Re: X display shift with disabled console blanking

From: James Pearson
Date: Mon Apr 30 2007 - 08:59:22 EST

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 ...

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?

And this does not happen if there is no previous setterm -blank 0

It doesn't happen if there is no previous 'setterm -blank 0' - so, arguably, this is the 'fix' ...

James Pearson
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