[PATCH 2/5] drm/display: bridge-connector: Preserve max bpc across connector reset

[Igor Paunovic <royalnet026@gmail.com>]

drm_connector_attach_max_bpc_property() initializes max_requested_bpc
and max_bpc in the connector state, but nothing restores them when the
connector state is later thrown away and re-created:
drm_bridge_connector_create_state() returns a zeroed state for
non-HDMI bridge connectors, and drm_mode_config_reset() installs such
a fresh state on every connector that implements
&drm_connector_funcs.atomic_create_state instead of a .reset hook.

This matters because drivers can attach the property before
drm_mode_config_reset() runs. Rockchip, for example, binds its
component drivers (which create connectors and attach properties)
before calling drm_mode_config_reset(). After that reset,
max_requested_bpc is 0, so drm_atomic_connector_check() computes
max_bpc = min(info->bpc ?: 8, 0) = 0. Any driver that filters output
formats on conn_state->max_bpc then rejects every format, and clients
that never set the "max bpc" property - fbcon in particular - end up
with a black screen. meson already attaches the property on a
drm_bridge_connector before drm_mode_config_reset() and so already
boots with max_requested_bpc = 0 today. dw-hdmi's format negotiation
does consume the value there, but meson attaches the property with a
maximum of 8 and the 8-bit fallback formats are not gated on it, so
negotiation yields the same result for 0 and 8 - the change is a
no-op for meson.

HDMI bridge connectors are immune: drm_bridge_connector_create_state()
calls __drm_atomic_helper_connector_hdmi_state_init(), which
initializes both fields from connector->max_bpc. amdgpu likewise
re-initializes max_requested_bpc in its own .reset implementation.
Non-HDMI bridge connectors have no equivalent.

Mirror the HDMI helper on the non-HDMI path: record the upper attach
limit in connector->max_bpc (drmm_connector_hdmi_init() already stores
the same value there) and restore max_requested_bpc and max_bpc from
it in drm_bridge_connector_create_state() whenever the "max bpc"
property is attached. Connectors without the property behave exactly
as before.

Signed-off-by: Igor Paunovic <royalnet026@gmail.com>
---
 drivers/gpu/drm/display/drm_bridge_connector.c | 6 +++++-
 drivers/gpu/drm/drm_connector.c                | 5 +++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/display/drm_bridge_connector.c b/drivers/gpu/drm/display/drm_bridge_connector.c
index 632cc3ae3b54..d33c4fc42fbf 100644
--- a/drivers/gpu/drm/display/drm_bridge_connector.c
+++ b/drivers/gpu/drm/display/drm_bridge_connector.c
@@ -282,9 +282,13 @@ drm_bridge_connector_create_state(struct drm_connector *connector)
 	if (IS_ERR(conn_state))
 		return conn_state;
 
-	if (bridge_connector->bridge_hdmi)
+	if (bridge_connector->bridge_hdmi) {
 		__drm_atomic_helper_connector_hdmi_state_init(connector,
 							      conn_state);
+	} else if (connector->max_bpc_property) {
+		conn_state->max_requested_bpc = connector->max_bpc;
+		conn_state->max_bpc = connector->max_bpc;
+	}
 
 	return conn_state;
 }
diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
index 8b4baed060f3..0400a6a92e2b 100644
--- a/drivers/gpu/drm/drm_connector.c
+++ b/drivers/gpu/drm/drm_connector.c
@@ -2866,6 +2866,10 @@ EXPORT_SYMBOL(drm_connector_set_link_status_property);
  * @max: The maximum bit depth supported by the connector.
  *
  * This is used to add support for limiting the bit depth on a connector.
+ * @max is also recorded in &drm_connector.max_bpc, so that
+ * &drm_connector_funcs.atomic_create_state and &drm_connector_funcs.reset
+ * implementations can restore the property default when re-creating the
+ * connector state.
  *
  * Returns:
  * Zero on success, negative errno on failure.
@@ -2888,6 +2892,7 @@ int drm_connector_attach_max_bpc_property(struct drm_connector *connector,
 	drm_object_attach_property(&connector->base, prop, max);
 	connector->state->max_requested_bpc = max;
 	connector->state->max_bpc = max;
+	connector->max_bpc = max;
 
 	return 0;
 }
-- 
2.43.0