Message-Id: <20251128-drm-bridge-alloc-getput-drm_of_find_bridge-v2-2-88f8a107eca2@bootlin.com>
Date: Fri, 28 Nov 2025 17:50:12 +0100
From: Luca Ceresoli <luca.ceresoli@...tlin.com>
To: Andrzej Hajda <andrzej.hajda@...el.com>, 
 Neil Armstrong <neil.armstrong@...aro.org>, Robert Foss <rfoss@...nel.org>, 
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Cc: Hui Pu <Hui.Pu@...ealthcare.com>, 
 Thomas Petazzoni <thomas.petazzoni@...tlin.com>, 
 Louis Chauvet <louis.chauvet@...tlin.com>, dri-devel@...ts.freedesktop.org, 
 linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org, 
 imx@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org, 
 linux-renesas-soc@...r.kernel.org, linux-amlogic@...ts.infradead.org, 
 linux-mediatek@...ts.infradead.org, linux-samsung-soc@...r.kernel.org, 
 Luca Ceresoli <luca.ceresoli@...tlin.com>
Subject: [PATCH v2 02/26] drm/bridge: deprecate of_drm_find_bridge()

of_drm_find_bridge() does not increment the returned bridge
refcount. of_drm_get_bridge() is to be used as a replacement.

Suggested-by: Maxime Ripard <mripard@...nel.org>
Link: https://lore.kernel.org/dri-devel/20250319-stylish-lime-mongoose-0a18ad@houat/
Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>

---

Changes in v2:
- expand comment to mention why this function is dangerous and what users
  should do about refcounting
---
 drivers/gpu/drm/drm_bridge.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index 367b7a3d8aa3..21a84715d221 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -299,7 +299,7 @@ EXPORT_SYMBOL(__devm_drm_bridge_alloc);
  * @bridge: bridge control structure
  *
  * Add the given bridge to the global list of bridges, where they can be
- * found by users via of_drm_find_bridge().
+ * found by users via of_drm_get_bridge().
  *
  * The bridge to be added must have been allocated by
  * devm_drm_bridge_alloc().
@@ -360,7 +360,7 @@ EXPORT_SYMBOL(devm_drm_bridge_add);
  * @bridge: bridge control structure
  *
  * Remove the given bridge from the global list of registered bridges, so
- * it won't be found by users via of_drm_find_bridge(), and add it to the
+ * it won't be found by users via of_drm_get_bridge(), and add it to the
  * lingering bridge list, to keep track of it until its allocated memory is
  * eventually freed.
  */
@@ -1448,6 +1448,20 @@ EXPORT_SYMBOL(of_drm_get_bridge);
  *
  * @np: device node
  *
+ * This function is deprecated. Convert to of_drm_get_bridge() instead for
+ * proper refcounting.
+ *
+ * The bridge returned by this function is not refcounted. This is
+ * dangerous because the bridge might be deallocated even before the caller
+ * has a chance to use it. To use this function you have to do one of:
+ * - get a reference with drm_bridge_get() as soon as possible to
+ *   minimize the race window, and then drm_bridge_put() when no longer
+ *   using the pointer
+ * - not call drm_bridge_get() or drm_bridge_put() at all, which used to
+ *   be the correct practice before dynamic bridge lifetime was introduced
+ * - again, convert to of_drm_get_bridge(), which is the only safe thing
+ *   to do
+ *
  * RETURNS:
  * drm_bridge control struct on success, NULL on failure
  */

-- 
2.51.1

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