DVI output for GeoForce 8400 GS to ViewSonic VX2235wm LCD monitor doesn't work

[Adam Williamson]

On Tue, 2010-01-12 at 10:27 -0500, Jonathan Kamens wrote:
> My old 32-bit system, on which I was running bleeding-edge Rawhide,
> died when folks working in my house fried my motherboard by plugging a
> sheetrock saw into my UPS (brilliant!).  I've replaced my old system
> with a home-built system with an Asus P5Q SE2 motherboard and a
> GeoForce 8400 GS graphics controller.  To allow things to stabilize
> for a while before going back to bleeding-edge, I'm now running F12
> with updates-testing rather than Rawhide.  Also, I've switched to
> 64-bit.
> 
>  
> 
> My monitor, a ViewSonic VX2235wm LCD, has both VGA and DVI inputs.
> With my old system, I was using VGA, since my graphics controller
> didn't have DVI out.  The GeoForce, however, has DVI out, so I'd like
> to start using it.  The problem is that when I try to switch from VGA
> to DVI, the monitor claims that it's getting no digital signal.  This
> could be due to any number of different problems, e.g.:
> 
>  
> 
> ·        Broken monitor
> 
> ·        Broken graphics controller
> 
> ·        Bad / incompatible DVI cable
> 
> ·        Incompatibility between the kind of DVI signal the controller
> is generating and the kind of signal the monitor accepts (I hear
> rumors that this kind of thing happens, but I know very little about
> the various video out formats and therefore have no insight into it or
> how to diagnose it)

The DVI cable / connector standard is actually capable of transmitting
analog signals, as well as digital. (DVI-A, rather than DVI-D). A
connector which can do both is referred to as DVI-I. You could,
theoretically, wind up with a situation where you're trying to connect a
video card that can only do DVI-A to a monitor that only does DVI-D, or
something. It's really rather unlikely, though. Wikipedia's page is
reasonable:

http://en.wikipedia.org/wiki/Digital_Visual_Interface

> ·        Known bug / non-support for DVI output in kernel and/or X
> driver
> 
> ·        Regression in kernel and/or X driver
> 
> ·        Me doing something stupid
> 
>  
> 
> I'm hoping there's somebody here who can help me narrow down the
> possibilities so that I know whether I should file a bug about this,
> replace some hardware, just live with it, or what.

It's very likely a driver issue, to be honest. I would usually diagnose
this just by swapping bits out - try a different cable, a different
monitor, and a different computer (and possibly a different operating
system). Of course, that gets much trickier if you don't have spares of
all the above available. :)

For a start, you could send us your /var/log/Xorg.0.log files - one from
a successful X start with an old-style VGA cable, and one from a failed
attempt with a DVI connection. So we can compare.

If we can't diagnose it very easily, you may want to just give up and
not worry about it. Theoretically a digital signal should give you a
better picture, particularly on an LCD, than an analog (which results in
_two_ unnecessary conversion stages, one D-A then one A-D). In practice,
however, it's bloody hard to tell the difference in most situations. I
have two monitors on my home setup, one connected by an old VGA cable
and one by DVI, and I really don't see a difference.

> ·        When I connect the monitor with just DVI and reboot the
> system, the BIOS and GRUB screens (well, I'm sure about the BIOS
> screens but less certain about the GRUB screen) render just fine
> through DVI, but then the screen goes blank.

This makes it even *more* likely to be a driver issue. In fact I'd say
it's 99.99% certain at that point.

> ·        Judging from my Xorg.0.log
> (http://stuff.mit.edu./~jik/misc/Xorg.0.log.txt), the X server notices
> when the DVI cable is inserted or removed.

Oh hey I should've spotted that! I'll take a look.

-- 
Adam Williamson
Fedora QA Community Monkey
IRC: adamw | Fedora Talk: adamwill AT fedoraproject DOT org
http://www.happyassassin.net