Re: [PATCH] ARM: gic: Document Power and Clock Domain optional properties

[Geert Uytterhoeven]

Hi Mark,

On Mon, Apr 27, 2015 at 7:15 PM, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> On Mon, Apr 27, 2015 at 05:43:13PM +0100, Geert Uytterhoeven wrote:
>> On Mon, Apr 27, 2015 at 5:54 PM, Mark Rutland <mark.rutland@xxxxxxx> wrote:
>> > On Mon, Apr 27, 2015 at 04:00:11PM +0100, Geert Uytterhoeven wrote:
>> >> On some SoCs, the GIC may be part of a PM Domain (hardware Power and/or
>> >> Clock Domain).  Document the related optional DT properties.
>> >>
>> >> Note: As the current GIC driver doesn't support Runtime PM yet, PM
>> >> Domain constraints must be handled elsewhere in e.g. platform code.
>> >
>> > I'm worried that without a current user these properties aren't going to
>> > see any testing...
>>
>> Still, DT is supposed to describe the hardware. Even if the driver doesn't
>> use the description.
>
> I appreciate that...
>
>> >> Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
>> >> ---
>> >> To preserve DT stability, we would like to add these properties to the
>> >> affected shmobile dtsi files.
>> >
>> > ... which means that they could be wrong, and will get in the way of
>> > stability rather than aiding it.
>>
>> We do know the GIC is part of the power domain, and has a controllable
>> clock (on the affected SoCs).
>
> ... my concern is that the data we place into the DT will be untested
> given that we don't have software relying on it. If said data is not
> correct, it is harmful to have, especially for such fundamental
> properties.

Your statement challenges the viability of Stable DT Requirements, as we
can thus never write a DTS until the full software implementation has been
completed ;-)

We do know switching off that clock disables the GIC, as per documentation
(and experimentation: lock up), and have used the same strategy with
other Renesas interrupt controllers (renesas,intc-irqpin and renesas,irqc).

>> > I'm also concerned that the carving up of clock inputs, power domains,
>> > and other physical details is implementation-specific. I imagine that
>> > pretty much every user that will care about this is using GIC-400, so
>> > could we make this specific to GIC-400?
>>
>> I have no idea which GIC version is being used.
>
> This is unfortunate.
>
>> This is for Renesas R-Mobile APE6 (r8a73a4) and various R-Car Gen2.
>> According to the DTS, they're compatible with "arm,cortex-a15-gic" and
>> "arm,cortex-a7-gic", and work with that value.
>
> Who put the DT together in the first place?

Magnus (added to CC).

> If it's a multi-cluster SoC then we know that we're not using any
> built-in distributor...

R-Mobile APE6 has 2 clusters (4xCA15 and 4xCA7).
R-Car Gen2 has 1 or 2 clusters (2/4xCA15 and/or 2/4xCA7).

