[Bug 94248] MegaChips MCDP2800 LSPCON and ASRock Fatal1ty Z170 Gaming-ITX/ac issues (Broadcast RGB, HDMI output, triple monitor not working)

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=94248

--- Comment #94 from Ville Syrjala <ville.syrjala at linux.intel.com> ---


[   12.431104] [drm:intel_atomic_check [i915]] [CONNECTOR:56:HDMI-A-2] checking
for sink bpp constrains
[   12.431151] [drm:intel_atomic_check [i915]] clamping display bpp (was 36) to
default limit of 24
[   12.431201] [drm:intel_hdmi_compute_config [i915]] picking bpc to 8 for HDMI
output
[   12.431246] [drm:intel_hdmi_compute_config [i915]] forcing pipe bpc to 24
for HDMI
[   12.431293] [drm:intel_atomic_check [i915]] hw max bpp: 36, pipe bpp: 24,
dithering: 0
[   12.431359] [drm:intel_dump_pipe_config [i915]] [CRTC:31:pipe A][modeset]
[   12.431403] [drm:intel_dump_pipe_config [i915]] cpu_transcoder: A, pipe bpp:
24, dithering: 0
[   12.431446] [drm:intel_dump_pipe_config [i915]] audio: 0, infoframes: 0

That "infoframes: 0" part is telling me that your display likely doesn't
identify itself as supporting HDMI, so we treat it as DVI and decide to send it
full range data instead of limited range data. But for DP (which is what LSPCON
is for us) we don't make that distinction. Unfortunately the DP spec doesn't
make any useful provisions for these kinds of combinations. so we're basically
left to guess what we should be doing.

I think what we might want to do is send full range data regardless of the
timings when dealing with a DVI/HDMI downstream port and the monitor doesn't
identify itself as HDMI compatible (essentially using the same logic as we use
for native HDMI). DP++ downstream port might more difficult since I don't
recall if we can somehow tell if the plugged in monitor is actually DVI/HDMI or
DP (I'll need to look into this a bit more). For VGA/NO_EDID/whatnot downstream
ports sending full range all the time might be the right choice.

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