linux-kernel - Re: Multiple consoles

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Message-ID: <20120109222234.6d34a7ad@pyramind.ukuu.org.uk>
Date:	Mon, 9 Jan 2012 22:22:34 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Phillip Susi <psusi@....rr.com>
Cc:	Greg KH <gregkh@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: Multiple consoles

> It occurred to me that there ought to be an entirely separate set of 
> virtual consoles bound to the second seat, and the second X server ought 
> to run on one of those vcs.  Or of course, you could choose to log into 
> the console and not bother with X.

X can do it without consoles.

> Looking at drivers/tty/vt/vt.c, it appears that it was written assuming 
> that there is just one linux console.  It appears to use global 
> variables for keeping track of which vc is active, etc, rather than 
> creating one or more console devices, and store the vc multiplexing 
> information in those devices.  So to fix this, vt.c and keyboard.c would 
> need significantly refactored to remove the global variables and create 
> a console device to bind vcs, keyboards, and displays to, and then you 
> could create a second one if you wanted.

Yes - in theory (you'd also have to sort out some of the passing of
variables around so that the code knew which 'console group' it belonged
to.

> Does this make sense or am I missing something?

It makes sense but it's one of a set of three problems with the tty/vt
layer IMHO

- A vt/tty is linked to a framebuffer of some sort, so you can't just
  create abstract ones plumbed into arbitary objects. Really the vt
  emulator ought to be separate from the consoles. 

- You can only have one set of vts

- The locking needs a rework, particularly the handling of crashes where
  sometimes we deadlock on the oops reporting.

and the framebuffer layer has some problems with locking and mode setting,
with hotplug, and with a complete inability to handle scatter-gather
backed consoles.

Alan
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