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

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Message-ID: <CANejiEUNAZsaAJBXrfi8+5E5GCK4KFUcOSiZarGEwqz2idT2EA@mail.gmail.com>
Date:	Tue, 11 Oct 2011 10:26:47 +0800
From:	Shaohua Li <shli@...nel.org>
To:	Ming Lei <tom.leiming@...il.com>
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?

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

> - 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?
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