Improving VT switching with KMS

Eric Anholt eric at
Thu Mar 4 11:35:45 PST 2010

On Wed, 24 Feb 2010 15:54:16 -0500 (EST), Ari Entlich <atrigent at> wrote:
> Hey all!
> So I'm that annoying guy who keeps asking questions on the irc channel
> about VT switching and KMS. I suppose I am now moving that discussion
> here so as to (hopefully) get more responses.
> So I made a kernel patch which allows (at least as far as the VT
> subsystem is concerned) a VT switch to happen even if the VT being
> switched away from is managed by a process. Since this new VT mode is
> basically a cross between VT_AUTO and VT_PROCESS, I just called it
> VT_PROCESS_AUTO. My impression (and the impression of a couple of
> other people who I've asked) is that this is now safe to do with KMS.

For working X Servers, we need to be able to get the master handoff
correct, so this doesn't seem safe.  But I think the real problem there
is that we need multi-master support, not serialized-master-handoff
support -- some client opening in an X Server while switched away should
get GL and be able to use it just like on an active X Server.

Beyond master setting, there's nothing left in my Enter/LeaveVT.  Hide
cursors on Leave, set video mode on Enter.  There are two other function
calls left in there, but I don't think they particularly deserve to live.

> Even if the answer to question #1 is yes, the X server still needs to
> be able to decide whether to use VT_PROCESS_AUTO, i.e. it needs to be
> able to check whether KMS is being used. Not having done any X
> hacking, this is proving to have quite a learning curve for me. Here
> are some of the preliminary assumptions I'm making and some of the
> things I've determined by grepping through the code (lots of these
> things will be obvious to most of you, just bear with me):

Is your VT switching code going to handle resetting my modes correctly?
I don't think that's the case today.
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