meson: Enabling full-range HDMI output

[Francisco Ayala Le Brun]

Hello,

I am seeking some guidance regarding enabling full-range HDMI output.
Specifically, I am wondering which component in the video pipeline
could be clamping HDMI output values to 16-235? Further details
follow:

I have been trying to get full-range HDMI output from a Le Potato
s905x SBC. At the moment, HDMI output seems to be hard-coded to color
quantization limited range (16-235). This greatly diminishes the
contrast in full range displays.

So far setting the VIU_OSD1/2_FULL_RANGE bit in the
VIU_OSD1/2_CTRL_STAT2 register does not seem to work. I have also
looked at the color space conversion matrix in dw_hdmi in case that is
the culprit, but the values seem fine and setting that to zero does
produce a signal in the full range (0). The buffer from the VIU is
using a YUV format so dw_hdmi is not using the full to limited range
conversion matrix.

Thank you in advance,
Francisco