Re: [PATCH 2/2] arm64: dts: mt8173: add timer node

[Sudeep Holla]

On 17/09/15 15:56, Yingjoe Chen wrote:
> On Thu, 2015-09-17 at 14:51 +0100, Sudeep Holla wrote:
> > On 16/09/15 03:04, Yingjoe Chen wrote:
> > > From: Daniel Kurtz <djkurtz@xxxxxxxxxxxx>
> > >
> > > Add device node to enable GPT timer. This timer will be
> > > used as sched clock source.
> >
> > Interesting any known issues with or advantage over the arch timers
> > to prefer it as sched clock source. I see even arch timers are present
> > in DT, hence the question. Or is it just a incorrect commit log ?
> >
> > How does this get selected as sched clock source ? I don't see
> > sched_clock_register in mtk_timer.c
> >
> > To be clear, I am not against adding this timer support, but just want
> > to know is it preferred for sched clock source ? if yes why ? better
> > resolution ?
>
> Hi Sudeep,
>
> Thanks for your review.
>
> I hit the send too soon and missed cover letter, please see:
> http://lists.infradead.org/pipermail/linux-mediatek/2015-September/002303.html

OK

> The main reason to use GPT as sched clock is it won't stop during idle.

I think your are confusing the system counter with arch timers. System
counter is always-on, but the arch timers(logic implementing timers
comparators) might not be off when the processor is powered down.

I think you need this timer and are using it for low power idle states
in which case you will use this as a clock event and not clock source.
It will be used as a hardware broadcast event source.

There's no call to sched_clock_register in mtk_timer.c, so it can't be
the sched clock, so you need to fix the commit log.

[...]

> > > diff --git a/arch/arm64/boot/dts/mediatek/mt8173.dtsi b/arch/arm64/boot/dts/mediatek/mt8173.dtsi
> > > index d18ee42..d763803 100644
> > > --- a/arch/arm64/boot/dts/mediatek/mt8173.dtsi
> > > +++ b/arch/arm64/boot/dts/mediatek/mt8173.dtsi
> > > @@ -238,6 +238,15 @@
> > >    			reg = <0 0x10007000 0 0x100>;
> > >    		};
> > >
> > > +		timer: timer@10008000 {
> > > +			compatible = "mediatek,mt8173-timer",
> >
> > Missing documentation ? I am referring upstream and it might be in some
> > patches already queued perhaps ?
>
> This is documented in
> Documentation/devicetree/bindings/timer/mediatek,mtk-timer.txt.
> Do you mean I should add "mediatek,mt8173-timer" to that file?

Yes

Regards,
Sudeep
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