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Subject: Re: [PATCH v16 4/4] drm/bridge: dw-hdmi: fix bus formats negotiation
 for 8 bit modes
From:   "H. Nikolaus Schaller" <hns@goldelico.com>
In-Reply-To: <4cb08b5d-c1ec-f2b4-a107-63a771146ec0@baylibre.com>
Date:   Wed, 2 Mar 2022 23:24:42 +0100
Cc:     Paul Boddie <paul@boddie.org.uk>, Jonas Karlman <jonas@kwiboo.se>,
        David Airlie <airlied@linux.ie>,
        Robert Foss <robert.foss@linaro.org>,
        linux-mips <linux-mips@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Paul Cercueil <paul@crapouillou.net>,
        Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>,
        Maxime Ripard <maxime@cerno.tech>,
        Andrzej Hajda <andrzej.hajda@intel.com>,
        Discussions about the Letux Kernel 
        <letux-kernel@openphoenux.org>,
        Jernej Skrabec <jernej.skrabec@gmail.com>,
        Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
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References: <cover.1645895582.git.hns@goldelico.com>
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 <5da069b6-8a99-79c2-109c-c85715165857@baylibre.com>
 <E0D3B7E8-0C8D-4119-8267-0556AB921B24@goldelico.com>
 <fca28594-8d4e-dd2f-93a0-a052cb888d90@baylibre.com>
 <75CBD357-577A-402D-9E3B-DBE82A84BC43@goldelico.com>
 <4cb08b5d-c1ec-f2b4-a107-63a771146ec0@baylibre.com>
To:     Neil Armstrong <narmstrong@baylibre.com>
Precedence: bulk

Hi Neil,

> Am 02.03.2022 um 15:34 schrieb Neil Armstrong =
<narmstrong@baylibre.com>:
>=20
> Hi,
>=20
>> (cross-checked: RGB mode still works if I force hdmi->sink_is_hdmi =3D =
false)
>=20
> 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 =3D false and replaced

 	/* Default 8bit RGB fallback */
-	output_fmts[i++] =3D MEDIA_BUS_FMT_RGB888_1X24;
+	output_fmts[i++] =3D 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 =3D 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/24a27226a22adf5f5573f013e5d7d89b0ec73664.16458=
95582.git.hns@goldelico.com/=