[PATCH v16 4/4] drm/bridge: dw-hdmi: fix bus formats negotiation for 8 bit modes

[H. Nikolaus Schaller]

Hi Neil,

> Am 03.03.2022 um 09:35 schrieb Neil Armstrong <narmstrong at baylibre.com>:
> 
> Hi,
> 
> On 02/03/2022 23:24, H. Nikolaus Schaller wrote:
>> Hi Neil,
>>> Am 02.03.2022 um 15:34 schrieb Neil Armstrong <narmstrong at baylibre.com>:
>>> 
>>> Hi,
>>> 
>>>> (cross-checked: RGB mode still works if I force hdmi->sink_is_hdmi = false)
>>> 
>>> I don't understand what's wrong, can you try to make the logic select MEDIA_BUS_FMT_YUV8_1X24 instead of DRM_COLOR_FORMAT_YCBCR422 ?
>> I have forced hdmi->sink_is_hdmi = false and replaced
>>  	/* Default 8bit RGB fallback */
>> -	output_fmts[i++] = MEDIA_BUS_FMT_RGB888_1X24;
>> +	output_fmts[i++] = MEDIA_BUS_FMT_YUV8_1X24;
>> And then screen remains black. MEDIA_BUS_FMT_RGB888_1X24 works.
>> (MEDIA_BUS_FMT_VUY8_1X24 doesn't work either).
>> So this indicates that YUV conversion is not working properly. Maybe missing some special
>> setup.
>> What I have to test if it works on a different monitor.

Same effect on a Xiaomi monitor (user manual just telling HDMI1,4 compatible), an
older Acer video projector and a Sharp TV set.

The Xiaomi monitor does not say "No signal" but shows a black screen. The others
do not even report any HDMI signals. All work well with MEDIA_BUS_FMT_RGB888_1X24.

This means the transcoding to YUV does not work properly on the jz4780 SoC setup.

So it looks as if we have to disable it (at least unless someone finds a fix).

>> Not that this specific panel
>> (a 7 inch waveshare touch with HDMIinput) is buggy and reports YUV capabilities
>> but does not handle them...
>> On the other hand this panel works on RasPi and OMAP5 (where I admit I do not know in
>> which mode).
> 
> Pretty sure they don't support YUV HDMI output.
> 
> If you can try on a certified HDMI devices like a TV, it would here figuring out where comes the issue.

I am not sure if the Sharp TV is fully certified but would assume...

> 
>>> If your CSC is broken, we'll need to disable it on your platform.
>> Indeed.
>> So it seems as if we need a mechanism to overwrite dw_hdmi_bridge_atomic_get_output_bus_fmts()
>> in our ingenic-dw-hdmi platform specialization [1] to always return MEDIA_BUS_FMT_RGB888_1X24.
>> Or alternatively set sink_is_hdmi = false there (unfortunately there is no direct access to
>> struct dw_hdmi in a specialization drivers).
>> Is this already possible or how can it be done?
> 
> It's not handled yet, but we may add the logic to handle the lack of CSC config bit and
> add a glue config bit to override this like we already did for CEC.
> 
> I wrote an initial support to disable CSC (only compile-tested), could you try on your platform with setting disable_csc = 1 in your dw-hdmi glue code ?

This works!

So how can we get that merged? IMHO your proposal should be before we add ingenic-dw-hdmi.
If you have a version with proper commit message I can add it to the beginning of my
seried and include it in a v17. Or if you get yours merged to drm-misc/drm-misc-next I
can build on top.

BR and thanks,
Nikolaus