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Received: from AM7PR04MB7046.eurprd04.prod.outlook.com (2603:10a6:20b:113::22) by GV2PR04MB11836.eurprd04.prod.outlook.com (2603:10a6:150:2fe::20) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9700.12; Thu, 12 Mar 2026 09:14:22 +0000 Received: from AM7PR04MB7046.eurprd04.prod.outlook.com ([fe80::4609:64af:8a4b:fd64]) by AM7PR04MB7046.eurprd04.prod.outlook.com ([fe80::4609:64af:8a4b:fd64%3]) with mapi id 15.20.9700.010; Thu, 12 Mar 2026 09:14:27 +0000 Message-ID: Date: Thu, 12 Mar 2026 17:15:33 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v11 2/3] drm/bridge: imx: Add i.MX93 parallel display format configuration support To: Marco Felsch , Luca Ceresoli Cc: Rob Herring , Krzysztof Kozlowski , Conor Dooley , Shawn Guo , Sascha Hauer , Pengutronix Kernel Team , Fabio Estevam , Peng Fan , Andrzej Hajda , Neil Armstrong , Robert Foss , Laurent Pinchart , Jonas Karlman , Jernej Skrabec , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , Frank Li , devicetree@vger.kernel.org, imx@lists.linux.dev, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org References: <20260303-v6-18-topic-imx93-parallel-display-v11-0-1b03733c8461@pengutronix.de> <20260303-v6-18-topic-imx93-parallel-display-v11-2-1b03733c8461@pengutronix.de> <33e09cc9-69bd-468b-8361-b92a50686f74@nxp.com> From: Liu Ying Content-Language: en-US In-Reply-To: Content-Type: text/plain; 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Np at all, it's good to go out to meet people sometimes :) > > On 26-03-10, Luca Ceresoli wrote: >> Hi Liu, Marco, >> >> On Tue Mar 10, 2026 at 3:57 AM CET, Liu Ying wrote: >>> Hi Marco, Luca, >>> >>> On Tue, Mar 03, 2026 at 11:34:27AM +0100, Marco Felsch wrote: >>> >>> [...] >>> >>>> + next_bridge = devm_drm_of_get_bridge(dev, dev->of_node, 1, 0); >>>> + if (IS_ERR(next_bridge)) >>>> + return dev_err_probe(dev, PTR_ERR(next_bridge), >>>> + "failed to get next bridge\n"); >>>> + pdfc->dev = dev; >>>> + pdfc->bridge.of_node = dev->of_node; >>>> + pdfc->bridge.type = DRM_MODE_CONNECTOR_DPI; >>>> + pdfc->bridge.next_bridge = next_bridge; >>> >>> When I was reviewing another patch[1], I was aware of the necessity of >>> calling drm_bridge_get() for next_bridge to balance the next bridge's >>> refcount put from __drm_bridge_free() for this bridge. I'd be good if >>> Luca may confirm this is correct. Sorry for bringing this up late. >> >> Indeed you have a good point. > > At which stage did you faced this issue? During driver probe, because of > EPROBE_DEFER? I just want to balance the get/put for the next bridge's refcount by calling drm_bridge_get() for next_bridge, as I said above. There could be some way to trigger some particular Use-After-Free issues if you don't do that. I did take some time today to try to trigger UAF, but no luck, though I found a potential bug in encoder_bridges_show() and generated a fix[1](Cc'ed Marco) when I played with debugfs to read the refcounts. My idea to trigger UAF is to remove imx_lcdif/imx93_pdfc/simple_panel modules when doing pageflips. Dunno if EPROBE_DEFER may trigger UAF. [1] https://lore.kernel.org/all/20260312-drm-misc-next-2026-03-05-fix-encoder-bridges-refcount-v1-1-b9ba3d844732@nxp.com/ > > That's the reason for having the local next_bridge variable since I > faced with the same issue. In other words this driver is correct and > it's on purpose to not assign it directly. Albeit I could/should have > added a comment. What's the issue your faced? How would using next_bridge variable impact? Without knowing these info, I presume that you still need to call drm_bridge_get() for next_bridge, since it's kind of obvious if you take a look at __drm_bridge_free(). > >> After re-checking devm_drm_of_get_bridge(), as I wrote on the other thread >> you pointed to, you should call drm_bridge_get(): >> >> - pdfc->bridge.next_bridge = next_bridge; >> + pdfc->bridge.next_bridge = drm_bridge_get(next_bridge); >> >> Marco, you can keep my R-by if you resend with just this change. >> >> Sorry about the confusion here. >> >> As mention on the other thread, devm_drm_of_get_bridge() is unable to >> support bridge hotplug. So it should be deprecated, but as of now there is >> no alternative. > > Sorry I need a bit more context. What's the issue? How can I trigger the > issue? Why is bridge hotplug required at this stage? Why is only this > bridge affecte by the hotplug issue? Maybe, Luca can comment here. > > Regards, > Marco > > > >> >> Luca >> >> -- >> Luca Ceresoli, Bootlin >> Embedded Linux and Kernel engineering >> https://bootlin.com/ >> > -- Regards, Liu Ying