could use help debugging black screen on RS780 (xf86-video-ati-6.13.0, radeon)

[Larry Doolittle]

Hi -

I got an ATI 790GX motherboard (Biostar TA790GX XE) specifically
to check out the "fully documented" ATI chipset capabilities.

Using either Debian Squeeze (xserver-xorg-video-radeon, built
from xf86-video-ati-6.12.6) or home-built binary built directly
from xf86-video-ati-6.13.0, screens go black when I start the X
server.  I can still change back to a text console, at least if
I use the VGA port.  Using the DVI port, it seems the monitor
gets confused and I need to cycle the computer and/or the monitor
to recover.

When the monitor (a Samsung 2048x1152 LCD) is in this blank state,
its menu controls will at least pop up and tell me that it has detected
the intended resolution.  

The computer stays running and can produce any files output needed to
help troubleshoot, and I can ssh in for run-time access to this mode.
I won't spam the list with the full Xorg.0.log file; if someone is
interested, I can make that monsterous thing available privately.
A possibly interesting fragment, the last set of messages printed out
as the X server starts, is:

Blank CRTC 0 success
Disable CRTC memreq 0 success
Disable CRTC 0 success
Mode 2048x1152 - 2208 1185 9
(II) RADEON(0): RADEONRestoreMemMapRegisters() : 
(II) RADEON(0):   MC_FB_LOCATION   : 0x00d700c0 0x00d700c0
(II) RADEON(0):   MC_AGP_LOCATION  : 0x00030000
Picked PLL 0
before 15675
after 15675
best_freq: 156838
best_feedback_div: 230
best_frac_feedback_div: 0
best_ref_div: 3
best_post_div: 7
(II) RADEON(0): crtc(0) Clock: mode 156750, PLL 1568380
(II) RADEON(0): crtc(0) PLL  : refdiv 3, fbdiv 0xE6(230), fracfbdiv 0, pdiv 7
Set CRTC 0 PLL success
Set CRTC Timing success
Set CRTC 0 Overscan success
Not using RMX
scaler 0 setup success
Set CRTC 0 Source success
crtc 0 YUV disable setup success
Output DAC1 setup success
Output CRT1 enable success
Enable CRTC 0 success
Enable CRTC memreq 0 success
Unblank CRTC 0 success

And the output of xrandr -q -d :0 is

Screen 0: minimum 320 x 200, current 2048 x 1152, maximum 2048 x 2048
VGA-0 connected 2048x1152+0+0 (normal left inverted right x axis y axis) 510mm x 287mm
   2048x1152      59.9*+
   1680x1050      60.0  
   1280x1024      60.0  
   1440x900       59.9  
   1280x960       60.0  
   1280x800       59.8  
   1024x768       60.0  
   800x600        60.3     56.2  
   640x480        59.9  
HDMI-0 disconnected (normal left inverted right x axis y axis)

Oh, one more tidbit: this is an amd64 installation.  The hardware
is reported by the X server as an RS780 DDR2 200e/500m
Chipset: "ATI Radeon HD 3300 Graphics" (ChipID = 0x9614)

I am a C programmer who has been using Linux and X since 1993,
so I'm quite capable of tackling this sort of thing in general,
but I would really appreciate some guidance as to how to start.

Thanks in advance!

   - Larry