meson: Enabling full-range HDMI output

[Francisco Ayala Le Brun]

Thank you for your response. I can confirm reverting the commit solves
the issue. However, it only leaves me more confused as to the root
cause.

If dw-hdmi is set to output RGB, and you set the CSC matrix to zeroes,
then it will show a correctly completely black output. However, if you
set the first column of the YUV-to-RGB matrix to ones, and show a
black image via DRM, then the tone is in the limited range. This
seemed to indicate dw-hdmi can output full range RGB, and the problem
was farther upstream.

Perhaps the problem is the CSC matrices? I was under the assumption
0x2000 corresponded to 1, even though i.MX6 manual chapter 33 seems to
suggest with a csc_scale of 1, then 0x800 would correspond to 1.
Still, I would expect at least fully black to be correct as that would
give a multiplication by zero.

Or the state of dw_hdmi is affecting the VIU in some way? Perhaps the
VIU hardware is made to clamp luminance when dw_hdmi is outputting
RGB? I noticed for the CSC there is only a special matrix for
RGB-to-limited-RGB, but no special matrix for YUV-to-limited-RGB. That
seems to suggest the clamping may be done upstream in those cases.

Francisco


On Wed, Oct 23, 2024 at 8:30 AM <neil.armstrong at linaro.org> wrote:
>
> Hi,
>
> On 23/10/2024 10:00, Francisco Ayala Le Brun wrote:
> > 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.
>
> Indeed the internal VPU pipeline is in 10bit YUV444, so it requires enabling
> the dw-hdmi CSC, so perhaps those are not designed to output full-range RGB ?
>
> Could you revert:
> d3d6b1bf85ae drm: bridge: dw_hdmi: fix preference of RGB modes over YUV420
>
> This would give you direct YUV444 HDMI output without CSC involved, reverting
> is not an options because of low-cost HDMI panels lying in their EDID
> and not really supporting YUV HDMI...
>
> So if the original YUV is in full range, the culprit is in the DW-HDMI CSC
> table.
>
> Neil
>
> >
> > Thank you in advance,
> > Francisco
> >
> > _______________________________________________
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>
>
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