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

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  Re: [PATCH 2/2] arm64: dts: mt8173: add timer node Yingjoe Chen <yingjoe.chen@mediatek.com> - 2015-10-01 16:40 +0200
    Re: [PATCH 2/2] arm64: dts: mt8173: add timer node Sudeep Holla <sudeep.holla@arm.com> - 2015-10-01 17:40 +0200
      Re: [PATCH 2/2] arm64: dts: mt8173: add timer node Yingjoe Chen <yingjoe.chen@mediatek.com> - 2015-10-02 16:10 +0200

#1237456 — Re: [PATCH 2/2] arm64: dts: mt8173: add timer node

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


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#1237520

On 01/10/15 15:33, Yingjoe Chen wrote:
> On Thu, 2015-09-17 at 17:13 +0100, Sudeep Holla wrote:
>>

[...]

>>
>> 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
>

The commit message makes no sense to me. The counters should continue to
work as long as they are in always-on domain. Only timers are lost
when you enter deeper idle states. So I agree with using MTK timer as
broadcast timer/eventsource. You still didn't answer what's the need
to use MTK timer as sched clocksource ?

Regards,
Sudeep
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#1238310

On Thu, 2015-10-01 at 16:32 +0100, Sudeep Holla wrote:
> 
> On 01/10/15 15:33, Yingjoe Chen wrote:
> > On Thu, 2015-09-17 at 17:13 +0100, Sudeep Holla wrote:
> >>
> 
> [...]
> 
> >>
> >> 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
> >
> 
> The commit message makes no sense to me. The counters should continue to
> work as long as they are in always-on domain. Only timers are lost
> when you enter deeper idle states. So I agree with using MTK timer as
> broadcast timer/eventsource. You still didn't answer what's the need
> to use MTK timer as sched clocksource ?


Hi, Sudeep,

ARM ARM said the counter should be in always-on domain, but
unfortunately that not true for mt8173. The last CPU enter idle can
choose to enter deep idle mode and the counter value would be lost. Our
firmware backup/recover the counter so it looks like it is stopped.
That's why I thought we need to use it as sched clocksource.

On mt8173, we will fix the firmware to add missing counts, so it will
looks like the counter keep counting. But other mediatek platform have
similar issue, and the 2 counter have same resolution, so I still want
to keep using GPT as sched clocksource.

Joe.C


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