Intel driver accesses reserved register bit SDVOC/SDVO_DETECTED

Eric Anholt eric at
Thu Dec 11 12:23:56 PST 2008

On Tue, 2008-11-18 at 09:43 +1100, garrone wrote:
> Using the intel driver on a GM45 device, with monitors
> connected to the TMDS1/2 outputs, it was found that
> the xserver was refusing to accept the option 
>     option "Monitor-TMDS-2" "<monitor-id>"
> and also leaving the second screen rather blank.
> Eventually, it was found that at xf86-video-intel::src/i830_driver.c,
> function I830SetupOutputs, line 922, the register SDVOC is anded with
> the SDVO_DETECTED mask in order to detect if a monitor was connected at
> startup. This register bit was clear, causing the screen to be missed.
> A careful reading of the intel documentation shows that while this bit
> indicates an initially detected connected monitor for SDVOB, 
> in fact it is reserved in the SDVOC register, and should therefore be
> ignored. 
> So to cause the second monitor to be enabled, the
> option ForceSDVODetect, false by default, should be set to true.
> Perhaps consideration could be given to defaulting this option to true,
> or even ignoring such initialization bits as SDVO_DETECTED altogether.

It's true that the bit is reserved for G/GM965, but not for G/GM45.
Also, (still) by my reading, it's not about a monitor being connected at
startup, but the SDVO chip being present.

Eric Anholt
eric at                         eric.anholt at

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