linux-kernel - Re: [RFD] Automatic suspend

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Message-Id: <200903020017.52390.rjw@sisk.pl>
Date:	Mon, 2 Mar 2009 00:17:50 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Arve Hjønnevåg <arve@...roid.com>
Cc:	Pavel Machek <pavel@....cz>,
	Alan Stern <stern@...land.harvard.edu>,
	"Woodruff, Richard" <r-woodruff2@...com>,
	Arjan van de Ven <arjan@...radead.org>,
	Kyle Moffett <kyle@...fetthome.net>,
	Oliver Neukum <oliver@...kum.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Nigel Cunningham <nigel@...el.suspend2.net>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	mark gross <mgross@...ux.intel.com>,
	Uli Luckas <u.luckas@...d.de>,
	Igor Stoppa <igor.stoppa@...ia.com>,
	Brian Swetland <swetland@...gle.com>,
	Len Brown <lenb@...nel.org>
Subject: Re: [RFD] Automatic suspend

On Sunday 01 March 2009, Arve Hjønnevåg wrote:
> On Sat, Feb 28, 2009 at 2:53 PM, Rafael J. Wysocki <rjw@...k.pl> wrote:
> > On Saturday 28 February 2009, Arve Hjønnevåg wrote:
> >> Can you summarize what the problems with my current api are? I get the
> >> impression that you think the overhead of using a list is too high,
> >> and that timeout support should be removed because you think all
> >> drivers that use it are broken.
> >
> > In no particular order:
> > 1. One user space process can create an unlimited number of wakelocks.  This
> >   shouldn't be possible.  Moreover, it is not even necessary for any process
> >   to have more than one wakelock held at any time.
> 
> This has been addressed. A user space process cannot create more
> wakelocks than it has filedescriptors.
> 
> > 2. Timeouts are wrong, because they don't really _solve_ any problem.  They are
> >   useful for working around the fact that you can't or you don't want to
> >   modify every piece of code that in principle should take a wakelock and
> >   that's it.
> 
> Yes, timeouts are sometimes wrong, but they are not always wrong. I
> gave two examples where the use of timeouts was not incorrect.

There still is a problem that the same operation can take time X on one
platform and time Y on another, so how are you going to determine the timeouts
that will be suitable for all platforms?

> >  However,  entire concept of having one code path acting on
> >   behalf of another one on a hunch that it might be doing something making
> >   suspend undesirable is conceptually broken IMO.
> 
> OK. Do you have an alternative?

Well, IMO every code path doing something that makes automatic suspend
undesirable should use a suspend blocker of some sort.  I'm afraid any other
approach will be unreliable and racy.

> I my opinion this is how the entire system works if you do autosuspend
> without a mechanism like wakelocks.

It surely hasn't been designed with automatic suspend in mind.

Thanks,
Rafael
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