[PATCH v6 06/16] drm/bridge: synopsys: dw-dp: Support MEDIA_BUS_FMT_FIXED

[Sebastian Reichel]

Add support for MEDIA_BUS_FMT_FIXED, which is e.g. requested for USB-C
DP chains as the last bridge in the chain (aux-hpd-bridge) does not
implement atomic_get_output_bus_fmts(), which results in the generic
drm_atomic_bridge_chain_select_bus_fmts() code using MEDIA_BUS_FMT_FIXED
instead. For decent support of this, two areas are changed:

1. In atomic_check, resolving MEDIA_BUS_FMT_FIXED output format by
   using the negotiated input format.

2. Implementing a custom .atomic_get_input_bus_fmts hook that, on
   MEDIA_BUS_FMT_FIXED, advertises all bandwidth-validated formats
   from dw_dp_bridge_atomic_get_output_bus_fmts(). This lets the
   upstream encoder negotiate the best mutually supported format.

Signed-off-by: Sebastian Reichel <sebastian.reichel@xxxxxxxxxxxxx>
---
 drivers/gpu/drm/bridge/synopsys/dw-dp.c | 34 +++++++++++++++++++++++++++++++--
 1 file changed, 32 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/bridge/synopsys/dw-dp.c b/drivers/gpu/drm/bridge/synopsys/dw-dp.c
index 7f6506f8d4bd..259efc25b56e 100644
--- a/drivers/gpu/drm/bridge/synopsys/dw-dp.c
+++ b/drivers/gpu/drm/bridge/synopsys/dw-dp.c
@@ -1530,6 +1530,7 @@ static int dw_dp_bridge_atomic_check(struct drm_bridge *bridge,
 				     struct drm_connector_state *conn_state)
 {
 	struct drm_display_mode *adjusted_mode = &crtc_state->adjusted_mode;
+	unsigned int out_bus_format = bridge_state->output_bus_cfg.format;
 	struct dw_dp *dp = bridge_to_dp(bridge);
 	struct dw_dp_bridge_state *state;
 	const struct dw_dp_output_format *fmt;
@@ -1540,7 +1541,10 @@ static int dw_dp_bridge_atomic_check(struct drm_bridge *bridge,
 	state = to_dw_dp_bridge_state(bridge_state);
 	mode = &state->mode;
 
-	fmt = dw_dp_get_output_format(bridge_state->output_bus_cfg.format);
+	if (out_bus_format == MEDIA_BUS_FMT_FIXED)
+		out_bus_format = bridge_state->input_bus_cfg.format;
+
+	fmt = dw_dp_get_output_format(out_bus_format);
 	if (!fmt)
 		return -EINVAL;
 
@@ -1807,6 +1811,32 @@ static u32 *dw_dp_bridge_atomic_get_output_bus_fmts(struct drm_bridge *bridge,
 	return output_fmts;
 }
 
+static u32 *
+dw_dp_bridge_atomic_get_input_bus_fmts(struct drm_bridge *bridge,
+				       struct drm_bridge_state *bridge_state,
+				       struct drm_crtc_state *crtc_state,
+				       struct drm_connector_state *conn_state,
+				       u32 output_fmt,
+				       unsigned int *num_input_fmts)
+{
+	/*
+	 * MEDIA_BUS_FMT_FIXED means the downstream bridge does not constrain
+	 * the bus format. In that case, advertise all formats supported by the
+	 * DP link so the upstream encoder can negotiate the best match.
+	 */
+	if (output_fmt == MEDIA_BUS_FMT_FIXED)
+		return dw_dp_bridge_atomic_get_output_bus_fmts(bridge,
+							       bridge_state,
+							       crtc_state,
+							       conn_state,
+							       num_input_fmts);
+
+	return drm_atomic_helper_bridge_propagate_bus_fmt(bridge, bridge_state,
+							 crtc_state, conn_state,
+							 output_fmt,
+							 num_input_fmts);
+}
+
 static struct drm_bridge_state *dw_dp_bridge_atomic_duplicate_state(struct drm_bridge *bridge)
 {
 	struct dw_dp_bridge_state *state;
@@ -1824,7 +1854,7 @@ static const struct drm_bridge_funcs dw_dp_bridge_funcs = {
 	.atomic_duplicate_state = dw_dp_bridge_atomic_duplicate_state,
 	.atomic_destroy_state = drm_atomic_helper_bridge_destroy_state,
 	.atomic_create_state = drm_atomic_helper_bridge_create_state,
-	.atomic_get_input_bus_fmts = drm_atomic_helper_bridge_propagate_bus_fmt,
+	.atomic_get_input_bus_fmts = dw_dp_bridge_atomic_get_input_bus_fmts,
 	.atomic_get_output_bus_fmts = dw_dp_bridge_atomic_get_output_bus_fmts,
 	.atomic_check = dw_dp_bridge_atomic_check,
 	.mode_valid = dw_dp_bridge_mode_valid,

-- 
2.53.0