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

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Message-ID: <20111010172557.332bfe30@lxorguk.ukuu.org.uk>
Date:	Mon, 10 Oct 2011 17:25:57 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Ming Lei <tom.leiming@...il.com>
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 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
is not the same thing as IRQ latency.
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