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

[Yingjoe Chen <yingjoe.chen@mediatek.com>]

On Thu, 2015-09-17 at 17:13 +0100, Sudeep Holla wrote:
> 
> 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@chromium.org>
> >>>
> >>> 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.

Hi Sudeep,

Sorry for late reply.

For sched_clock_register, please see
http://lists.infradead.org/pipermail/linux-mediatek/2015-July/001547.html
which was accepted in
https://git.linaro.org/people/daniel.lezcano/linux.git/shortlog/refs/heads/clockevents/4.4


You are right it is also used as clock event. I think we don't need to
mention those detail in commit message, so I'll change to just:

"Add device node to enable GPT timer."

> 
> [...]
> 
> >>> 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

Will do in next round.
Thanks

Joe.C