linux-kernel - Re: [Question] PM-QoS: PM_QOS_CPU_DMA

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Date:	Tue, 11 Oct 2011 11:40:21 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Shaohua Li <shli@...nel.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	mark gross <mgross@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux PM List <linux-pm@...r.kernel.org>
Subject: Re: [Question] PM-QoS: PM_QOS_CPU_DMA_LATENCY == interrupt latency?

Hi,

On Tue, Oct 11, 2011 at 10:26 AM, Shaohua Li <shli@...nel.org> wrote:
> 2011/10/11 Ming Lei <tom.leiming@...il.com>:
>> Hi,
>>
>> On Tue, Oct 11, 2011 at 9:49 AM, Shaohua Li <shli@...nel.org> wrote:
>>
>>> As Alan explained, PM_QOS_CPU_DMA_LATENCY is for dma snooping. For example,
>>> in x86, cpu snoop dma. when cpu is in idle state, cpu need snoop
>>> device dma activity, there
>>> is latency involved for idle state.
>>>
>>
>> I see, thanks for your clarification.
>>
>> I also have two further questions about it:
>>
>> - Except for dma snooping purpose, are there any other cases in which
>> PM_QOS_CPU_DMA_LATENCY is required?
> it's the main motivation, IIRC, don't know other platforms

If so, maybe all device drivers which support DMA transfer should
have used PM_QOS_CPU_DMA_LATENCY, but why only few drivers
did it?

>
>> - Are all CPUs required to be involved to dma snoop? Or only one CPU
>> is enough? If one is enough, maybe we can allow other CPUs to reach
>> deeper idle state.
> then how can you make cache coherency between the cpus?
>

Seems ARM supports cache maintenance operations from software[1],
but I don't know how to do it on x86, :-)


[1], http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dai0228a/index.html


thanks,
-- 
Ming Lei
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