Re: Valid DP connection without EDID?

[Adam Jackson <ajax@redhat.com>]

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?  Does it claim 
to be a branch device?  We don't currently get that case right, try for 
instance plugging in a DP->VGA pigtail.

> A quick patch below adds (moves) a check of EDID and returns the
> disconnected state when no valid EDID is returned.  An open question
> is whether this is really safe.  Is there any case where no valid EDID
> is returned but the connection is still valid?

DisplayPort requires EDID.  There may be a platform method for obtaining 
it for eDP, but for normal DP it has to exist over AUXCH.

DVI and HDMI require it too, in fact.  The only one that doesn't is VGA.

- ajax