Re: X display shift with disabled console blanking

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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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