DVI output not visible on LCD with DVI, VGA output works -- (MergedFB on Ubuntu Edgy, Powerbook G4 ATI radeon 9600 M10)

Martin Langhoff martin.langhoff at gmail.com
Wed Jan 3 01:16:21 PST 2007

[I'm not on the list, please CC ;-) ]

Hi! I have been playing with the external monitor output and I have 2
LCDs behaving differently. One (at work) has no problem with the DVI
monitor output directly to the DVI input. The otherone, at home,
accepts the signal only through the VGA adapter -- the direct DVI
signal does not work, and the monitor claims it sees no signal at all.

So I am after a hint or two on how to debug this one. I don't have any
gear to read the DVI signal to understand whether it is correct or

I have been working for a few days already on my MergedFB
configuration on this aluminium PowerBook G4, 17' with a Radeon 9600
M10 (RV350). Software is vanilla Ubuntu Edgy (plus a few things from
edgy backports). I initially followed the advise laid out at
http://ozlabs.org/~jk/docs/mergefb/ and had things going _perfectly
fine_ using MergedFB with the monitor at work (LM950), using the DVI
signal directly. It took 10 minutes and it was all go. Sweet.

The LCD at home (a cheap 19', 1440x900), on the other hand, does _not_
like the signal from DVI, and yet runs correctly with the DVI->VGA
adaptor (it does show some ghosting, though).

 - The DDC info is read correctly regardless of whether I start with
DVI or VGA cable connected. And swapping the cables around doesn't
help. Regardless of what connector is on at X.org startup, I always
get signal via VGA, and never get anything via DVI.

 - There has been some discussion about reduced screen blanking, and
how that can help LCDs at hi resolution over DVI. Setting
ReducedBlanking to true didn't help, so fetched & compiled the cvt
utility and setup a Modeline. It was a bit of challenge to get
xorg.conf to acknowledge that the monitor definition wasn't there just
dangling (as mergedfb does its own monitor setup, even if you never
name it). Eventually, putting 2 monitor entries for the Screen section
that corresponds to screen 0 got xorg to report in X.log that it is
using the right Modeline:

# from cvt   1440 900 60 -r
Modeline "1440x900"  88.75  1440 1488 1520 1600  900 903 909 926  +HSync

And still no dice. I don't know how to check WFT is happening on the
DVI port itself. I have the modified radeontool that dumps registers,
but I'm wary of messing about with them blind. Maybe there's a magic
one that says "go" ;-) ?

I did play with the modes proposed in the xmode script above, and with
the "bioskeys" -- both work correctly when I am using VGA output.

To be frank, I am not sure where to place the blame -- though I
suspect that the monitor isn't top of the line HW and its DVI input
reading may be subpar. On the other hand, it _is_ possible to make it
go with the radeon -- as it runs correctly under OSX (10.3) just
autodetected on boot or on plugging the cable in.

any hints for further debugging welcome!

pd: and happy new year!

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