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.ceres...@bootlin.com>
---
Changes in v9:
- change warning trigger from "refcount != 1" to "container not NULL"
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..0b450b334afd82e0460f18fdd248f79d0a2b153d
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 (!bridge->container)
+ 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
