Re: [PATCH v2] drm/bridge: dw-hdmi: use safe format when first in bridge chain

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Hi,

On 19/01/2022 13:28, Neil Armstrong wrote:
When the dw-hdmi bridge is in first place of the bridge chain, this
means there is no way to select an input format of the dw-hdmi HW
component.

Since introduction of display-connector, negotiation was broken since
the dw-hdmi negotiation 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 negotiation.

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

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

Reported-by: Biju Das <biju.das.jz@xxxxxxxxxxxxxx>
Bisected-by: Kieran Bingham <kieran.bingham+renesas@xxxxxxxxxxxxxxxx>
Tested-by: Kieran Bingham <kieran.bingham+renesas@xxxxxxxxxxxxxxxx>
Fixes: 0656d1285b79 ("drm/bridge: display-connector: implement bus fmts callbacks").
Signed-off-by: Neil Armstrong <narmstrong@xxxxxxxxxxxx>
---
Changes since v1:
- Remove bad fix in dw_hdmi_bridge_atomic_get_input_bus_fmts
- Fix typos in commit message

  drivers/gpu/drm/bridge/synopsys/dw-hdmi.c | 5 +++--
  1 file changed, 3 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..97cdc61b57f6 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;

Can a bridge reviewer get an eye on this ? this actually solves an issue reported on a Renesas platform.

Neil



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