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

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Message-ID: <20250929-enigmatic-delicate-mussel-f36b89@houat>
Date: Mon, 29 Sep 2025 14:46:18 +0200
From: Maxime Ripard <mripard@...nel.org>
To: Luca Ceresoli <luca.ceresoli@...tlin.com>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, 
	Simona Vetter <simona@...ll.ch>, 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>, Hui Pu <Hui.Pu@...ealthcare.com>, 
	Thomas Petazzoni <thomas.petazzoni@...tlin.com>, dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/7] drm/bridge: lock the encoder bridge chain mutex
 during insertion

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@...tlin.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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