linux-kernel - [PATCH 3/3] drm/bridge: add warning for bridges not using devm_drm_bridge

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Message-Id: <20250620-drm-bridge-alloc-getput-drm-bridge-c-v1-3-bad7eba5d117@bootlin.com>
Date: Fri, 20 Jun 2025 11:32:08 +0200
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>, 
 Laurent Pinchart <Laurent.pinchart@...asonboard.com>, 
 Jonas Karlman <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>, 
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, 
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>
Cc: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>, 
 Anusha Srivatsa <asrivats@...hat.com>, 
 Thomas Petazzoni <thomas.petazzoni@...tlin.com>, 
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
 Luca Ceresoli <luca.ceresoli@...tlin.com>
Subject: [PATCH 3/3] drm/bridge: add warning for bridges not using
 devm_drm_bridge_alloc()

To the best of my knowledge, all drivers in the mainline kernel adding a
DRM bridge are now converted to using devm_drm_bridge_alloc() for
allocation and initialization. Among others this ensures initialization of
the bridge refcount, allowing dynamic allocation lifetime.

devm_drm_bridge_alloc() is now mandatory for all new bridges. Code using
the old pattern ([devm_]kzalloc + filling the struct fields +
drm_bridge_add) is not allowed anymore.

Any drivers that might have been missed during the conversion, patches in
flight towards mainline and out-of-tre drivers still using the old pattern
will already be caught by a warning looking like:

  ------------[ cut here ]------------
  refcount_t: addition on 0; use-after-free.
  WARNING: CPU: 2 PID: 83 at lib/refcount.c:25 refcount_warn_saturate+0x120/0x148
  [...]
  Call trace:
   refcount_warn_saturate+0x120/0x148 (P)
   drm_bridge_get.part.0+0x70/0x98 [drm]
   drm_bridge_add+0x34/0x108 [drm]
   sn65dsi83_probe+0x200/0x480 [ti_sn65dsi83]
   [...]

This warning comes from the refcount code and happens because
drm_bridge_add() is increasing the refcount, which is uninitialized and
thus initially zero.

Having a warning and the corresponding stack trace is surely useful, but
the warning text does not clarify the root problem nor how to fix it.

Add a DRM_WARN() just before increasing the refcount, so the log will be
much more readable:

  [drm] DRM bridge corrupted or not allocated by devm_drm_bridge_alloc()
  ------------[ cut here ]------------
  refcount_t: addition on 0; use-after-free.
  [...etc...]

A DRM_WARN is used because drm_warn and drm_WARN require a struct
drm_device pointer which is not yet available when adding a bridge.

Do not print the dev_name() in the warning because struct drm_bridge has no
pointer to the struct device. The affected driver should be easy to catch
based on the following stack trace however.

Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>

---

This patch was added in v8
---
 drivers/gpu/drm/drm_bridge.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index f001bbe95559aabf0aac9f25f89250ad4e1ad9c8..7d511c28608f1d8ea8fbb81d00efa9e227b02a13 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -295,6 +295,9 @@ EXPORT_SYMBOL(__devm_drm_bridge_alloc);
  */
 void drm_bridge_add(struct drm_bridge *bridge)
 {
+	if (kref_read(&bridge->refcount) == 0)
+		DRM_WARN("DRM bridge corrupted or not allocated by devm_drm_bridge_alloc()\n");
+
 	drm_bridge_get(bridge);

 	mutex_init(&bridge->hpd_mutex);

-- 
2.49.0