Wrong DDC information for external monitor with i810
ajax at nwnk.net
Thu Sep 14 13:15:41 PDT 2006
On Thursday 14 September 2006 05:30, Alan Hourihane wrote:
> Your monitor's DDC is broken. See this line....
> (II) I810(0): clock: 81.0 MHz Image Size: 0 x 0 mm
Strictly speaking that's legal if the monitor earlier reported itself as being
variable-sized. It might even be legal for fixed size monitors; the
implementor's guide's conformance notes aren't clear on the subject.
> and then this....
> (II) I810(0): Ranges: V min: 60 V max: 75 Hz, H min: 31 H max: 60 kHz,
> PixClock max 80 MHz
> The modeline requires 81.0MHz (or in the calculated case 80.14MHz), but
> the monitor's DDC says it's got a maximum of 80.0MHz. Therefore the mode
> is rejected.
This is an EDID limitation. The maximum pixel clock in the ranges section is
encoded in decamegahertz, so the next highest possible limit would be 90MHz.
Given the choice between the two, 80MHz is less wrong. This is actually
pretty common on 20" LCDs, where 1600x1200 works out to 162MHz but the
advertised max is 160MHz.
DI-EXT is better and encodes the maximum dotclock in megahertz, but in the
absence of that it's probably sensible to round up to the dotclock of the
preferred mode. This is part of the work I'm planning to land in 7.2.
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