linux-kernel - Re: [PATCH 1/7] ARM: imx: add timer stop flag to ARM power off state

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Date:   Tue, 09 Jan 2018 23:21:45 +0100
From:   Stefan Agner <stefan@...er.ch>
To:     Lucas Stach <l.stach@...gutronix.de>
Cc:     Anson Huang <anson.huang@....com>,
        Dong Aisheng <dongas86@...il.com>, mark.rutland@....com,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        robh+dt@...nel.org, dl-linux-imx <linux-imx@....com>,
        kernel@...gutronix.de, Fabio Estevam <fabio.estevam@....com>,
        shawnguo@...nel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/7] ARM: imx: add timer stop flag to ARM power off state

On 2018-01-09 15:04, Lucas Stach wrote:
> Am Dienstag, den 09.01.2018, 14:37 +0100 schrieb Stefan Agner:
>> On 2018-01-09 11:13, Lucas Stach wrote:
>> > Am Dienstag, den 09.01.2018, 09:25 +0000 schrieb Anson Huang:
>> > >
>> > > Best Regards!
>> > > Anson Huang
>> > >
>> > >
>> > > > -----Original Message-----
>> > > > From: Dong Aisheng [mailto:dongas86@...il.com]
>> > > > Sent: 2018-01-09 5:23 PM
>> > > > To: Stefan Agner <stefan@...er.ch>
>> > > > Cc: shawnguo@...nel.org; kernel@...gutronix.de; Fabio Estevam
>> > > > <fabio.estevam@....com>; robh+dt@...nel.org; mark.rutland@....c
>> > > > om;
>> > > > linux-arm-kernel@...ts.infradead.org; devicetree@...r.kernel.or
>> > > > g;
>> > > > linux-
>> > > > kernel@...r.kernel.org; Anson Huang <anson.huang@....com>; dl-
>> > > > linux-imx
>> > > > <linux-imx@....com>
>> > > > Subject: Re: [PATCH 1/7] ARM: imx: add timer stop flag to ARM
>> > > > power
>> > > > off state
>> > > >
>> > > > On Tue, Jan 02, 2018 at 05:42:17PM +0100, Stefan Agner wrote:
>> > > > > When the CPU is in ARM power off state the ARM architected
>> > > > > timers
>> > > > > are
>> > > > > stopped. The flag is already present in the higher power WAIT
>> > > > > mode.
>> > > > >
>> > > > > This allows to use the ARM generic timer on i.MX 6UL/6ULL
>> > > > > SoC.
>> > > > > Without the flag the kernel freezes when the timer enters the
>> > > > > first
>> > > > > time ARM power off mode.
>> > > > >
>> > > > > Cc: Anson Huang <anson.huang@....com>
>> > > > > Signed-off-by: Stefan Agner <stefan@...er.ch>
>> > > >
>> > > > It seems ok at my side.
>> > > > Did you meet the real issue? If yes, how to reproduce?
>> > > >
>> > > > Both mx6sx and mx6ul are using GPT which do not need that flag,
>> > > > suppose we
>> > > > should remove it, right?
>> > > > Anson can help confirm it.
>> > >
>> > > For UP system like i.MX6SX, we do NOT enable "cortex-a9-twd-
>> > > timer",
>> > > so local
>> > > timer is NOT used, GPT is used instead, GPT's clock is NOT
>> > > disabled
>> > > when cpuidle,
>> > > so I think we should remove all these Timer stop flag for 6SX
>> > > CPUIDLE.
>> >
>> > It's correct to set the flag even on UP systems, as the flag means
>> > the
>> > CPU _local_ timer is stopped in this sleep mode. Also there are
>> > systems
>> > out there which are using the TWD on UP, as it operates at a higher
>> > frequency leading to better wakeup granularity.
>>
>> Documentation/devicetree/bindings/arm/twd.txt states that TWD
>> provides
>> "per-cpu local timer". But as far as I can see TWD still uses SPI
>> interrupts, routed through GIC, so is this the differentiation?
> 
> Maybe what I wrote wasn't entirely clear. I completely agree with this
> patch.
> 
> The TWD on Cortex-A9 is a CPU local timer, same as the architected
> timer in later cores. It doesn't provide all the benefits of the
> architected timer (the clock frequency varies with CPU core clock and
> it's not virt capable), but some systems still prefer it over the i.MX
> GPT, as it provides much better wakeup granularity.
> 
> So annotating the CPU idle states with the timer stop flag is the right
> thing to do. This flag has nothing to with the usage of GPT or TWD on a
> specific system.

Can I take that as an Acked-by?

--
Stefan