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

[H. Nikolaus Schaller]

Hi Neil,

> Am 02.03.2022 um 15:34 schrieb Neil Armstrong <narmstrong@xxxxxxxxxxxx>:
> 
> 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. 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).

> 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?

BR and thanks,
Nikolaus

[1]: https://lore.kernel.org/all/24a27226a22adf5f5573f013e5d7d89b0ec73664.1645895582.git.hns@xxxxxxxxxxxxx/