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

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Message-ID: <CACVXFVODp5o0=+6pwG+K_LV=rtCJo-5J3Os10orwijZJq4r4Ng@mail.gmail.com>
Date:	Tue, 11 Oct 2011 18:56:03 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	"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?

On Tue, Oct 11, 2011 at 6:07 PM, Alan Cox <alan@...rguk.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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