[PATCH 0/4] drm/bridge: enforce drm_bridge_add() before drm_bridge_attach()

[PATCH 0/4] drm/bridge: enforce drm_bridge_add() before drm_bridge_attach()

[Luca Ceresoli]

This small series enforces that DRM bridges must be added before they are
attached as discussed in [1].

This is part of the work towards removal of bridges from a still existing
DRM pipeline without use-after-free. The grand plan was discussed in [0].
Here's the work breakdown (➜ marks the current series):

 1. ➜ add refcounting to DRM bridges (struct drm_bridge)
    (based on devm_drm_bridge_alloc() [0])
    A. ✔ add new alloc API and refcounting (v6.16)
    B. ✔ convert all bridge drivers to new API (v6.17)
    C. ✔ kunit tests (v6.17)
    D. ✔ add get/put to drm_bridge_add/remove() + attach/detach()
         and warn on old allocation pattern (v6.17)
    E. ➜ add get/put on drm_bridge accessors
       1. ✔ drm_bridge_chain_get_first_bridge() + add a cleanup action
            (drm-misc-next)
       2. ✔ drm_bridge_get_prev_bridge() (drm-misc-next)
       3. ✔ drm_bridge_get_next_bridge() (drm-misc-next)
       4. ✔ drm_for_each_bridge_in_chain() (drm-misc-next)
       5. ✔ drm_bridge_connector_init (drm-misc-next)
       6. … protect encoder bridge chain with a mutex
       7. of_drm_find_bridge
       8. drm_of_find_panel_or_bridge, *_of_get_bridge
       9. ➜ enforce drm_bridge_add before drm_bridge_attach
    F. ✔ debugfs improvements
       1. ✔ add top-level 'bridges' file (v6.16)
       2. ✔ show refcount and list removed bridges (drm-misc-next)
 2. … handle gracefully atomic updates during bridge removal
 3. … DSI host-device driver interaction
 4. ✔ removing the need for the "always-disconnected" connector
 5. finish the hotplug bridge work, moving code to the core and potentially
    removing the hotplug-bridge itself (this needs to be clarified as
    points 1-3 are developed)

Series layout:
 - patches 1-2: add missing drm_bridge_add() to the 2 drivers known for not
                calling it
 - patch 3: document that drm_bridge_add() is required before attach
 - patch 4: add a warning in drm_bridge_attach() is drm_bridge_add() was
            not called

The added warning might reveal more non-compliant drivers, in that case
they will be fixed as they are found.

[0] https://lore.kernel.org/lkml/20250206-hotplug-drm-bridge-v6-0-9d6f2c9c3058@bootlin.com/#t
[1] https://lore.kernel.org/all/20250709-sophisticated-loon-of-rain-6ccdd8@houat/

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
---
Luca Ceresoli (4):
      drm/sti: hda: add bridge before attaching
      drm/sti: hdmi: add bridge before attaching
      drm/bridge: document that adding a bridge is mandatory before attach
      drm/bridge: add warning for bridges attached without being added

 drivers/gpu/drm/drm_bridge.c   | 6 ++++++
 drivers/gpu/drm/sti/sti_hda.c  | 5 +++++
 drivers/gpu/drm/sti/sti_hdmi.c | 2 ++
 3 files changed, 13 insertions(+)
---
base-commit: 0f2efbe6d8305b91c9b2c92ebaf8c24a614bc305
change-id: 20251003-b4-drm-bridge-alloc-add-before-attach-ef312c0b8a8c

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

[PATCH 1/4] drm/sti: hda: add bridge before attaching

[Luca Ceresoli]

DRM bridges should be always added to the global bridge list before being
attached.

