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

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Message-ID: <55FAE69C.2000906@arm.com>
Date:	Thu, 17 Sep 2015 17:13:16 +0100
From:	Sudeep Holla <sudeep.holla@....com>
To:	Yingjoe Chen <yingjoe.chen@...iatek.com>
Cc:	Sudeep Holla <sudeep.holla@....com>,
	Matthias Brugger <matthias.bgg@...il.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Michael Turquette <mturquette@...libre.com>,
	James Liao <jamesjj.liao@...iatek.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	"linux-mediatek@...ts.infradead.org" 
	<linux-mediatek@...ts.infradead.org>,
	Sascha Hauer <kernel@...gutronix.de>,
	Olof Johansson <olof@...om.net>,
	"srv_heupstream@...iatek.com" <srv_heupstream@...iatek.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Daniel Kurtz <djkurtz@...omium.org>,
	"linux-clk@...r.kernel.org" <linux-clk@...r.kernel.org>,
	Eddie Huang <eddie.huang@...iatek.com>
Subject: Re: [PATCH 2/2] arm64: dts: mt8173: add timer node



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@...omium.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.

[...]

>>> 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@...08000 {
>>> +			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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