[PATCH 2/2] arm64: dts: rockchip: add eMMC's power domain support for rk3399

[Ulf Hansson]

On 1 September 2016 at 23:50, Doug Anderson <dianders at chromium.org> wrote:
> Hi,
>
> On Thu, Sep 1, 2016 at 6:45 AM, Ulf Hansson <ulf.hansson at linaro.org> wrote:
>> I was reading the discussion regarding this change and browsing the DT
>> documentation around this... Can you guys explain what really goes on
>> here, please.
>>
>> To me, it seems like you are managing one device's resources in one
>> separate genpd. One genpd per device. Is that correct?
>>
>> Perhaps each device actually has its own PM domain and thus it makes
>> sense to assign one genpd per device?
>
> I'm not as familiar with genpd as I should be, so hopefully this makes sense.
>
> ...in hardware there is a "pd_emmc" that is the power domain for just
> eMMC.  That will be referenced hooked up via device tree, like:
>
> power-domains = <&power RK3399_PD_EMMC>;

Yes, I noticed that and this is what puzzles me a bit.

>
> I believe that means that power will automatically be removed whenever
> the device is runtime suspended or suspended.

Well, it depends if the genpd has a subdomain or other devices in it
being runtime resumed.
The genpd will not power off unless all devices within it are runtime
enabled+suspended and that its subdomains are also powered off.

So, in case you only have one device and no subdomains, then your
statement is correct.

>
> If w're not supporting "autosuspend" and nobody is tweaking anything

I guess you mean runtime PM autosuspend? Then why don't you support this?

Wouldn't that allow you to avoid wasting power in runtime when the
device is idle?

> manually, then it's possible (I think) that runtime suspend happens at
> exactly the same time as suspend.  ...but my point was that it was

I am not sure I follow you here. You must not rely on that the device
always becomes runtime suspended during system suspend, as there are
no guarantees for this.

Instead that is something you need to take care of in the
subsystem/driver for the device, of course.

> cleaner to actually do it any restoring in the "runtime resume" hooks
> to match what genpd does.  This matches what you say: use runtime PM.

Yes!

Using genpd without deploying runtime PM for the devices doesn't make
much sense, at least to me.

>
> ...but it also sounds like it might not be terribly important to
> restore these values since they're a bit silly to begin with.  If
> that's true then I guess we don't need to do anything special here.
>
>
> Did that all make sense (it's entirely possible it didn't since
> somehow my brain still hasn't absorbed all runtime PM and genpd
> concepts)

No worries. I understand this might be a bit tricky, that's why I also
tries to help review related changes.

Kind regards
Uffe