Link: https://lore.kernel.org/all/20250709-sophisticated-loon-of-rain-6ccdd8@houat/
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
---
 drivers/gpu/drm/sti/sti_hda.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/gpu/drm/sti/sti_hda.c b/drivers/gpu/drm/sti/sti_hda.c
index 2c015f563de96ae58959801493ead870c49f70e5..b7397827889c94d6f8e8c2a48b09f06e29ceeaf7 100644
--- a/drivers/gpu/drm/sti/sti_hda.c
+++ b/drivers/gpu/drm/sti/sti_hda.c
@@ -779,6 +779,8 @@ static int sti_hda_probe(struct platform_device *pdev)
 		return PTR_ERR(hda->clk_hddac);
 	}
 
+	drm_bridge_add(&hda->bridge);
+
 	platform_set_drvdata(pdev, hda);
 
 	return component_add(&pdev->dev, &sti_hda_ops);
@@ -786,7 +788,10 @@ static int sti_hda_probe(struct platform_device *pdev)
 
 static void sti_hda_remove(struct platform_device *pdev)
 {
+	struct sti_hda *hda = platform_get_drvdata(pdev);
+
 	component_del(&pdev->dev, &sti_hda_ops);
+	drm_bridge_remove(&hda->bridge);
 }
 
 static const struct of_device_id hda_of_match[] = {

-- 
2.51.0

[PATCH 2/4] drm/sti: hdmi: add bridge before attaching

[Luca Ceresoli]

DRM bridges should be always added to the global bridge list before being
attached.

Link: https://lore.kernel.org/all/20250709-sophisticated-loon-of-rain-6ccdd8@houat/
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
---
 drivers/gpu/drm/sti/sti_hdmi.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/gpu/drm/sti/sti_hdmi.c b/drivers/gpu/drm/sti/sti_hdmi.c
index 4e7c3d78b2b971f8083deae96f3967b44a6499cb..f8222e60b1e01afb6d93f816915f17056c060f22 100644
--- a/drivers/gpu/drm/sti/sti_hdmi.c
+++ b/drivers/gpu/drm/sti/sti_hdmi.c
@@ -1459,6 +1459,7 @@ static int sti_hdmi_probe(struct platform_device *pdev)
 
 	platform_set_drvdata(pdev, hdmi);
 
+	drm_bridge_add(&hdmi->bridge);
 	return component_add(&pdev->dev, &sti_hdmi_ops);
 
  release_adapter:
@@ -1475,6 +1476,7 @@ static void sti_hdmi_remove(struct platform_device *pdev)
 	if (hdmi->audio_pdev)
 		platform_device_unregister(hdmi->audio_pdev);
 	component_del(&pdev->dev, &sti_hdmi_ops);
+	drm_bridge_remove(&hdmi->bridge);
 }
 
 struct platform_driver sti_hdmi_driver = {

-- 
2.51.0

[PATCH 3/4] drm/bridge: document that adding a bridge is mandatory before attach

[Luca Ceresoli]

At the moment it's not documented that you need to add a bridge before
attaching it. Clarify that.

Suggested-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/all/20250709-sophisticated-loon-of-rain-6ccdd8@houat/
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
---
 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 53e7ece36dd940aabd1c0880f296fce7224a12ac..1246a52f8767b52c5f10139aa897824b3c2f28da 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -422,6 +422,9 @@ static bool drm_bridge_is_atomic(struct drm_bridge *bridge)
  * If non-NULL the previous bridge must be already attached by a call to this
  * function.
  *
+ * The bridge to be attached must have been previously added by
+ * drm_bridge_add().
+ *
  * Note that bridges attached to encoders are auto-detached during encoder
  * cleanup in drm_encoder_cleanup(), so drm_bridge_attach() should generally
  * *not* be balanced with a drm_bridge_detach() in driver code.

-- 
2.51.0

[PATCH 4/4] drm/bridge: add warning for bridges attached without being added

[Luca Ceresoli]

DRM bridges must be added before they are attached. Add a warning to catch
violations.

The warning is based on the bridge not being part of any list, so it will
trigger if the bridge is being attached without ever having been added.

It won't catch cases of bridges attached after having been added and then
removed, because in that case the bridge will be in
bridge_lingering_list. However such a case is both more demanding to detect
and less likely to happen, so it can be left unchecked, at least for now.

