[PATCH v9 0/3] drm/bridge: get/put the bridge reference in drm_bridge.c, warn on old alloc pattern

[PATCH v9 0/3] drm/bridge: get/put the bridge reference in drm_bridge.c, warn on old alloc pattern

[Luca Ceresoli]

This small series adds drm_bridge_get/put() calls where applicable in the
bridge core code, and adds a warning when the old allocation patter is
used.

This is part of the work towards removal of bridges from a still existing
DRM pipeline without use-after-free.

The steps in the grand plan [1] are:

 1. ➜ add refcounting to DRM bridges (struct drm_bridge)
 2. handle gracefully atomic updates during bridge removal
 3. avoid DSI host drivers to have dangling pointers to DSI devices
 4. finish the hotplug bridge work, removing the "always-disconnected"
    connector, moving code to the core and potentially removing the
    hotplug-bridge itself (this needs to be clarified as points 1-3 are
    developed)

This series is part of step 1 of the grand plan.

Current tasks in step 1 of the grand plan:

 A. ✔ add new alloc API and refcounting (in v6.16-rc1)
 B. ✔ convert all bridge drivers to new API (now in drm-misc-next)
 C. ✔ kunit tests (now in drm-misc-next)
 D. ➜ after (B), add get/put to drm_bridge_add/remove() + attach/detech()
      and warn on old allocation pattern (this series)
 E. after (B), convert accessors; this is a large work and can be done
    in chunks
 F. debugfs improvements

Specifically, this series does two things related to the recently
introduced devm_drm_bridge_alloc() [0]:

 A) add drm_bridge_get/put() in the two places within drm_bridge.c where a
    bridge reference is taken/relased
 B) add a warning on bridges using the old allocation pattern instead of
    devm_drm_bridge_alloc()

These are loosely related topics, but I'm sending them in a single series
to avoid trivial patch conflicts, as they touch the same lines. This will
holefully help getting the patches applied more efficiently and quickly. I
hope this is fine being 3 small patches.

Part A) is made of patches 1 and 2. Those have been previously sent in
"[PATCH v7 00/11] drm/bridge: add devm_drm_bridge_alloc() with bridge
refcount" [2] (as patches 3 and 4), and already Reviewed-by Maxime at that
time. However they could not be applied until the conversion of all bridge
drivers to devm_drm_bridge_alloc(), which got completed a few days
ago. Hence the reason this is v8.

[0] https://gitlab.freedesktop.org/drm/misc/kernel/-/commit/0cc6aadd7fc1e629b715ea3d1ba537ef2da95eec
[1] https://lore.kernel.org/lkml/20250206-hotplug-drm-bridge-v6-0-9d6f2c9c3058@bootlin.com/t/#u
[2] https://lore.kernel.org/all/20250314-drm-bridge-refcount-v7-0-152571f8c694@bootlin.com/

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
---
Changes in v9:
- patch 3: change warning trigger from "refcount != 1" to "container not NULL"
- Link to v8 (counted as v1 by mistake): https://lore.kernel.org/r/20250620-drm-bridge-alloc-getput-drm-bridge-c-v1-0-bad7eba5d117@bootlin.com

---
Luca Ceresoli (3):
      drm/bridge: get/put the bridge reference in drm_bridge_add/remove()
      drm/bridge: get/put the bridge reference in drm_bridge_attach/detach()
      drm/bridge: add warning for bridges not using devm_drm_bridge_alloc()

 drivers/gpu/drm/drm_bridge.c | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)
---
base-commit: a59a271769149f0b8258507276f3d2a24370cbdb
change-id: 20250620-drm-bridge-alloc-getput-drm-bridge-c-0ab2ca481c37

Best regards,
-- 
Luca Ceresoli <luca.ceresoli@bootlin.com>

[PATCH v9 1/3] drm/bridge: get/put the bridge reference in drm_bridge_add/remove()

[Luca Ceresoli]

drm_bridge_add() adds the bridge to the global bridge_list, so take a
reference for that. Vice versa in drm_bridge_remove().

Reviewed-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
---

Changes in v9: none
Changes in v8: none

Changes in v7:
- in v6 this was part of "drm/bridge: add support for refcounted DRM
  bridges", now split to a separate patch
---
 drivers/gpu/drm/drm_bridge.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index d6ce7b4c019f415400bab8aa3d032638cba6cdc5..fa2b2457b16e145e3ace70c53984937096d310d0 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -295,6 +295,8 @@ EXPORT_SYMBOL(__devm_drm_bridge_alloc);
  */
 void drm_bridge_add(struct drm_bridge *bridge)
 {
+	drm_bridge_get(bridge);
+
 	mutex_init(&bridge->hpd_mutex);
 
 	if (bridge->ops & DRM_BRIDGE_OP_HDMI)
@@ -342,6 +344,8 @@ void drm_bridge_remove(struct drm_bridge *bridge)
 	mutex_unlock(&bridge_lock);
 
 	mutex_destroy(&bridge->hpd_mutex);
+
+	drm_bridge_put(bridge);
 }
 EXPORT_SYMBOL(drm_bridge_remove);
 

-- 
2.49.0

[PATCH v9 2/3] drm/bridge: get/put the bridge reference in drm_bridge_attach/detach()

[Luca Ceresoli]

drm_bridge_attach() adds the bridge to the encoder chain, so take a
reference for that. Vice versa in drm_bridge_detach().

