meson: Enabling full-range HDMI output

[Neil Armstrong]

On 24/10/2024 10:38, neil.armstrong at linaro.org wrote:
> On 23/10/2024 14:12, Francisco Ayala Le Brun wrote:
>> 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.
> 
> So seems the current CSC tables outputs limited range from YUV, could you try:
> 
> =================><==================================
> diff --git a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
> index 0031f3c54882..9e1cb9be4622 100644
> --- a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
> +++ b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
> @@ -1769,8 +1769,11 @@ static void hdmi_config_AVI(struct dw_hdmi *hdmi,
>          drm_hdmi_avi_infoframe_from_display_mode(&frame, connector, mode);
> 
>          if (hdmi_bus_fmt_is_rgb(hdmi->hdmi_data.enc_out_bus_format)) {
> +               bool is_input_rgb = hdmi_bus_fmt_is_rgb(hdmi->hdmi_data.enc_in_bus_format);
> +               bool is_limited_range = hdmi->hdmi_data.rgb_limited_range |!is_input_rgb;

Of course:

              bool is_limited_range = hdmi->hdmi_data.rgb_limited_range || !is_input_rgb;

Sorry for the typo.

> +
>                  drm_hdmi_avi_infoframe_quant_range(&frame, connector, mode,
> -                                                  hdmi->hdmi_data.rgb_limited_range ?
> +                                                  is_limited_range ?
>                                                     HDMI_QUANTIZATION_RANGE_LIMITED :
>                                                     HDMI_QUANTIZATION_RANGE_FULL);
>          } else {
> =================><==================================
> 
> It won't output full range, but at least the monitor will know the real
> signal quant range.
> 
> The real fix would be to calculate some new parameters for full range,
> but I'm not able to do that personally.
> 
> Neil
> 
>>>
>>> Neil
>>>
>>>>
>>>> Thank you in advance,
>>>> Francisco
>>>>
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>>>
>>>
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> 
> 
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