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Mon, 29 Sep 2025 16:53:22 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bootlin.com; s=dkim; t=1759157609; h=from:subject:date:message-id:to:cc:mime-version:content-type: content-transfer-encoding:in-reply-to:references; bh=3pVpwTaco7yFHAe69ZBiTvXHcvn2qIWppyNoGTqmjTQ=; b=ca/E6wAhdfUjViADrg2w0zrDIpOzyj9i+1rFiYXvEsOH2M68QaaZA+3+H3QHiodUZFLosf pWmH4K7NLln+RKv4S+dpr39P3DQVy6c5AVOHnAVM3hcytr4PfOtnjoKRJ8eCZv2Zz55By0 C0l5ai8pg7IqUWqXQcCUb3KdIcUAVkQLwlJlXHInfvNB2TK/BLIXqj7+d+PrMLy5Pmyp8N LZU+xgQWrqpTBXtMYLK1a9+BvMqvCnzCXrMBQZISA1PB6yWUrnksSZT8dRTB8wBy5oamt/ 7VnR4UIL3oSEFH5jZ/O0FsKkGD5I0sy1SKItyLw6WlT4YTxyRolMQO0tKOOj6A== Date: Mon, 29 Sep 2025 16:53:21 +0200 From: Luca Ceresoli To: Maxime Ripard Cc: Maarten Lankhorst , Thomas Zimmermann , David Airlie , Simona Vetter , Andrzej Hajda , Neil Armstrong , Robert Foss , Laurent Pinchart , Jonas Karlman , Jernej Skrabec , Hui Pu , Thomas Petazzoni , dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 3/7] drm/bridge: lock the encoder bridge chain mutex during insertion Message-ID: <20250929165321.28b8bbcd@booty> In-Reply-To: <20250929-enigmatic-delicate-mussel-f36b89@houat> References: <20250926-drm-bridge-alloc-encoder-chain-mutex-v1-0-23b62c47356a@bootlin.com> <20250926-drm-bridge-alloc-encoder-chain-mutex-v1-3-23b62c47356a@bootlin.com> <20250929-enigmatic-delicate-mussel-f36b89@houat> Organization: Bootlin X-Mailer: Claws Mail 4.3.1 (GTK 3.24.49; x86_64-redhat-linux-gnu) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Last-TLS-Session-Version: TLSv1.3 On Mon, 29 Sep 2025 14:46:18 +0200 Maxime Ripard wrote: > 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 > > --- > > 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? Because this is what drm_bridge_attach() says O:-) > * drm_bridge_attach - attach the bridge to an encoder's chain ... > * 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. Also, it's what the code does. > Also, it looks entirely unrelated to the rest of the patch. Sure, I can split it. It is also redundant given that's repeating what drm_bridge_attach() says. I wrote this comment for future people looking at this code. If _attach() takes a lock and _detach() does not, it could look like a potential mistake, and someone could spend precious hours in trying to fix it. Maybe replace with: /* Must be called with the encoder bridge chain locked */ ? Luca -- Luca Ceresoli, Bootlin Embedded Linux and Kernel engineering https://bootlin.com