).

Now we do have hardware that would benefit from all three of the above.
AFAIK a device can be a member of only one power domain, through
the struct device.pm_domain pointer. Can/should this be extended,
cfr. the RPM_GET_CALLBACK() stack for Runtime PM (but we do want
to call into more than one of them).

I'm still too pm_domain-illiterate to say much more about this though...

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds