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SubjectRe: [Question] PM-QoS: PM_QOS_CPU_DMA_LATENCY == interrupt latency?
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On Tue, Oct 11, 2011 at 6:07 PM, Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:
>> > No. Well it may be on some platforms but it isn't the same thing. On some
>> > devices a DMA transfer doesn't need the CPU involved but needs the CPU to
>> > respond within a set timescale (eg for coherency or bus arbitration). It
>>
>> I understand only the CPU can respond after it is notified by a
>> interrupt event, don't I?
>
> The instruction stream being executed maybe, but not things like the cache
>
>> Also could you give a example about how the CPU responds to a DMA transfer
>> within a set timescale if it is required?
>
> The kind of thing you are dealing with is
>
>        DMA engine requests a cache line of data
>        CPU wakes out of sleep, completes bus transaction

I think the CPU should be woken up by interrupt from DMA engine, so
it is still a kind of interrupt latency?

Also looks like it is a bit odd that why CPU is involved to complete the
bus transaction which should have been done by DMA engine only.
Is there a practical example about this?

>        CPU goes back to sleep
>        DMA engine starts outputting data bits over SPI bus or similar
>
>        repeat until done
>
> so it's not instruction level stuff, merely bus traffic.
>


thanks,
--
Ming Lei
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