linux-kernel - Re: [PATCH 2/2] ARM: vf610: Use ARM Global Timer as clocksource

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Date:	Thu, 25 Sep 2014 10:25:13 +0200
From:	Stefan Agner <stefan@...er.ch>
To:	Shawn Guo <shawn.guo@...escale.com>
Cc:	kernel@...gutronix.de, festevam@...il.com,
	jingchang.lu@...escale.com, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] ARM: vf610: Use ARM Global Timer as clocksource

Am 2014-09-25 09:50, schrieb Shawn Guo:
> On Thu, Sep 11, 2014 at 02:06:15PM +0200, Stefan Agner wrote:
>> Use ARM Global Timer as clocksource instead of the PIT timer. This
>> leaves the PIT timer for other users e.g. the secondary Cortex-M4
>> core. Also, the Global Timer has double the precission (running at
>> pheripheral clock compared to IPG clock) and a 64-bit incrementing
>> counter register.
> 
> I just think of one thing.  Will this change cause a problem of the low
> power idle support in case we want to power down ARM core in there?
> 

I'm not sure what really happend to the Global Timer when we power down
the ARM core. We use a clocksoure of different power domain now, so it
might make a difference in low power modes, but I think it will improve
things: The PIT timer's clock currently have been clock gated even in
STOP mode, which does not power down the ARM core. And it would be shut
down completely in LP-Mode 1-3 which since PIT is part of the big power
domain 1. 

But AFAIK, its not required that the clocksource is running while in low
power modes. The time should just not jump, and if the timers registers
are lost during suspend, a proper suspend/resume support need to be
implemented. 


> 
>>
>> Signed-off-by: Stefan Agner <stefan@...er.ch>
>> ---
>> Theoretically we could remove the PIT driver now. But we could also
>> enable both drivers, but is having two clock source useful for the
>> Kernel at all?
>>
>>  arch/arm/mach-imx/Kconfig | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm/mach-imx/Kconfig b/arch/arm/mach-imx/Kconfig
>> index 64161aa..adc77180 100644
>> --- a/arch/arm/mach-imx/Kconfig
>> +++ b/arch/arm/mach-imx/Kconfig
>> @@ -656,7 +656,8 @@ config SOC_VF610
>>  	bool "Vybrid Family VF610 support"
>>  	select ARM_GIC
>>  	select PINCTRL_VF610
>> -	select VF_PIT_TIMER
>> +	select ARM_GLOBAL_TIMER
>> +	select CLKSRC_ARM_GLOBAL_TIMER_SCHED_CLOCK
>>  	select PL310_ERRATA_769419 if CACHE_L2X0
>>
>>  	help
>> --
>> 2.1.0
>>
--
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