drm/bridge: Extend bridge API to disable connector creation

Most bridge drivers create a DRM connector to model the connector at the
output of the bridge. This model is historical and has worked pretty
well so far, but causes several issues:

- It prevents supporting more complex display pipelines where DRM
connector operations are split over multiple components. For instance a
pipeline with a bridge connected to the DDC signals to read EDID data,
and another one connected to the HPD signal to detect connection and
disconnection, will not be possible to support through this model.

- It requires every bridge driver to implement similar connector
handling code, resulting in code duplication.

- It assumes that a bridge will either be wired to a connector or to
another bridge, but doesn't support bridges that can be used in both
positions very well (although there is some ad-hoc support for this in
the analogix_dp bridge driver).

In order to solve these issues, ownership of the connector should be
moved to the display controller driver (where it can be implemented
using helpers provided by the core).

Extend the bridge API to allow disabling connector creation in bridge
drivers as a first step towards the new model. The new flags argument to
the bridge .attach() operation allows instructing the bridge driver to
skip creating a connector. Unconditionally set the new flags argument to
0 for now to keep the existing behaviour, and modify all existing bridge
drivers to return an error when connector creation is not requested as
they don't support this feature yet.

The change is based on the following semantic patch, with manual review
and edits.

@ rule1 @
identifier funcs;
identifier fn;
@@
 struct drm_bridge_funcs funcs = {
 	...,
 	.attach = fn
 };

@ depends on rule1 @
identifier rule1.fn;
identifier bridge;
statement S, S1;
@@
 int fn(
 	struct drm_bridge *bridge
+	, enum drm_bridge_attach_flags flags
 )
 {
 	... when != S
+	if (flags & DRM_BRIDGE_ATTACH_NO_CONNECTOR) {
+		DRM_ERROR("Fix bridge driver to make connector optional!");
+		return -EINVAL;
+	}
+
 	S1
 	...
 }

@ depends on rule1 @
identifier rule1.fn;
identifier bridge, flags;
expression E1, E2, E3;
@@
 int fn(
 	struct drm_bridge *bridge,
 	enum drm_bridge_attach_flags flags
 ) {
 <...
 drm_bridge_attach(E1, E2, E3
+	, flags
 )
 ...>
 }

@@
expression E1, E2, E3;
@@
 drm_bridge_attach(E1, E2, E3
+	, 0
 )

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Tested-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200226112514.12455-10-laurent.pinchart@ideasonboard.com

1 files changed, 18 insertions, 2 deletions

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index 5a8f7d7e05f3..96eace94fea8 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -149,6 +149,7 @@ static const struct drm_private_state_funcs drm_bridge_priv_state_funcs = {
  * @encoder: DRM encoder
  * @bridge: bridge to attach
  * @previous: previous bridge in the chain (optional)
+ * @flags: DRM_BRIDGE_ATTACH_* flags
  *
  * Called by a kms driver to link the bridge to an encoder's chain. The previous
  * argument specifies the previous bridge in the chain. If NULL, the bridge is
@@ -166,7 +167,8 @@ static const struct drm_private_state_funcs drm_bridge_priv_state_funcs = {
  * Zero on success, error code on failure
  */
 int drm_bridge_attach(struct drm_encoder *encoder, struct drm_bridge *bridge,
-		      struct drm_bridge *previous)
+		      struct drm_bridge *previous,
+		      enum drm_bridge_attach_flags flags)
 {
 	int ret;
 
@@ -188,7 +190,7 @@ int drm_bridge_attach(struct drm_encoder *encoder, struct drm_bridge *bridge,
 		list_add(&bridge->chain_node, &encoder->bridge_chain);
 
 	if (bridge->funcs->attach) {
-		ret = bridge->funcs->attach(bridge);
+		ret = bridge->funcs->attach(bridge, flags);
 		if (ret < 0)
 			goto err_reset_bridge;
 	}
@@ -312,6 +314,20 @@ void drm_bridge_detach(struct drm_bridge *bridge)
  *   allows providing a single static const &drm_bridge_funcs instance in
  *   bridge drivers, improving security by storing function pointers in
  *   read-only memory.
+ *
+ *   In order to ease transition, bridge drivers may support both the old and
+ *   new models by making connector creation optional and implementing the
+ *   connected-related bridge operations. Connector creation is then controlled
+ *   by the flags argument to the drm_bridge_attach() function. Display drivers
+ *   that support the new model and create connectors themselves shall set the
+ *   %DRM_BRIDGE_ATTACH_NO_CONNECTOR flag, and bridge drivers shall then skip
+ *   connector creation. For intermediate bridges in the chain, the flag shall
+ *   be passed to the drm_bridge_attach() call for the downstream bridge.
+ *   Bridge drivers that implement the new model only shall return an error
+ *   from their &drm_bridge_funcs.attach handler when the
+ *   %DRM_BRIDGE_ATTACH_NO_CONNECTOR flag is not set. New display drivers
+ *   should use the new model, and convert the bridge drivers they use if
+ *   needed, in order to gradually transition to the new model.
  */
 
 /**