[Bug 97774] [SKL] Can we support YCbCr 420 output in HDMI 2.0 port?

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=97774

--- Comment #4 from Ville Syrjala <ville.syrjala at linux.intel.com> ---
(In reply to Ville Syrjala from comment #3)
> (In reply to Jun Zhao from comment #0)
> > We also found some TVs just support 4:2:0 YCbCr. Has Linux driver not
> > support it yet?
> 
> No support as of now.

Just to elaborat ea bit. For SKL and whatnot the only way to get it would be
via an external DP->HDMI 2.0 dongle/cable, or via LSPCON (which is the same
thing pretty much).

Adding the support would involve several parts:

1) Add all the generic HDMI forum VSDB parsing and whatnot to the EDID parser

This is needed so that we'll know whether the sink supports YCbCr 4:2:0. There
have been some patches towards HDMI 2.0 support for the EDID parser, but I
can't recall how much was still missing from them. Not much merged so far.

3) Implement YCbCr 4:4:4 output support

This would involve doing RGB->YCbCr conversion with the pipe CSC. I started to
implement this at some point, but got sidetracked and never actually finished.
I should probably have a branch with some half finished code somewhere.

2) Extend out DP->HDMI dongle support to allow YCbCr 4:2:0 output.

Looking at the DP 1.4 spec, I see a PROTOCOL_CONVERTER_CONTROL_1 register that
would allow you to enable YCbCr 4:2:0 conversion. If I'm reading things
correctly we need to output YCbCr 4:4:4 to the dongle, and then dongle will
downsample that to 4:2:0 when instructed to do so via this register. We
probably have to parse a few other things from the DPCD as well so that we'll
know whether the dongle even supports this.

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