Re: Valid DP connection without EDID?

[Takashi Iwai <tiwai@suse.de>]

At Mon, 17 Sep 2012 18:21:47 +1000,
Dave Airlie wrote:
> 
> On Mon, Sep 17, 2012 at 6:16 PM, Takashi Iwai <tiwai@suse.de> wrote:
> > Hi Adam,
> >
> > At Fri, 14 Sep 2012 11:25:03 -0400,
> > Adam Jackson wrote:
> >>
> >> On 9/14/12 10:19 AM, Takashi Iwai wrote:
> >> > Hi,
> >> >
> >> > we've got a machine showing a ghost DP2 output on a docking station.
> >> > The docking station has only one DP port and it's connected to DP1.
> >> > As a result, we get an DP2 active output containing the bogus VESA
> >> > standard modes 1024x768, 800x600, 640x480 although it's not connected
> >> > at all.
> >> >
> >> > Looking a bit deeply on it, it seems that the connector gives actually
> >> > the valid DPCD.  So intel_dp_detect() returns
> >> > connector_status_connected.  But since there is no real connection,
> >> > EDID isn't obtained.  Thus of course no valid modes set.
> >>
> >> Can you be more specific here?  What DPCD does it return?
> >
> > It shows "DPCD: 110a820100030181"
> 
> I don't see how it can be a floating port if something is answering
> DPCD, or how it could be a hardware problem. Like it doesn't seem
> likely they terminated the port with a special DP chip.

The problem is that you have no DP output there in reality.  So, when
you plug the docking station, GNOME sets up the screen with a bogus DP
output, either in an extended mode with a non-existing screen or a
clone mode with a wrong low resolution.

> Though you'd have to decode the DPCD to see what it is.

Hmm, but isn't it the check of EDID existence enough?
Checking DPCD bits might work, but I have no exact spec of DPCD, so no
idea whether the above is really wrong or not.


thanks,

Takashi