Modeline autoadjusting problem

Marc Ferland mferland at
Thu Dec 18 14:23:40 PST 2008

On Thursday 18 December 2008 16:34:11 Olivier Galibert wrote:
> On Thu, Dec 18, 2008 at 03:47:23PM -0500, Marc Ferland wrote:
> > Seems like the modeline is adjusted by the driver _even_ if I specified
> > the LVDSFixedMode "false" option. I really don't know what to do next...
> > How can I force my modeline to the driver?
> You can't.  Looking at the code, there is no way to tell the system
> you know what you're doing.  ->mode_fixup is called unconditionally,
> and i830_lvds_mode_fixup has no toggle to switch itself off.  That
> mode fixup function takes lvds_fixed_mode as gospel, and seems to do
> part of the chipset programming, so you can't just disable it anyway.
> Strange.  lvds_fixed_mode itself can only come from bios or ddc, not
> config from what I can see.
> It's weird there isn't a generic high level way to tell X not to trust
> the roms and trust the config instead, when it comes to modelines,
> dpi, things like that.

Thanks for the info Oliver! I looked up the i830_lvds_mode_fixup function and 
added a dummy "return TRUE;" just after the :

>  /* If we don't have a panel mode there's not much we can do */
>     if (pI830->lvds_fixed_mode == NULL)
>         return TRUE;


Now the modeline seems to be taken into account (from what I see from the 
register dump log). At least it works for me now...

I don't know if anyone from Intel could tell me if this is the default 
behavior? Like Oliver, I also think this is pretty strange. What's the 
purpose of having modelines in xorg.conf if there overwritten by the BIOS....

In my case, I use an LCD with no EDID info (this is for an embedded platform). 
And the BIOS that comes with the board has wrong modelines hardcoded into it.



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