Re: [PATCH 3/7] drm/bridge: lock the encoder bridge chain mutex during insertion

Maxime Ripard
2025-09-29T12:46:21Z

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On Fri, Sep 26, 2025 at 05:59:44PM +0200, Luca Ceresoli wrote:
> drm_bridge_attach() modifies the encoder bridge chain, so take a mutex
> around such operations to allow users of the chain to protect themselves
> from chain modifications while iterating.
>
> This change does not apply to drm_bridge_detach() because:
> * only the drm_encoder.c calls it, not bridge drivers (unlike
> drm_bridge_attach())
> * the only drm_bridge_detach() caller is drm_encoder_cleanup() which
> already locks the mutex for the entire cleanup loop, thus additionally
> locking it here would deadlock
> * drm_bridge_detach() is recursively calling itself along the chain, so
> care would be needed to avoid deadlocks
> Add a comment to clarify that is intended.
>
> Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
> ---
> drivers/gpu/drm/drm_bridge.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
> index 630b5e6594e0affad9ba48791207c7b403da5db8..90e467cf91a134342c80d2f958b928472aaf0d8b 100644
> --- a/drivers/gpu/drm/drm_bridge.c
> +++ b/drivers/gpu/drm/drm_bridge.c
> @@ -453,10 +453,12 @@ int drm_bridge_attach(struct drm_encoder *encoder, struct drm_bridge *bridge,
> bridge->dev = encoder->dev;
> bridge->encoder = encoder;
>
> + drm_encoder_chain_lock(encoder);
> if (previous)
> list_add(&bridge->chain_node, &previous->chain_node);
> else
> list_add(&bridge->chain_node, &encoder->bridge_chain);
> + drm_encoder_chain_unlock(encoder);
>
> if (bridge->funcs->attach) {
> ret = bridge->funcs->attach(bridge, encoder, flags);
> @@ -487,7 +489,9 @@ int drm_bridge_attach(struct drm_encoder *encoder, struct drm_bridge *bridge,
> err_reset_bridge:
> bridge->dev = NULL;
> bridge->encoder = NULL;
> + drm_encoder_chain_lock(encoder);
> list_del(&bridge->chain_node);
> + drm_encoder_chain_unlock(encoder);
>
> if (ret != -EPROBE_DEFER)
> DRM_ERROR("failed to attach bridge %pOF to encoder %s: %d\n",
> @@ -503,6 +507,11 @@ int drm_bridge_attach(struct drm_encoder *encoder, struct drm_bridge *bridge,
> }
> EXPORT_SYMBOL(drm_bridge_attach);
>
> +/*
> + * Invoked by the encoder during encoder cleanup in drm_encoder_cleanup(),
> + * so should generally *not* be called by driver code.

Why not? Also, it looks entirely unrelated to the rest of the patch.

> As opposed to + * drm_bridge_attach(), does not lock the encoder chain mutex. + */ void
> drm_bridge_detach(struct drm_bridge *bridge) { if (WARN_ON(!bridge))

Maxime

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