linux-kernel - Re: [suspend/resume] Re: userspace notification from module

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Date:	Mon, 04 Jan 2010 00:35:31 +0100
From:	Bartłomiej Zimoń <uzi18@...pl>
To:	linux-kernel@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, awalls@...ix.net,
	danborkmann@...glemail.com, linux-pm@...ts.linux-foundation.org,
	stern@...land.harvard.edu, rjw@...k.pl
Subject: Re: [suspend/resume] Re: userspace notification from module

Dnia 4 stycznia 2010 0:30 "Rafael J. Wysocki" <rjw@...k.pl> napisał(a):

> On Sunday 03 January 2010, Bartłomiej Zimoń wrote:
> > Dnia 3 stycznia 2010 22:29 "Rafael J. Wysocki" <rjw@...k.pl> napisał(a):
> ...
> > 
> > It could be even UIO module but there are no pm events reachable there?
> > 
> > If it is not clean, we must extend pm-utils or write something new
> > (with backends dbus, ipc, scripts, ...)
> > But You see? We still have no information from kernel about events 
> > (especialy resume) or maybe i dont see this ;/
> 
> They are available to the process that tells the kernel to suspend.
> 
> Namely, to tell the kernel to suspend to RAM, the process (call it a power
> manager) needs to (for example) fprintf() "mem" to /sys/power/state.  As soon
> as this happens, the kernel will start to freeze processes (except for the
> power manager itself), so the power manager knows that everything should be
> ready for the suspend before it writes to /sys/power/state.  It doesn't need
> the kernel to tell it when the suspend is going to start, because it _knows_
> that in advance.
> 
> Now, the fprintf() used to trigger the suspend will not return until the resume
> is complete.  So, again, when the fprintf() returns, the power manager will know
> that the resume has just finished (more precisely, the kernel side of it has
> just finished).
> 
> There simply is no need for any special communication between the kernel and
> the power manager.
> 

Thx Rafael - now it clear to me.
And what do You think about sending extra signals to processes?

Best regards.
Bartłomiej Zimoń
PLD Linux, Kadu Team, FreeRunner user
http://kadu-im.blogspot.com/

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