Link: https://lore.kernel.org/all/20250709-sophisticated-loon-of-rain-6ccdd8@houat/
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
---
 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 1246a52f8767b52c5f10139aa897824b3c2f28da..6dba601a056bb3cff8b8dd5b1ec46299235b2d85 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -441,6 +441,9 @@ int drm_bridge_attach(struct drm_encoder *encoder, struct drm_bridge *bridge,
 	if (!encoder || !bridge)
 		return -EINVAL;
 
+	if (list_empty(&bridge->list))
+		DRM_WARN("Missing drm_bridge_add() before attach\n");
+
 	drm_bridge_get(bridge);
 
 	if (previous && (!previous->dev || previous->encoder != encoder)) {

-- 
2.51.0

Re: [PATCH 1/4] drm/sti: hda: add bridge before attaching

[Raphaël Gallais-Pou]

Le 03/10/2025 à 10:59, Luca Ceresoli a écrit :
> DRM bridges should be always added to the global bridge list before being
> attached.
> 
> Link: https://lore.kernel.org/all/20250709-sophisticated-loon-of-rain-6ccdd8@houat/
> Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
> ---
Hi Luca,

Acked-by: Raphael Gallais-Pou <rgallaispou@gmail.com>

Best regards,
Raphaël>

Re: [PATCH 2/4] drm/sti: hdmi: add bridge before attaching

[Raphaël Gallais-Pou]

Le 03/10/2025 à 10:59, Luca Ceresoli a écrit :
> DRM bridges should be always added to the global bridge list before being
> attached.
> 
> Link: https://lore.kernel.org/all/20250709-sophisticated-loon-of-rain-6ccdd8@houat/
> Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
> ---
Hi Luca,

Acked-by: Raphael Gallais-Pou <rgallaispou@gmail.com>

Best regards,
Raphaël>

Re: [PATCH 3/4] drm/bridge: document that adding a bridge is mandatory before attach

[Raphaël Gallais-Pou]

Le 03/10/2025 à 10:59, Luca Ceresoli a écrit :
> At the moment it's not documented that you need to add a bridge before
> attaching it. Clarify that.
> 
> Suggested-by: Maxime Ripard <mripard@kernel.org>
> Link: https://lore.kernel.org/all/20250709-sophisticated-loon-of-rain-6ccdd8@houat/
> Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
> ---
Hi Luca,

Reviewed-by: Raphael Gallais-Pou <rgallaispou@gmail.com>

Best regards,
Raphaël

Re: [PATCH 4/4] drm/bridge: add warning for bridges attached without being added

[Raphaël Gallais-Pou]

Le 03/10/2025 à 10:59, Luca Ceresoli a écrit :
> DRM bridges must be added before they are attached. Add a warning to catch
> violations.
> 
> The warning is based on the bridge not being part of any list, so it will
> trigger if the bridge is being attached without ever having been added.
> 
> It won't catch cases of bridges attached after having been added and then
> removed, because in that case the bridge will be in
> bridge_lingering_list. However such a case is both more demanding to detect
> and less likely to happen, so it can be left unchecked, at least for now.
> 
> Link: https://lore.kernel.org/all/20250709-sophisticated-loon-of-rain-6ccdd8@houat/
> Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
> ---

Reviewed-by: Raphael Gallais-Pou <rgallaispou@gmail.com>

Best regards,
Raphaël>

Re: [PATCH 0/4] drm/bridge: enforce drm_bridge_add() before drm_bridge_attach()

[Raphaël Gallais-Pou]

