[PATCH] drm/i915: customize DPCD brightness control for specific panel

[Adam Jackson]

On Mon, 2019-10-07 at 12:08 +0300, Jani Nikula wrote:

> The problem with the EDID quirks is that exposing the quirks sticks out
> like a sore thumb. Thus far all of it has been contained in drm_edid.c
> and they affect how the EDID gets parsed, for all drivers. Obviously
> this could be changed, but is it the right thing to do?
> 
> What I suggested was, check the OUI only, and if it matches, do
> more. Perhaps there's something in the 0x300 range of DPCD offsets that
> you can read? Or perhaps you need to write the source OUI first, and
> then do that.

My issue isn't really with identifying the panel from EDID rather than
DPCD, whichever identifier is most specific is probably the best thing
to use. It's more that this quirk is identified in common code but only
applied in one driver. If this panel were ever to be attached to some
other source, they might well want to apply the same kind of fix. My
(admittedly naïve) reading of the OUI handshake process is that when
the source device writes an OUI to DP_SOURCE_OUI it is telling the sink
"I'm about to issue commands that conform to _this_ vendor's own
conventions". If that convention communicates information that is
entirely contained within AUXCH transactions (and doesn't, for example,
require looking at some other strapping pin or external device) then in
principle it doesn't matter if the source device "matches" that OUI; it
would be legal for an AMD GPU to write the same sequence and expect the
same reaction, should that panel be attached to an AMD GPU.

So, it would be nice to know exactly what that protocol is meant to do,
if it applies only to this specific panel or anything else with the
same TCON, how one would identify such TCONs in the wild other than
EDID, if it relies on an external PWM or something, etc. And it might
make sense for now to make this a (shudder) driver-specific EDID quirk
rather than match by DPCD, at least until we know if the panel is ever
seen attached to other source devices and if the OUI convention is
self-contained.

- ajax