Re: X display shift with disabled console blanking

[James Pearson <james-p@moving-picture.com>]

Antonino A. Daplas wrote:
> On Mon, 2007-04-30 at 13:58 +0100, James Pearson wrote:
>
> > Antonino A. Daplas wrote:
> >
> > > On Fri, 2007-04-27 at 18:08 +0100, James Pearson wrote:
> > >
> > >
> > > > I have a problem whereby the X display 'shifts' to left when anything 
> > > > writes to /dev/console - where console screen blanking has been disabled 
> > > > i.e. doing something like:
> > > >
> > > > boot to run level 3
> > > >
> > > > If not root, then make sure /dev/console is writeable
> > > >
> > > > login and type:
> > > >
> > > > setterm -blank 0
> > > >
> > > > start X
> > > >
> > > > type into an xterm:
> > > >
> > > > echo "some random text" > /dev/console
> > > > (may have to repeat the echo above a few times)
> > > >
> > > > ... and the whole X display jumps (and wraps) to the left
> > > >
> > > > I'm using a RHEL4 based distro with a vanilla 2.6.21 x86_64 kernel 
> > > > (although I've seen the problem with various x86_64 and i686 2.6.X kernels).
> > > >
> > > > I've seen this problem on a number of different nVidia cards - using 
> > > > the vesa driver (same problem occurs with nVidia's binary driver). I 
> > > > haven't tried using other makes of graphics cards.
> > > >
> > > >
> > > > OK, this may be a strange combination of disabling the text console 
> > > > blanking and running X, but something isn't right somewhere ...
> > >
> > >
> > > Yep, it's strange because I can't reproduce this. And the console write
> > > should not succeed if the current console is in KD_GRAPHICS mode, which
> > > is done by X (unless your version is different).
> >
> > I've just installed a vanilla CentOS 4.4 on an i686 SMP machine - with 
> > an nVidia Quadro4 980 XGL card.
> >
> > By default, this sets up X using the 'nv' driver (using RedHat's 
> > xorg-x11-6.8.2-1.EL.13.37).
> >
> > If I follow my 'recipe' above, then the screen shifts - note: it looks 
> > like you have to write several lines of text to /dev/console (at least 
> > 30) to trigger the problem (e.g. run the echo to /dev/console in a loop) 
> > - also, I've found that switching to the console and back to X 
> > (Ctrl-Alt-F1 then Ctrl-Alt-F7) while this echo loop is running can force 
> > the shift to start ...
>
>
> I would understand that switching from text to graphics and vice versa
> can trigger display problems (it shouldn't, but it happens), but not
> while you are only echoing text to the system console in graphics mode.

It looks like switching from graphics -> text -> graphics definitely 
plays a part in the problem ... see below

> > This is with the RedHat based 2.6.9-42.ELsmp kernel - but I also get the 
> > problem with a vanilla 2.6.21 kernel.
> >
> >
> > > > Any ideas?
> > >
> > >
> > > I don't.  But, what is your current console?  Is it VGA, or framebuffer?
> > > Can you try doing this again in both VGA and vesafb?
> >
> > I'm not sure what the current console is - whatever is the default with 
> > RHEL4/CentOS4 - how do I select a different type of console?
>
>
> dmesg | grep "Console:"

Console: colour VGA+ 80x25

> > > And this does not happen if there is no previous setterm -blank 0
> > > command?
> >
> > It doesn't happen if there is no previous 'setterm -blank 0' - so, 
> > arguably, this is the 'fix' ...
>
>
> Weird. The only thing I can think of is that console blanking is being
> triggered while the console is in graphics mode, which is not legal. 
>
> How about 'setterm -blank 1', do an infinite echo loop and wait for at
> least 1 minute?

Setting the blank time to anything other than 0 is fine (no screen shift)

> Also, can you open drivers/char/vt.c and look for the function
> do_blank_screen?  You should have this particular segment.
>
> Change this
>
> if (console_blanked) {
> 	if (blank_state == blank_vesa_wait) {
> 		blank_state = blank_off;
> 		vc->vc_sw->con_blank(vc, vesa_blank_mode + 1, 0);
> 	}
> 	return;
> }
>
> to
>
> if (console_blanked && vc->vc_mode == KD_TEXT && !entering_gfx ) {
> 	if (blank_state == blank_vesa_wait) {
> 		blank_state = blank_off;
> 		vc->vc_sw->con_blank(vc, vesa_blank_mode + 1, 0);
> 	}
> 	return;
> }
>
> Let me know if that even makes a difference.

Made no difference, although I can't see how it would as console_blanked 
is 0 when the problem happens.

It does indeed seem that the switching back and forth between text and 
graphics does appear to be part of the issue - in my previous testing I 
probably did do this (but didn't include this in my recipe above) - so 
here is a new 'recipe' that shows the problem (for me)

boot to run level 3

if not root, then make sure /dev/console is writeable

login and type:

setterm -blank 0

start X

type into an xterm:

while true; do echo "" > /dev/console; usleep 100000; done

while the above loop is running switch to the text console and back 
again (Ctrl-Alt-F1 then Ctrl-Alt-F7)

... and the screen will be shifting (and wrapping) to the left.

James Pearson

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