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Thu, 9 Apr 2026 11:57:09 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bootlin.com; s=dkim; t=1775728632; h=from:subject:date:message-id:to:cc:mime-version:content-type: content-transfer-encoding:content-language:in-reply-to:references; bh=eEchKB+r7i4/YwY7RKiJKjKP3eW5V6E5trl50E3B2ys=; b=efS0wf+b5LyoTXx6AgeQVMuWcNoHXNjqE40jxSkExUYTGCfkzzZFaO1Gx7bxZqUIMsWjl7 oa1op6wx5+cI881XhwpS4wwWSYamU0GWdstL5ujSrsq25HVcMyMCVMSfhbecfs32vgSGsS 2ZAqBhfG4qFRGd8CKTSEE9KaUcydWLH2z+Y4k1ePW5V95cOeOIzb+loASLu8BvAG8RYDCe ZXOY6HynVnN/yG5Z0vITzrjtK5TlsGB6wPmKrY0Q80S0IyL7qpa5YpVCknj+EdGiEf9Bda /RK35elxDWRwCDfZk+UnQiV0EpcJNNlXiQrXJJVkHpRFULN8rI2cbaM9jpgnmw== Message-ID: <9e517e8d-f944-4934-9f97-07101ec98711@bootlin.com> Date: Thu, 9 Apr 2026 11:57:09 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 6/7] drm/bridge: prevent encoder chain changes while iterating with list_for_each_entry_reverse() To: Luca Ceresoli , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , Andrzej Hajda , Neil Armstrong , Robert Foss , Laurent Pinchart , Jonas Karlman , Jernej Skrabec Cc: Hui Pu , Thomas Petazzoni , dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, Ian Ray References: <20260324-drm-bridge-alloc-encoder-chain-mutex-v5-0-8bf786c5c7e6@bootlin.com> <20260324-drm-bridge-alloc-encoder-chain-mutex-v5-6-8bf786c5c7e6@bootlin.com> Content-Language: en-US From: Louis Chauvet In-Reply-To: <20260324-drm-bridge-alloc-encoder-chain-mutex-v5-6-8bf786c5c7e6@bootlin.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Last-TLS-Session-Version: TLSv1.3 On 3/24/26 9:58 AM, Luca Ceresoli wrote: > These loops in drm_bridge.c iterate over the encoder chain using > list_for_each_entry_reverse(), which does not prevent changes to the bridge > chain while iterating over it. > > Take the encoder chain mutex while iterating to avoid chain changes while > iterating. > > All the "simple" loops are converted. drm_atomic_bridge_chain_pre_enable() > and drm_atomic_bridge_chain_post_disable() are handled by a separate > commit. > > Signed-off-by: Luca Ceresoli > Reviewed-by: Louis Chauvet > --- > > Changes in v3: > - Lock encoder->bridge_chain_mutex directly, no wrappers > > Changes in v2: none > --- > drivers/gpu/drm/drm_bridge.c | 40 ++++++++++++++++++++++------------------ > 1 file changed, 22 insertions(+), 18 deletions(-) > > diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c > index 37feb8143de4..c76f3451af39 100644 > --- a/drivers/gpu/drm/drm_bridge.c > +++ b/drivers/gpu/drm/drm_bridge.c > @@ -808,6 +808,7 @@ void drm_atomic_bridge_chain_disable(struct drm_bridge *bridge, > return; > > encoder = bridge->encoder; > + mutex_lock(&encoder->bridge_chain_mutex); > list_for_each_entry_reverse(iter, &encoder->bridge_chain, chain_node) { > if (iter->funcs->atomic_disable) { > iter->funcs->atomic_disable(iter, state); > @@ -818,6 +819,7 @@ void drm_atomic_bridge_chain_disable(struct drm_bridge *bridge, > if (iter == bridge) > break; > } > + mutex_unlock(&encoder->bridge_chain_mutex); > } > EXPORT_SYMBOL(drm_atomic_bridge_chain_disable); > > @@ -1322,25 +1324,27 @@ int drm_atomic_bridge_chain_check(struct drm_bridge *bridge, > return ret; > > encoder = bridge->encoder; > - list_for_each_entry_reverse(iter, &encoder->bridge_chain, chain_node) { > - int ret; > - > - /* > - * Bus flags are propagated by default. If a bridge needs to > - * tweak the input bus flags for any reason, it should happen > - * in its &drm_bridge_funcs.atomic_check() implementation such > - * that preceding bridges in the chain can propagate the new > - * bus flags. > - */ > - drm_atomic_bridge_propagate_bus_flags(iter, conn, > - crtc_state->state); > - > - ret = drm_atomic_bridge_check(iter, crtc_state, conn_state); > - if (ret) > - return ret; > + scoped_guard(mutex, &encoder->bridge_chain_mutex) { > + list_for_each_entry_reverse(iter, &encoder->bridge_chain, chain_node) { > + int ret; > + > + /* > + * Bus flags are propagated by default. If a bridge needs to > + * tweak the input bus flags for any reason, it should happen > + * in its &drm_bridge_funcs.atomic_check() implementation such > + * that preceding bridges in the chain can propagate the new > + * bus flags. > + */ > + drm_atomic_bridge_propagate_bus_flags(iter, conn, > + crtc_state->state); > + > + ret = drm_atomic_bridge_check(iter, crtc_state, conn_state); > + if (ret) > + return ret; > > - if (iter == bridge) > - break; > + if (iter == bridge) > + break; > + } > } > > return 0; >