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SubjectRe: [Question] PM-QoS: PM_QOS_CPU_DMA_LATENCY == interrupt latency?

On Tue, Oct 11, 2011 at 10:26 AM, Shaohua Li <> wrote:
> 2011/10/11 Ming Lei <>:
>> Hi,
>> On Tue, Oct 11, 2011 at 9:49 AM, Shaohua Li <> 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, :-)


Ming Lei

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