>> >> On Thu, Mar 26, 2015 at 11:39 AM, Marc Zyngier <marc.zyngier@xxxxxxx> wrote:
>> >> > On 25/03/15 21:19, Geert Uytterhoeven wrote:
>> >> >> I would like to add the clock and GIC dependency on the clock in the DTS now,
>> >> >> for reasons of DTS stability. But that means I need a temporary workaround
>> >> >> to avoid the clock from being disabled, until the GIC driver handles this.
>> >> >>
>> >> >> I don't expect a fix for the GIC code to just show up magically. I just wanted
>> >> >> you to be aware of the problem. GIC is not the only problematic module here,
>> >> >> there are others, cfr. the last slide of [2].
>> >> >
>> >> > As long as there is an agreement from the DT people on the presence of
>> >> > that extra property in the GIC node, I'm happy with that. I'd like it to
>> >> > be documented though.
>> >>
>> >> Full thread at
>> >> http://lists.infradead.org/pipermail/linux-arm-kernel/2015-March/331876.html
>> >>
>> >>  Documentation/devicetree/bindings/arm/gic.txt | 8 ++++++++
>> >>  1 file changed, 8 insertions(+)
>> >>
>> >> diff --git a/Documentation/devicetree/bindings/arm/gic.txt b/Documentation/devicetree/bindings/arm/gic.txt
>> >> index 2da059a4790cb3c6..b21113b35f085f27 100644
>> >> --- a/Documentation/devicetree/bindings/arm/gic.txt
>> >> +++ b/Documentation/devicetree/bindings/arm/gic.txt
>> >> @@ -58,6 +58,14 @@ Optional
>> >>    regions, used when the GIC doesn't have banked registers. The offset is
>> >>    cpu-offset * cpu-nr.
>> >>
>> >> +- power-domains : A phandle and PM domain specifier as defined by bindings of
>> >> +               the power controller specified by phandle, used when the GIC
>> >> +               is part of a Power or Clock Domain.
>> >
>> > I imagine that it's possible for the distributor and CPU interfaces to
>> > be in separate power domains in some implementaitons.
>>
>> Possibly (then we're gonna need subnodes, each with their own
>> power-domains properties?).
>
> Potentially for such implementations, yes. To the best of my knowledge a
> GIC-400 is a single block, which is one reason why it would be nice to
> focus on that particular implementation if that's applicable.
>
>> > Which components of the GIC does the apply to? The whole thing? Just the
>> > GICD?
>>
>> Unfortunately that's not documented...
>
> That worries me somewhat, given my comments above, though I'd very much
> suspect this is a GIC-400.
>
>> >> +- clocks        : A phandle and clock specifier as defined by bindings of
>> >> +               the clock controller specified by phandle, used when the GIC
>> >> +               is part of a Clock Domain.
>> >
>> > Depending on implementation, a GIC could require multiple clocks, and
>> > their names would be implementation-specific (that said, GIC-400 has a
>> > single "CLK" input).
>>
>> Documentation/devicetree/bindings/arm/gic.txt doesn't mention any clocks
>> for GIC-400?
>
> I'm going by the TRM. There are other items not in the bindign that are
> implementation-specific.
>
>> > Assuming that you're using GIC-400, could we use clock-names to make
>> > that explicit?
>>
>> You can. But you have to be careful to avoid conflicts, as the controller
>> for the clock domain needs to know which clock to use.
>
> Could you elborate on that? I don't see why a node's clock-names
> property should be relevant to the clock controller.

If there are multiple clocks listed, the clock controller needs to know which
clocks is to be used for power management.

If we standardize on a name for the clock to be used for power management
(a platform property), that could conflict with the naming in the binding
for the device.
Some device bindings don't mandate clock-names as only a single clock
is used.
Other device bindings do mandate clock-names. You mentioned "CLK" for
GIC-400, others use e.g. "fck", or "ick".

>> On Renesas SoCs, we always used the first clock for power control.
>> For the future, we plan to use "fck" if it exists.
>
> That's irrelevant from the PoV of the GIC; per the GIC architecture you
> can't assume anything about the set of input clocks, and GIC-400 happens
> to have a single clock input called "CLK". It's not IP specific to
> Renesas.
>
>> Yes, this can be complicated, as ideally it needs synchronization between
>> device DT bindings and platform (clock/power domain) DT bindings.
>>
>> Please also note that any device (hardware IP core) can be reused on an
>> SoC that supports clock and/or power domains, and suddenly starts to rely
>> on power-domains and clocks properties.
>
> Sorry, I don't follow this last part.

Devices may describe zero, one, or more clocks in the DT bindings. Usually
these clocks are functional, and they are described because the driver needs
to control them, and/or needs knowledge about them.

As all (current) logic is synchronous, there's always a clock that drives the
logic of the device. Gating this clock can be used for power management.
This clock may or may not be the one described in the bindings (obviously
it isn't if the binding doesn't describe any clock, but there can be other
cases).

Any IP core for a device may be reused. On some SoCs, the logic may be
driven by a fixed, non-gateable clock. On others the logic may be driven by
a gateable clock to save power. Whether or not there is a gateable clock
is a property of the platform (SoC), not of the IP core.

Please see slides 37-40 of my presentation "Last one out, turn off the lights"
at ELC2015:
http://events.linuxfoundation.org/sites/events/files/slides/elc2015_uytterhoeven.pdf
They show examples of devices with zero, one or more described clocks.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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
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