[ltp] Re: Unstable Radeon output at 1920x1200

[Alex Deucher]

On Thu, Mar 27, 2008 at 12:40 AM, Shem Multinymous
<multinymous@gmail.com> wrote:
> On Mon, Mar 24, 2008 at 3:01 PM, Shem Multinymous <multinymous@gmail.com> wrote:
>  >  I have a problem driving LCD monitors at 1920x1200 via DVI on my T43
>  >  with Radeon X300. The image shows up, but is unstable: at erratic
>  >  times but generally once per 20-60 seconds, I get two disruptive
>  >  effects: the image disappears for a couple of seconds, or there is a
>  >  momentary image tearing effect. Basically, it looks like the monitor
>  >  is temporarily losing sync with the DVI signal. This occurs with 3
>  >  different LCD monitors and 3 different DVI cables.
>  >  Now, I know 1920x1200 24bpp is pushing the limit of DVI, but here's
>  >  the thing: when dual-booting into Windows, the display is rock-solid
>  >  at 1920x1200 24bpp DVI, using exactly the same hardware.
>
>  Update:
>
>  I was able to get a stable 1920x1200 on DVI by forcing a custom
>  modeline with a slightly lower pixel clock:
>         Modeline  "1920x1200-dvi"  150.0   1920 1940 2007 2027   1200 1201
>  1204 1233   +hsync -vsync
>  For reference, Xorg's EDID default for this monitor is as follows:
>         Modeline  "1920x1200"      154.0   1920 1968 2000 2080   1200 1203
>  1209 1235   +hsync -vsync
>
>  Still no idea why it works by default under Windows.

Could be several things.  the monitor may be lying about it's
preferred timing, or it may not like the clock the radeon is sending
it.  Windows may have a quirk table for broken edids, or the ati
windows driver may know that 154 Mhz clocks causes problems on DVI, so
it backs it off a bit.  Hard to say.  There used to be an option in
the ati windows driver to use alternate timings for DVI, perhaps we've
made that automatic.

Alex