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

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Message-ID: <CACVXFVOcSOe+_XpNdKGBCMzdFcs4=ViZbf7hPLnieZUhPewzmA@mail.gmail.com>
Date:	Tue, 11 Oct 2011 10:18:19 +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 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?

- 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.


thanks,
-- 
Ming Lei
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