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

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Message-ID: <CANejiEXACbyvq=_ChAsb7D3ZoBQtqs6uS-s7NPU26e3X-tyPLA@mail.gmail.com>
Date:	Tue, 11 Oct 2011 13:32:08 +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 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?
depends on your device. Say cpu takes 1s to snoop dma in idle state, so device
dma will cost more than 1s. if your device can work with such latency, no
problem then.
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