linux-kernel - Re: X display shift with disabled console blanking

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Message-ID: <4635E810.80809@moving-picture.com>
Date:	Mon, 30 Apr 2007 13:58:56 +0100
From:	James Pearson <james-p@...ing-picture.com>
To:	"Antonino A. Daplas" <adaplas@...il.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: X display shift with disabled console blanking

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
> command?

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

James Pearson
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