[Intel-gfx] Valid DP connection without EDID?

[Dave Airlie]

On Mon, Sep 17, 2012 at 6:28 PM, Takashi Iwai <tiwai at suse.de> wrote:
> At Mon, 17 Sep 2012 18:21:47 +1000,
> Dave Airlie wrote:
>>
>> On Mon, Sep 17, 2012 at 6:16 PM, Takashi Iwai <tiwai at 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.

Yes I understand the problem, I'm disagreeing with your diagnosis of
what the problem is, since DPCD response means something is there.
DPCD comes from the other end of the cable.
>
>> Though you'd have to decode the DPCD to see what it is.
>
> Hmm, but isn't it the check of EDID existence enough?

No thats a hack, that we might use, but I can't imagine a DP->VGA with
a EDID-less VGA device producing an EDID, whereas it will produce a
DPCD.

> Checking DPCD bits might work, but I have no exact spec of DPCD, so no
> idea whether the above is really wrong or not.

Is there a VGA port on the docking station, it might have a DP->VGA in
it, or dual-link DVI port.

Dave.