[Bug 106291] drm.edid_firmware override does not work

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=106291

--- Comment #21 from Ville Syrjala <ville.syrjala at linux.intel.com> ---
(In reply to Wolfgang Haupt from comment #20)
> (In reply to Jani Nikula from comment #19)
> > It seems to me the drm.edid_firmware option (that this bug was originally
> > about) works as expected.
> 
> Yes we can agree on that one.
> 
> > Obviously everything about DP link fail when you don't have a cable
> > connected. I'm not sure what your expectation is. That can't work.
> 
> I want to force display on digital output and hardcode the edid.
> May I ask about the "why" it can't work

In order to light up a DP link we have to talk to the device at the other end
of the cable. There's a whole slew of parameters that have to be negotiated
between the source and sink devices. If there's nothing connected we won't get
an answer to any of the queries and things will probably go badly hence forth.
In order to do this properly, we'd basically have to implement a simulated DP
sink in the driver and talk to that instead of the real thing. And once the
real thing would get plugged in we'd have to switch over to talking to it
instead of the simulation, and renegotiate everything.

In the case of LSPCON the DP device is actually right there on the motherboard,
so in theory we should be able to talk to it. However I believe the LSPCON
devices may snoop the EDID of the actual display and change their behaviour
accordingly. If there's nothing to snoop they may just fail to respond in a
meaningful manner to our queries.

In this case it looks like the LSPCON devices is maybe telling us that it
doesn't support any DP link rates, and we print out a warning since that isn't
a valid response as far as the DP spec is concerned.

It migth be interesting to see what it's reporting actually:
dd if=/dev/drm_dp_aux0 > dpcd

and attach the file to the bug. Use ddrescue instead of you get an error and
resulting dump isn't the full 1 MiB in size.

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