linux-kernel - Re: [PATCH]PM QOS refresh against next-20100430

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Message-ID: <20100503104250.7605d2bc@bike.lwn.net>
Date:	Mon, 3 May 2010 10:42:50 -0600
From:	Jonathan Corbet <corbet@....net>
To:	Kevin Hilman <khilman@...prootsystems.com>
Cc:	mgross@...ux.intel.com, aili@...eaurora.org,
	dwalker@...eaurora.org, tiwai@...e.de, bruce.w.allan@...el.com,
	davidb@...cinc.com, mcgrof@...il.com, pavel@....cz,
	linux-pm <linux-pm@...ts.linux-foundation.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH]PM QOS refresh against next-20100430

On Mon, 03 May 2010 09:40:11 -0700
Kevin Hilman <khilman@...prootsystems.com> wrote:

> > One question, though...  one clear use of this API is for drivers to
> > say "don't go into C3 or deeper because things go wrong"; I'm about to
> > add another one of those.  It works, but the use of a
> > PM_QOS_CPU_DMA_LATENCY requirement with a hard-coded number that one
> > hopes is small enough seems a bit...indirect.  I wonder if it would be
> > clearer and more robust to add a new requirement^Wrequest type saying
> > "the quality of service I need is shallow sleeps only"?  
> 
> The problem with that is portability.
> 
> What does "shallow" mean?  

Well, shallow could mean that the state lacks the CPUIDLE_FLAG_DEEP
flag; that should be relatively portable.  In any case, it seems more
so than "if I put in a 55us latency requirement, I'll stay out of C3".

Just a thought, anyway; it's not like I've really worked through a
plausible alternative API.

jon
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