Reviewed-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
---

Changes in v9: none
Changes in v8: none

Changes in v7:
- in v6 this was part of "drm/bridge: add support for refcounted DRM
  bridges", now split to a separate patch
---
 drivers/gpu/drm/drm_bridge.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index fa2b2457b16e145e3ace70c53984937096d310d0..f001bbe95559aabf0aac9f25f89250ad4e1ad9c8 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -411,11 +411,17 @@ int drm_bridge_attach(struct drm_encoder *encoder, struct drm_bridge *bridge,
 	if (!encoder || !bridge)
 		return -EINVAL;
 
-	if (previous && (!previous->dev || previous->encoder != encoder))
-		return -EINVAL;
+	drm_bridge_get(bridge);
 
-	if (bridge->dev)
-		return -EBUSY;
+	if (previous && (!previous->dev || previous->encoder != encoder)) {
+		ret = -EINVAL;
+		goto err_put_bridge;
+	}
+
+	if (bridge->dev) {
+		ret = -EBUSY;
+		goto err_put_bridge;
+	}
 
 	bridge->dev = encoder->dev;
 	bridge->encoder = encoder;
@@ -464,6 +470,8 @@ int drm_bridge_attach(struct drm_encoder *encoder, struct drm_bridge *bridge,
 			      "failed to attach bridge %pOF to encoder %s\n",
 			      bridge->of_node, encoder->name);
 
+err_put_bridge:
+	drm_bridge_put(bridge);
 	return ret;
 }
 EXPORT_SYMBOL(drm_bridge_attach);
@@ -484,6 +492,7 @@ void drm_bridge_detach(struct drm_bridge *bridge)
 
 	list_del(&bridge->chain_node);
 	bridge->dev = NULL;
+	drm_bridge_put(bridge);
 }
 
 /**

-- 
2.49.0

[PATCH v9 3/3] drm/bridge: add warning for bridges not using devm_drm_bridge_alloc()

[Luca Ceresoli]

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@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

Re: [PATCH v9 3/3] drm/bridge: add warning for bridges not using devm_drm_bridge_alloc()

[Maxime Ripard]

On Fri, 20 Jun 2025 17:59:55 +0200, Luca Ceresoli wrote:
> 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.
> 
> 
> [ ... ]

Reviewed-by: Maxime Ripard <mripard@kernel.org>

Thanks!
Maxime

Re: [PATCH v9 3/3] drm/bridge: add warning for bridges not using devm_drm_bridge_alloc()

[Dmitry Baryshkov]

On 20/06/2025 18:59, Luca Ceresoli wrote:
> 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@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)

Can we rename this function, making sure that noone can call it 
directly? E.g. adding two underscores might point out that is is an 
internal API.

>   {
> +	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);
> 


-- 
With best wishes
Dmitry

Re: [PATCH v9 3/3] drm/bridge: add warning for bridges not using devm_drm_bridge_alloc()

[Luca Ceresoli]

Hello Dmitry,

thanks for reviewing!

On Mon, 23 Jun 2025 14:06:43 +0300
Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com> wrote:

> On 20/06/2025 18:59, Luca Ceresoli wrote:
> > 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@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)  
> 
> Can we rename this function, making sure that noone can call it 
> directly? E.g. adding two underscores might point out that is is an 
> internal API.

I'm afraid I don't think this would be correct. Every bridge driver is
expected to call drm_bridge_add() at the end of probe/initialization,
to make the DRM subsystem aware that this bridge is ready for being
used.

The point of this patch, which is a completion to [1], is to ensure
that all drivers use the "new" process:

 1. bridge = devm_drm_bridge_alloc()
 2. drm_bridge_add(bridge)

and there are no users of the old process:

 1. bridge = [devm_]kzalloc()
 2. drm_bridge_add(bridge)

Does this addresses your concern?

[1] https://cgit.freedesktop.org/drm/drm-misc/commit/?id=b3f13e00a8de351832df7d628a15ca4db49ca94f

Luca

-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

Re: [PATCH v9 0/3] drm/bridge: get/put the bridge reference in drm_bridge.c, warn on old alloc pattern

[Luca Ceresoli]

On Fri, 20 Jun 2025 17:59:52 +0200, Luca Ceresoli wrote:
> This small series adds drm_bridge_get/put() calls where applicable in the
> bridge core code, and adds a warning when the old allocation patter is
> used.
> 
> This is part of the work towards removal of bridges from a still existing
> DRM pipeline without use-after-free.
> 
> [...]

Applied, thanks!

[1/3] drm/bridge: get/put the bridge reference in drm_bridge_add/remove()
      commit: a7748dd127ea8d9cd2d9d942ef21c85e5569bb73
[2/3] drm/bridge: get/put the bridge reference in drm_bridge_attach/detach()
      commit: 94d50c1a2ca31d80f12d9c2bdbc41437751e320c
[3/3] drm/bridge: add warning for bridges not using devm_drm_bridge_alloc()
      commit: e6565e76e977422cb9e51fe872a2bcfdbc321b7b

Best regards,
-- 
Luca Ceresoli <luca.ceresoli@bootlin.com>