BIOS mirroring mode

Alex Deucher alexdeucher at
Tue Sep 16 14:47:18 PDT 2008

On Sat, Sep 6, 2008 at 7:24 PM, Protas Oleksiy < at> wrote:
> Hi, guys,
> Sorry for bothering that often but I need an advice from low-level X coders,
> because surfing the net only gave userish results. Let me outline the problem.
> We have a laptop with 1280x800 native resolution. It gets connected to
> projector with a 1024x768 resolution. What I need is a cloned display. RandR
> plays fine but there is a tricky thing. Because 1024x768 is smaller than
> 1280x800, the projector only displays a portion of the screen, not the whole.
> Setting 1024x768 on built-in LCD is a no-go, because it messes plasma
> workspace and looks awful on non-native resolution.
> Now the question part. When the laptop goes up it uses BIOS to mirror the
> display output onto external VGA channel. It handles all rescaling tasks, etc.
> In Linux it is true up to X start, because the driver disables this ability
> and manages VGA output on its own.
> I am wondering how can one enable this feature on a running X server? This
> *must* be possible, as Windows keeps this thing upon boot to GUI if the
> desktop is not expanded.
> Could you hint me how this can be achieved or what needs to be coded in order
> to?

Depending on your laptop, if the driver is randr 1.2 enabled, this
should just work.  Most randr-enabled X drivers can handle scaling and
centering of panels as well.  What the bios does depends on the
vendor.  It probably uses the scaler for the panel.


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