Date:   Wed, 20 Apr 2022 16:12:29 -0700
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Thierry Reding <thierry.reding@...il.com>,
        Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        Rob Clark <robdclark@...il.com>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] Revert "drm: of: Properly try all possible cases for bridge/panel detection"

Commit '80253168dbfd ("drm: of: Lookup if child node has panel or
bridge")' introduced the ability to describe a panel under a display
controller without having to use a graph to connect the controller to
its single child panel (or bridge).

The implementation of this would find the first non-graph node and
attempt to acquire the related panel or bridge. This prevents cases
where any other child node, such as a aux bus for a DisplayPort
controller, or an opp-table to find the referenced panel.

Commit '67bae5f28c89 ("drm: of: Properly try all possible cases for
bridge/panel detection")' attempted to solve this problem by not
bypassing the graph reference lookup before attempting to find the panel
or bridge.

While this does solve the case where a proper graph reference is
present, it does not allow the caller to distinguish between a
yet-to-be-probed panel or bridge and the absence of a reference to a
panel.

One such case is a DisplayPort controller that on some boards have an
explicitly described reference to a panel, but on others have a
discoverable DisplayPort display attached (which doesn't need to be
expressed in DeviceTree).

This reverts commit '67bae5f28c89 ("drm: of: Properly try all possible
cases for bridge/panel detection")', as a step towards reverting commit
'80253168dbfd ("drm: of: Lookup if child node has panel or bridge")'.

Signed-off-by: Bjorn Andersson <bjorn.andersson@...aro.org>
---
 drivers/gpu/drm/drm_of.c | 99 ++++++++++++++++++++--------------------
 1 file changed, 49 insertions(+), 50 deletions(-)

diff --git a/drivers/gpu/drm/drm_of.c b/drivers/gpu/drm/drm_of.c
index f4df344509a8..026e4e29a0f3 100644
--- a/drivers/gpu/drm/drm_of.c
+++ b/drivers/gpu/drm/drm_of.c
@@ -214,29 +214,6 @@ int drm_of_encoder_active_endpoint(struct device_node *node,
 }
 EXPORT_SYMBOL_GPL(drm_of_encoder_active_endpoint);
 
-static int find_panel_or_bridge(struct device_node *node,
-				struct drm_panel **panel,
-				struct drm_bridge **bridge)
-{
-	if (panel) {
-		*panel = of_drm_find_panel(node);
-		if (!IS_ERR(*panel))
-			return 0;
-
-		/* Clear the panel pointer in case of error. */
-		*panel = NULL;
-	}
-
-	/* No panel found yet, check for a bridge next. */
-	if (bridge) {
-		*bridge = of_drm_find_bridge(node);
-		if (*bridge)
-			return 0;
-	}
-
-	return -EPROBE_DEFER;
-}
-
 /**
  * drm_of_find_panel_or_bridge - return connected panel or bridge device
  * @np: device tree node containing encoder output ports
@@ -259,44 +236,66 @@ int drm_of_find_panel_or_bridge(const struct device_node *np,
 				struct drm_panel **panel,
 				struct drm_bridge **bridge)
 {
-	struct device_node *node;
-	int ret;
+	int ret = -EPROBE_DEFER;
+	struct device_node *remote;
 
 	if (!panel && !bridge)
 		return -EINVAL;
-
 	if (panel)
 		*panel = NULL;
-	if (bridge)
-		*bridge = NULL;
-
-	/* Check for a graph on the device node first. */
-	if (of_graph_is_present(np)) {
-		node = of_graph_get_remote_node(np, port, endpoint);
-		if (node) {
-			ret = find_panel_or_bridge(node, panel, bridge);
-			of_node_put(node);
-
-			if (!ret)
-				return 0;
-		}
-	}
 
-	/* Otherwise check for any child node other than port/ports. */
-	for_each_available_child_of_node(np, node) {
-		if (of_node_name_eq(node, "port") ||
-		    of_node_name_eq(node, "ports"))
+	/**
+	 * Devices can also be child nodes when we also control that device
+	 * through the upstream device (ie, MIPI-DCS for a MIPI-DSI device).
+	 *
+	 * Lookup for a child node of the given parent that isn't either port
+	 * or ports.
+	 */
+	for_each_available_child_of_node(np, remote) {
+		if (of_node_name_eq(remote, "port") ||
+		    of_node_name_eq(remote, "ports"))
 			continue;
 
-		ret = find_panel_or_bridge(node, panel, bridge);
-		of_node_put(node);
+		goto of_find_panel_or_bridge;
+	}
+
+	/*
+	 * of_graph_get_remote_node() produces a noisy error message if port
+	 * node isn't found and the absence of the port is a legit case here,
+	 * so at first we silently check whether graph presents in the
+	 * device-tree node.
+	 */
+	if (!of_graph_is_present(np))
+		return -ENODEV;
+
+	remote = of_graph_get_remote_node(np, port, endpoint);
+
+of_find_panel_or_bridge:
+	if (!remote)
+		return -ENODEV;
+
+	if (panel) {
+		*panel = of_drm_find_panel(remote);
+		if (!IS_ERR(*panel))
+			ret = 0;
+		else
+			*panel = NULL;
+	}
+
+	/* No panel found yet, check for a bridge next. */
+	if (bridge) {
+		if (ret) {
+			*bridge = of_drm_find_bridge(remote);
+			if (*bridge)
+				ret = 0;
+		} else {
+			*bridge = NULL;
+		}
 
-		/* Stop at the first found occurrence. */
-		if (!ret)
-			return 0;
 	}
 
-	return -EPROBE_DEFER;
+	of_node_put(remote);
+	return ret;
 }
 EXPORT_SYMBOL_GPL(drm_of_find_panel_or_bridge);
 
-- 
2.35.1

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