Re: [RESEND PATCH v16 4/4] ARM: dts: add the support power-domain node on RK3288 SoCs

[Doug Anderson]

Kevin,

On Tue, Aug 25, 2015 at 2:07 PM, Kevin Hilman <khilman@xxxxxxxxxx> wrote:
> Caesar Wang <wxt@xxxxxxxxxxxxxx> writes:
>
>> We can add more domains node in the future.
>> This patch add the needed clocks into power-controller.
>> As the discuess about all the device clocks being listed in
>> the power-domains itself.
>>
>> There are several reasons as follows:
>>
>> Firstly, the clocks need be turned off to save power when
>> the system enter the suspend state. So we need to enumerate
>> the clocks in the dts. In order to power domain can turn on and off.
>
> Yes, but this is the job of device drivers which are runtime PM adapted
> to gate their own clocks.  I agree these clocks need to be enumerated in
> the DTS, but they should be in the device nodes.

I _think_ what Caesar means is that the alternative to this patch is
to leave the clocks on all the time as they were during the early days
of Rockchip (AKA last year).  If the clocks are on all the time then
the power domain patches can work fine.  However, once you start
letting clocks turn off then you need to make sure that the power
domain code turns the back on temporarily.


>> Secondly, the reset-circuit should reset be synchronous on RK3288,
>> then sync revoked. So we need to enable clocks of all devices.
>> In other words, we have to enable the clocks before you operate them
>> if all the device clocks are included in someone domians.
>
> Yes, this is pretty common for reset.
>
>> Someone wish was to get the clocks by reading the clocks from the
>> device nodes, We can do that but we can solve the above issues.
>
> I don't follow this sentence.  Are you saying doing that will not solve
> the above issues?  Why not?  Please explain.
>
> If there are non-device clocks that also need to be enabled before
> asserting reset, then those are candidates for the power-domain node,
> but not device clocks.

It's been a long time and I don't know that I've reviewed every
revision of this series, but I think there was a proposal that we
shouldn't list clocks here.  Instead we should search through and find
all devices that refer to this power domain, reach in and find their
clocks, and turn them on.  Did I get that right?  To put things in a
concrete way, for pd_vio we'd go through the entire device tree
ourselves and find all properties that look like "power-domains =
<&power RK3288_PD_VIO>;".  We'd then find the parent of those
properties and look for a property named "clocks".  We'd then iterate
over all those clocks and turn those on.  Did I get that right?

The above doesn't seem like a terribly great idea to me for a number
of reasons, including:

1. If I remember correctly, it's important to turn on clocks for
devices even if they're not something you're using / have a driver
for.  If you don't then the device won't get reset properly and this
can affect things like suspend/resume because the hardware in the SoC
will query all devices at suspend time to make sure they're ready.  If
a device is wedged because its clock wasn't on at the right them then
it will cause problems.

2. If we absolutely need to turn all clocks and we get clocks from
device tree nodes on then it means we need device tree nodes for every
device in the domain.  These would be needed even if there are no
accepted bindings for this device yet.  So we'd need to do one of: A)
Block power domain patches on feature complete bindings for all
drivers; B) Make up non-approved compatible strings for all devices
and throw them into the DTS; C) Add nodes in the DTS without a
compatible string just to satisfy the power domain requirements.  None
of these seem terribly appealing.

3. It is entirely possible that there are clocks that will be listed
in the individual devices that aren't needed for powering on the power
domain.  I'd tend to believe that PCLK_EDP_CTRL (the pixel clock)
doesn't really need to be turned on when adjusting the "VIO" power
domain.  Right now Caesar has it listed, but it probably isn't needed
(Caesar: can you confirm?).

4. It seems just slightly brittle to be reaching into other device
nodes and making assumptions about their properties.  Yeah, it's
probably safe to assume that "clocks" has a list of clocks and
"power-domains" will point to something whose first entry is a
phandle, but it still seems just a tad bit like violating an
abstraction barrier.


Anyway, perhaps I'm misunderstanding, or perhaps my concerns are
simply not for important things.  If so feel free to yell at me.  ;)


>> Anyway, the best ideas we can fix it in the future SoCs.
>
> I don't think this is an SoC design issue as this is needed when you
> have synchronous reset.  My concern is primarily around how to describe
> this in the DT.

I suppose the SoC could override things and make sure clocks are on in
this case?  ...or if a clock is off it could defer powering it up
somehow until the clock came on?  ...dunno how this actually looks in
hardware.  In any case letting devices get wedged doesn't seem
ideal...

-Doug
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