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

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Date:   Sat, 26 Feb 2022 18:13:02 +0100
From:   "H. Nikolaus Schaller" <hns@...delico.com>
To:     Andrzej Hajda <andrzej.hajda@...el.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Robert Foss <robert.foss@...aro.org>,
        Paul Boddie <paul@...die.org.uk>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Paul Cercueil <paul@...pouillou.net>,
        Maxime Ripard <maxime@...no.tech>,
        "H. Nikolaus Schaller" <hns@...delico.com>,
        Kieran Bingham <kieran.bingham+renesas@...asonboard.com>
Cc:     Jonas Karlman <jonas@...boo.se>, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-mips@...r.kernel.org,
        letux-kernel@...nphoenux.org
Subject: [PATCH v16 4/4] drm/bridge: dw-hdmi: fix bus formats negotiation for 8 bit modes

Commit 7cd70656d1285b ("drm/bridge: display-connector: implement bus fmts callbacks")

introduced a new mechanism to negotiate bus formats between hdmi connectors
and bridges which is to be used e.g. for the jz4780 based CI20 board.

In this case dw-hdmi sets up a list of formats in
dw_hdmi_bridge_atomic_get_output_bus_fmts().

This includes e.g. MEDIA_BUS_FMT_UYVY8_1X16 which is chosen for the CI20 but
only produces a black screen.

Analysis revealed an omission in

Commit 6c3c719936dafe ("drm/bridge: synopsys: dw-hdmi: add bus format negociation")

to check for 8 bit with when adding UYVY8 or YUV8 formats.

This fix is based on the observation that max_bpc = 0 when running this
function while info->bpc = 8.

Adding the proposed patch makes the jz4780/CI20 panel work again with default
MEDIA_BUS_FMT_RGB888_1X24 mode.

Fixes: 7cd70656d1285b ("drm/bridge: display-connector: implement bus fmts callbacks")
Fixes: 6c3c719936dafe ("drm/bridge: synopsys: dw-hdmi: add bus format negociation")
Signed-off-by: H. Nikolaus Schaller <hns@...delico.com>
---
 drivers/gpu/drm/bridge/synopsys/dw-hdmi.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
index 43e375da131e8..c08e2cc96584c 100644
--- a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
+++ b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
@@ -2621,11 +2621,13 @@ static u32 *dw_hdmi_bridge_atomic_get_output_bus_fmts(struct drm_bridge *bridge,
 		output_fmts[i++] = MEDIA_BUS_FMT_RGB101010_1X30;
 	}
 
-	if (info->color_formats & DRM_COLOR_FORMAT_YCBCR422)
-		output_fmts[i++] = MEDIA_BUS_FMT_UYVY8_1X16;
+	if (max_bpc >= 8 && info->bpc >= 8) {
+		if (info->color_formats & DRM_COLOR_FORMAT_YCBCR422)
+			output_fmts[i++] = MEDIA_BUS_FMT_UYVY8_1X16;
 
-	if (info->color_formats & DRM_COLOR_FORMAT_YCBCR444)
-		output_fmts[i++] = MEDIA_BUS_FMT_YUV8_1X24;
+		if (info->color_formats & DRM_COLOR_FORMAT_YCBCR444)
+			output_fmts[i++] = MEDIA_BUS_FMT_YUV8_1X24;
+	}
 
 	/* Default 8bit RGB fallback */
 	output_fmts[i++] = MEDIA_BUS_FMT_RGB888_1X24;
-- 
2.33.0