Le 03/10/2025 à 10:59, Luca Ceresoli a écrit :
> This small series enforces that DRM bridges must be added before they are
> attached as discussed in [1].
> 
> This is part of the work towards removal of bridges from a still existing
> DRM pipeline without use-after-free. The grand plan was discussed in [0].
> Here's the work breakdown (➜ marks the current series):
> 
>   1. ➜ add refcounting to DRM bridges (struct drm_bridge)
>      (based on devm_drm_bridge_alloc() [0])
>      A. ✔ add new alloc API and refcounting (v6.16)
>      B. ✔ convert all bridge drivers to new API (v6.17)
>      C. ✔ kunit tests (v6.17)
>      D. ✔ add get/put to drm_bridge_add/remove() + attach/detach()
>           and warn on old allocation pattern (v6.17)
>      E. ➜ add get/put on drm_bridge accessors
>         1. ✔ drm_bridge_chain_get_first_bridge() + add a cleanup action
>              (drm-misc-next)
>         2. ✔ drm_bridge_get_prev_bridge() (drm-misc-next)
>         3. ✔ drm_bridge_get_next_bridge() (drm-misc-next)
>         4. ✔ drm_for_each_bridge_in_chain() (drm-misc-next)
>         5. ✔ drm_bridge_connector_init (drm-misc-next)
>         6. … protect encoder bridge chain with a mutex
>         7. of_drm_find_bridge
>         8. drm_of_find_panel_or_bridge, *_of_get_bridge
>         9. ➜ enforce drm_bridge_add before drm_bridge_attach
>      F. ✔ debugfs improvements
>         1. ✔ add top-level 'bridges' file (v6.16)
>         2. ✔ show refcount and list removed bridges (drm-misc-next)
>   2. … handle gracefully atomic updates during bridge removal
>   3. … DSI host-device driver interaction
>   4. ✔ removing the need for the "always-disconnected" connector
>   5. finish the hotplug bridge work, moving code to the core and potentially
>      removing the hotplug-bridge itself (this needs to be clarified as
>      points 1-3 are developed)
> 
> Series layout:
>   - patches 1-2: add missing drm_bridge_add() to the 2 drivers known for not
>                  calling it
>   - patch 3: document that drm_bridge_add() is required before attach
>   - patch 4: add a warning in drm_bridge_attach() is drm_bridge_add() was
>              not called
> 
> The added warning might reveal more non-compliant drivers, in that case
> they will be fixed as they are found.
> 
> [0] https://lore.kernel.org/lkml/20250206-hotplug-drm-bridge-v6-0-9d6f2c9c3058@bootlin.com/#t
> [1] https://lore.kernel.org/all/20250709-sophisticated-loon-of-rain-6ccdd8@houat/
> 
> Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
> ---
> Luca Ceresoli (4):
>        drm/sti: hda: add bridge before attaching
>        drm/sti: hdmi: add bridge before attaching
>        drm/bridge: document that adding a bridge is mandatory before attach
>        drm/bridge: add warning for bridges attached without being added
> 
>   drivers/gpu/drm/drm_bridge.c   | 6 ++++++
>   drivers/gpu/drm/sti/sti_hda.c  | 5 +++++
>   drivers/gpu/drm/sti/sti_hdmi.c | 2 ++
>   3 files changed, 13 insertions(+)
> ---
> base-commit: 0f2efbe6d8305b91c9b2c92ebaf8c24a614bc305
> change-id: 20251003-b4-drm-bridge-alloc-add-before-attach-ef312c0b8a8c
> 

If drm-misc maintainers agree I can merge the whole series.

Regards,
Raphaël> Best regards,

Re: [PATCH 0/4] drm/bridge: enforce drm_bridge_add() before drm_bridge_attach()

[Luca Ceresoli]

Hello Raphaël,

On Sat Oct 18, 2025 at 2:49 PM CEST, Raphaël Gallais-Pou wrote:
>
>
> Le 03/10/2025 à 10:59, Luca Ceresoli a écrit :
>> This small series enforces that DRM bridges must be added before they are
>> attached as discussed in [1].

Thanks for reviewing this series!

I just sent v2 which only adds a new patch with an additional check, which
I realized being useful after receiving a regression report.

All the details at
https://lore.kernel.org/r/20251021-b4-drm-bridge-alloc-add-before-attach-v2-0-c17cc1bbff72@bootlin.com

Luca

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