[RFC 0/2] Add AUX device entries for DP MST devices

[Li, Sun peng (Leo)]

>> Hmm. My MST-foo is admittedly weak so I'm not sure. A quick trawl through
>> the spec didn't provide any solid explanations either :( However eg.
>> "Figure 2-83: Example Multi-function MST Branch-Sink Device Enumeration"
>> in the DP 1.4 spec does appear to show kind of virtual DPCD thing behind
>> a logical port. But I'm not really sure what than means.
> 
> Good point, I'm gonna dig more into that. It sounds like we should be
> able to access that with the relative addressing as defined by the spec
> (2.11.5). I'll have to see why that's currently getting nacked though.
> 

It looks like DPCD reads on logical ports work on some devices, and not
others... I swapped my MST display out with a different one, and it read
just fine.

More specifically - in a daisy chain setup with 2 MST displays - with
the one that works at the end:

# auxrw.py read /dev/drm_dp_aux4_mst\:0-1-8 0x30-0x3f -> ACK
# auxrw.py read /dev/drm_dp_aux6_mst\:0-1 0x30-0x3f -> ACK
(The GUIDs returned are identical)

With the one that doesn't at the end:

# auxrw.py read /dev/drm_dp_aux4_mst\:0-1-8 0x30-0x3f -> *NAK*
# auxrw.py read /dev/drm_dp_aux6_mst\:0-1 0x30-0x3f -> ACK

So it seems there's some device dependent behavior here. I'm not sure if
there's a better way of handling this besides exposing all the
downstream ports: If it works, great. If not, just don't use it?

Leo