[Bug 100023] LSPCon and DisplayPort always output limited range RGB, even when it should output full range RGB

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=100023

--- Comment #1 from N. W. <nw9165-3201 at yahoo.com> ---
Relevant quote from https://bugs.freedesktop.org/show_bug.cgi?id=94248#c94 :

(In reply to Ville Syrjala from comment #94)
> 
> [   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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