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

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Message-Id: <201001060003.23419.rjw@sisk.pl>
Date:	Wed, 6 Jan 2010 00:03:23 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Bartłomiej Zimoń <uzi18@...pl>
Cc:	linux-kernel@...r.kernel.org,
	Anders Eriksson <aeriksson@...tmail.fm>,
	linux-pm@...ts.linux-foundation.org, awalls@...ix.net,
	danborkmann@...glemail.com
Subject: Re: [linux-pm] [suspend/resume] Re: userspace notification from module

On Tuesday 05 January 2010, Bartłomiej Zimoń wrote:
> Dnia 5 stycznia 2010 22:23 	"Rafael J. Wysocki" <rjw@...k.pl> napisał(a):
> > On Tuesday 05 January 2010, Bartłomiej Zimoń wrote:
> > > Dnia 5 stycznia 2010 10:07 	Anders Eriksson <aeriksson@...tmail.fm> napisał(a):
> > > > 
> > > > rjw@...k.pl said:
> > > > >> > I don't see a problem with this in principle, although I don't think signals
> > > > >> > are very suitable for this particular purpose, because you need two-way
> > > > >> > communication between the power manager and the processes it's going to
> > > > >> > notify (because it has to wait for the processes to finish their preparations
> > > > >> > and to tell it that they are ready). 
> > > > 
> > > > Wouldn't there need to be dependecy tracking for the userspace processes? A 
> > > > process couldn't signal "done" until it know there's no more work to do, which 
> > > > requires all other processes to finish up first.
> > > > 
> > > 
> > > Not all processes will need pm notification.
> > > 
> > > Just in case, could this "power manager" be provided as userspace
> > > driver for kernel (for example compiled as UIO)?
> > 
> > I don't really understand why you want to do that in the kernel.  Is there
> > any particular reason why this cannot be done in the user space?
> > 
> 
> Problem is with notification from pm-utils. Dbus for session is only 
> user session waid and it is almost imposible to send something 
> from systembus or it needs lots of magic.

Don't use dbus, then.

> Even if i'll use pm-utils, must provide way to notify my app.
> Looks like no standard way here, that's bad, every app has it's own?

Isn't there a possibility to introduce a standard way of doing that without
involving the kernel directly?

> I think this way - if kernel is processes manager and if it sends
> "some unknown" signal to every process before kernel suspend so why
> not send sepcific and known signal instread.

We _can_ do that, but what would a process be supposed to do after receiving
such a signal?

> But now we have DeviceKit-power/upower whatever but it doesn't provide
> this feature or something simmilar to relay on.

You're still not saying what you really need.  So, what's the application you
have in mind that needs a notification from the _kernel_ so badly?

Rafael
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