linux-kernel - Re: [PATCH] PM / i915: Skip kernel VT switch during suspend/resume if KMS is used

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Message-Id: <201001261947.46015.rjw@sisk.pl>
Date:	Tue, 26 Jan 2010 19:47:45 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Pavel Machek <pavel@....cz>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Len Brown <lenb@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	dri-devel@...ts.sourceforge.net, Eric Anholt <eric@...olt.net>,
	airlied@...ux.ie
Subject: Re: [PATCH] PM / i915: Skip kernel VT switch during suspend/resume if KMS is used

On Tuesday 26 January 2010, Pavel Machek wrote:
> On Mon 2010-01-25 22:54:37, Rafael J. Wysocki wrote:
> > On Monday 25 January 2010, Alan Cox wrote:
> > > > > But in that case we should be able to disable the VT switch disable
> > > > > path; we just have to check each driver as it's loaded.
> > > > 
> > > > OK, what the right sequence of checks would be in that case and where to place
> > > > them?
> > > 
> > > Why are we even driving a vt switch direct from the suspend/resume
> > > logic ? The problem starts there. If it was being handled off the device
> > > suspend/resume method then there wouldn't be a mess to start with ?
> > > 
> > > Start at the beginning
> > > 
> > > - Why do we switch to arbitarily chosen 'last vt'
> > > - Why isn't vt related suspend/resume handled by the device
> > 
> > Well, that was added long ago as a workaround for some problems people
> > reported (presumably).  I've never looked at that before, so I can't really
> > tell why someone did it this particular way.
> 
> As X drives hardware, it is/was neccessary to get control out of X and
> console switch was convenient.
> 
> Note that it needs to happen with userland still active -- before
> freezer.

Well, that's a bit cumbersome.

> And yes, it should be per-driver these days.

That would have to be done using suspend notifiers and should depend on what
driver actually controls the screen at the moment.  And I guess the only case
in which we actually _need_ to do the kernel VT switch is when the hardware
is controlled by X and without KMS.

Is there a simple way to determine if that's the case?

Rafael
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