[PATCH] drm/bridge: dw-hdmi: use safe format when first in bridge chain

[Neil Armstrong]

When the dw-hdmi bridge is in first place of the bridge chain, this
means there is now way to select an input format of the dw-hdmi HW
component.

Since introduction of display-connector, negociation was broken since
the dw-hdmi negociation code only worked when the dw-hdmi bridge was
in last position of the bridge chain or behind another bridge also
supporting input & output format negociation.

Commit 0656d1285b79 ("drm/bridge: display-connector: implement bus fmts callbacks")
was introduced to make negociation work again by making display-connector
act as a pass-through concerning input & output format negociation.

But in the case were the dw-hdmi was single in the bridge chain, for
example on Renesas SoCs, with the disply-connector bridge the dw-hdmi
is no more single, breaking output format.

Reported-by: Biju Das <biju.das.jz at bp.renesas.com>
Bisected-by: Kieran Bingham <kieran.bingham+renesas at ideasonboard.com>
Tested-by: Kieran Bingham <kieran.bingham+renesas at ideasonboard.com>
Fixes: 0656d1285b79 ("drm/bridge: display-connector: implement bus fmts callbacks").
Signed-off-by: Neil Armstrong <narmstrong at baylibre.com>
---
 drivers/gpu/drm/bridge/synopsys/dw-hdmi.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
index 54d8fdad395f..9f2e1cac0ae2 100644
--- a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
+++ b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
@@ -2551,8 +2551,9 @@ static u32 *dw_hdmi_bridge_atomic_get_output_bus_fmts(struct drm_bridge *bridge,
 	if (!output_fmts)
 		return NULL;
 
-	/* If dw-hdmi is the only bridge, avoid negociating with ourselves */
-	if (list_is_singular(&bridge->encoder->bridge_chain)) {
+	/* If dw-hdmi is the first or only bridge, avoid negociating with ourselves */
+	if (list_is_singular(&bridge->encoder->bridge_chain) ||
+	    list_is_first(&bridge->chain_node, &bridge->encoder->bridge_chain)) {
 		*num_output_fmts = 1;
 		output_fmts[0] = MEDIA_BUS_FMT_FIXED;
 
@@ -2673,6 +2674,10 @@ static u32 *dw_hdmi_bridge_atomic_get_input_bus_fmts(struct drm_bridge *bridge,
 	if (!input_fmts)
 		return NULL;
 
+	/* If dw-hdmi is the first bridge fall-back to safe output format */
+	if (list_is_first(&bridge->chain_node, &bridge->encoder->bridge_chain))
+		output_fmt = MEDIA_BUS_FMT_FIXED;
+
 	switch (output_fmt) {
 	/* If MEDIA_BUS_FMT_FIXED is tested, return default bus format */
 	case MEDIA_BUS_FMT_FIXED:
-- 
2.25.1