[PATCH RFC 13/19] drm/bridge: Add the drm_bridge_chain_get_prev_bridge() helper

[Boris Brezillon]

Will be useful for bridge drivers that want to do bus format
negotiation with their neighbours.

Signed-off-by: Boris Brezillon <boris.brezillon at collabora.com>
---
 drivers/gpu/drm/drm_bridge.c | 19 +++++++++++++++++++
 include/drm/drm_bridge.h     |  2 ++
 2 files changed, 21 insertions(+)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index dcad661daa74..9efb27087e70 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -271,6 +271,25 @@ drm_bridge_chain_get_next_bridge(struct drm_bridge *bridge)
 }
 EXPORT_SYMBOL(drm_bridge_chain_get_next_bridge);
 
+/**
+ * drm_bridge_chain_get_prev_bridge() - Get the previous bridge in the chain
+ * @bridge: bridge object
+ *
+ * RETURNS:
+ * the previous bridge in the chain, or NULL if there's @bridge is the
+ * last.
+ */
+struct drm_bridge *
+drm_bridge_chain_get_prev_bridge(struct drm_bridge *bridge)
+{
+	if (!bridge || !bridge->encoder ||
+	    list_is_first(&bridge->encoder->bridge_chain, &bridge->chain_node))
+                return NULL;
+
+	return list_prev_entry(bridge, chain_node);
+}
+EXPORT_SYMBOL(drm_bridge_chain_get_prev_bridge);
+
 /**
  * DOC: bridge callbacks
  *
diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
index 5d8fe3709bde..2f69adb7b0f3 100644
--- a/include/drm/drm_bridge.h
+++ b/include/drm/drm_bridge.h
@@ -485,6 +485,8 @@ struct drm_bridge *
 drm_bridge_chain_get_last_bridge(struct drm_encoder *encoder);
 struct drm_bridge *
 drm_bridge_chain_get_next_bridge(struct drm_bridge *bridge);
+struct drm_bridge *
+drm_bridge_chain_get_prev_bridge(struct drm_bridge *bridge);
 bool drm_bridge_chain_mode_fixup(struct drm_encoder *encoder,
 				 const struct drm_display_mode *mode,
 				 struct drm_display_mode *adjusted_mode);
-- 
2.21.0