i855 timing mismatch, xorg lying about frequencies ? [was: 'Moiree' patterns...]

[Barry Scott]

xorg at zevv.nl wrote:
> Hello,
>
>
> Short version: 
>
> I can't get my 855 to accept my modelines, and the frequencies as
> reported by X in the logs are not true. How can I force the i810 driver
> to get the timing from my modelines ?
>
>
> Long version :
>
> I've been struggling for some time to get my new 1680x1050 Dell display
> to work on a i855 adapter. Xorg is able to set the proper resolution,
> but the image is just not right - a description of the problem is in my
> previous posts.
>
> I've hooked up a scope on the VGA cable, and it seems that the VGA
> timing is quite a bit off, there's a mismatch between frequencies
> reported by xorg, and the actual timing of the hsync/vsync signals.
>
> When starting X, it reports the following in the logfile :
>
> (II) I810(0): Attempting to use 60.34Hz refresh for mode "1680x1050" (849)
>
> The vsync frequency is said to be 60.34 Hz. Measuring the actual signal
> shows that the actual frequency is 64.43 Hz. 
>
> I did some comparisons between the signals generated under windows XP
> (the monitor is ok there) and linux.
>
> Windows:
>
>    ___   _________   _____	hsync, 65.4 khz, negative pulse
>       | |         | |		time low = 1.21 u, time high = 14.09 u
>       |_|         |_|
>      __                          __ 
>      ||                          ||       vsync, 60.0 Hz, positive pulse
>    __||__________________________||__     time low = 1.58 ms, time high = 91.7 us
>
>
> Linux:
>      _             _		hsync, 70.4 khz, positive pulse
>     | |           | |		time low = 13.44 u, time high = 760n
>   __| |___________| |__
>      __                          __ 
>      ||                          ||       vsync, 64.43 Hz, positive pulse
>    __||__________________________||__     time low = 14.48 ms, time high=42.78 uS
>
>
> The most obvious difference between those two is the polarity of the
> horizontal sync - negative with windows, positive with Linux.
>
> All and all a long story, but now I know exactly what timing I need to
> properly drive the display. I hoped from here on it would be as simple
> as building some modelines and feeding those to xorg, but something's
> not right here: xorg just seems to ignore my modelines, and does it's
> own thing. I disabled DDC (option "DCC" "0"), added my own modelines in
> various forms, but I can't get this to work.
>
> What am I missing here ? Where does the 855 get it's timing from, and
> why does it not accept my modelines ? Any guru's here who can help me on
> this one ?
>   
Basically this does not work in the release i810 driver. You need to use the
modesetting branch of the driver from git.

I get the sources with:

git clone git://anongit.freedesktop.org/git/xorg/driver/xf86-video-intel 
xf86-video-intel

Barry