Re: [Linux-fbdev-devel] Some questions

[Antonino D. <ad...@po...>]

On Wed, 2003-03-12 at 00:29, James Simmons wrote:
> 
> > So, I think fbcon should also detect if the previous console is not
> > fbcon (ie previous con > last_fbvc || previous con < first_fbvc), and if
> > it isn't, force an fb_set_par.
> 
> This whole idea of mapping a specific VC to a specific piece of hardware 
> is just plain dumb and overly complex. The new console system will 
> allocate a set of 16 console to each independent VT.
> 

Well, you have info->currcon, fg_console etc...  No mapping is done,
fbcon will just keep track of the tty's it's currently switched to (vt.c
does it, and so does fbcon_switch), and fbcon will just look if the vc
has a valid PID (which means a process installed its own vc) and
therefore not trust it to completely preserve the graphics state.

Note that X does something like this to protect itself.  When switch
from console to X, it refreshes its own registers.

I think you may need some support for this.  Several have already
reported that switching from X to a console results in a corrupted
display which can be cleared by doing an fbset. 

The choices are:

1. refresh the registers at every switch - slowest but guarantees
hardware state is always preserved 
 
2. do not refresh unless var changed - fasted but can result in
corruption/crash if registers were changed behind the back of fbdev

3. selective refresh - do not refresh if switching between "trusted"
consoles, refresh if switching from "untrusted" consoles.

4. Find a method to detect if the registers were changed behind the back
of fbdev.

Tony