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

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Message-ID: <CACVXFVNAFTpZEOnH+6LxUk=dXtNKvmYVx5NtuJJ1bri4XEyt5Q@mail.gmail.com>
Date:	Tue, 11 Oct 2011 09:28:54 +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?

Hi,

On Tue, Oct 11, 2011 at 12:25 AM, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
> On Mon, 10 Oct 2011 22:31:34 +0800
> Ming Lei <tom.leiming@...il.com> wrote:
>
>> Hi,
>>
>> Looks like it is a bit difficult to understand PM_QOS_CPU_DMA_LATENCY
>> from the words' meaning.
>>
>> After searching from google, I don't find some useful information about
>> the root cause for introducing PM_QOS_CPU_DMA_LATENCY. I understand
>> it is very similar to interrupt latency. Also from the comment for
>> omap_pm_set_max_mpu_wakeup_lat in file[1], the description is basically same
>> with interrupt latency.
>>
>> >From comments of pm_qos_add_request usages in drivers, it can be understood
>> as interrupt latency too, IMO.
>>
>> So, could we think that PM_QOS_CPU_DMA_LATENCY is interrupt latency?
>
> 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?

Also could you give a example about how the CPU responds to a DMA transfer
within a set timescale if it is required?

> is not the same thing as IRQ latency.


thanks,
-- 
Ming Lei
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