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Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98 #2 (Red Hat Linux)) id 1tvAcY-0000000BRbx-39wv; Thu, 20 Mar 2025 07:47:18 +0000 Received: from relay7-d.mail.gandi.net ([2001:4b98:dc4:8::227]) by bombadil.infradead.org with esmtps (Exim 4.98 #2 (Red Hat Linux)) id 1tvAWw-0000000BQwH-29nj for linux-arm-kernel@lists.infradead.org; Thu, 20 Mar 2025 07:41:32 +0000 Received: by mail.gandi.net (Postfix) with ESMTPSA id 2E3C84436F; Thu, 20 Mar 2025 07:41:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bootlin.com; s=gm1; t=1742456486; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=fmPdfR67O2wcc6/RI+Tc9AyY4rjn6JD2dOIfEvFlJnQ=; b=Fajab6ofQy9GQKOL1lDcWQfm/0zyroqQRlF4ltPWxT9DK3Kz2DXP4fiTBAuRaWBovUj2Cq 7Xs65a44MCIogTqhY9yE+Pbjp+uATyM3uGz8ZXBovdasbw4nY74FcibRBPw15cVC5B//HL HoE79L729ZJJC2p0S2zTI6MKh5g6VFJAR5pV3gUeTNOpVL9Bk7zrb72rG67qGTRwhtUEmP doGblm+y8jR0MsywXUhEFuv1zt7JHUkmCFEJPi/JlSYAhD2EBPzsvfS9haVYm4wtkorHyP nZ64j9pnqZIyUM2/BSBuYi5mvWIKq1K7tmjyz/E7reZ1jTkV0dPNeOZ858B90w== Date: Thu, 20 Mar 2025 08:41:18 +0100 From: Luca Ceresoli To: Maxime Ripard Cc: Andrzej Hajda , Neil Armstrong , Robert Foss , Laurent Pinchart , Jonas Karlman , Jernej Skrabec , Maarten Lankhorst , Thomas Zimmermann , David Airlie , Simona Vetter , Marek Vasut , Stefan Agner , Shawn Guo , Sascha Hauer , Pengutronix Kernel Team , Fabio Estevam , Inki Dae , Jagan Teki , Marek Szyprowski , Thomas Petazzoni , Anusha Srivatsa , Paul Kocialkowski , Dmitry Baryshkov , =?UTF-8?B?SGVydsOp?= Codina , Hui Pu , dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, imx@lists.linux.dev, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v7 00/11] drm/bridge: add devm_drm_bridge_alloc() with bridge refcount Message-ID: <20250320084118.347bafc9@booty> In-Reply-To: <20250319-stylish-lime-mongoose-0a18ad@houat> References: <20250314-drm-bridge-refcount-v7-0-152571f8c694@bootlin.com> <20250314-daft-woodoo-cheetah-e029c5@houat> <20250317155607.68cff522@booty> <20250319-stylish-lime-mongoose-0a18ad@houat> Organization: Bootlin X-Mailer: Claws Mail 4.3.1 (GTK 3.24.43; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-GND-State: clean X-GND-Score: -100 X-GND-Cause: gggruggvucftvghtrhhoucdtuddrgeefvddrtddtgddugeejieegucetufdoteggodetrfdotffvucfrrhhofhhilhgvmecuifetpfffkfdpucggtfgfnhhsuhgsshgtrhhisggvnecuuegrihhlohhuthemuceftddunecusecvtfgvtghiphhivghnthhsucdlqddutddtmdenucfjughrpeffhffvvefukfgjfhhoofggtgfgsehtjeertdertddvnecuhfhrohhmpefnuhgtrgcuvegvrhgvshholhhiuceolhhutggrrdgtvghrvghsohhlihessghoohhtlhhinhdrtghomheqnecuggftrfgrthhtvghrnhepgeelffefgfehhfdtvdefueefieevkefggfelkeeiudetkeektedvhedukefgvddvnecuffhomhgrihhnpegsohhothhlihhnrdgtohhmnecukfhppedvrgdtvdemieejtdemvddtvddtmegvrgdtudemsggvgedumeelhegvjeemfeegfeemledufegvnecuvehluhhsthgvrhfuihiivgeptdenucfrrghrrghmpehinhgvthepvdgrtddvmeeijedtmedvtddvtdemvggrtddumegsvgegudemleehvgejmeefgeefmeeludefvgdphhgvlhhopegsohhothihpdhmrghilhhfrhhomheplhhutggrrdgtvghrvghsohhlihessghoohhtlhhinhdrtghomhdpnhgspghrtghpthhtohepfedtpdhrtghpthhtohepmhhrihhprghrugeskhgvrhhnvghlrdhorhhgpdhrtghpthhtoheprghnughriigvjhdrhhgrjhgurgesihhnthgvlhdrtghomhdprhgtphhtthhopehnvghilhdrrghrmhhsthhrohhngheslhhinhgrr hhordhorhhgpdhrtghpthhtoheprhhfohhssheskhgvrhhnvghlrdhorhhgpdhrtghpthhtohepnfgruhhrvghnthdrphhinhgthhgrrhhtsehiuggvrghsohhnsghorghrugdrtghomhdprhgtphhtthhopehjohhnrghssehkfihisghoohdrshgvpdhrtghpthhtohepjhgvrhhnvghjrdhskhhrrggsvggtsehgmhgrihhlrdgtohhmpdhrtghpthhtohepmhgrrghrthgvnhdrlhgrnhhkhhhorhhstheslhhinhhugidrihhnthgvlhdrtghomh X-GND-Sasl: luca.ceresoli@bootlin.com X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250320_004131_204416_A9F575EF X-CRM114-Status: GOOD ( 61.10 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Hello Mazime, On Wed, 19 Mar 2025 17:16:53 +0100 Maxime Ripard wrote: > On Mon, Mar 17, 2025 at 03:56:07PM +0100, Luca Ceresoli wrote: > > Hello Maxime, > > > > On Fri, 14 Mar 2025 19:21:01 +0100 > > Maxime Ripard wrote: > > > > > Hi, > > > > > > On Fri, Mar 14, 2025 at 11:31:13AM +0100, Luca Ceresoli wrote: > > > > This series improves the way DRM bridges are allocated and > > > > initialized and makes them reference-counted. The goal of reference > > > > counting is to avoid use-after-free by drivers which got a pointer > > > > to a bridge and keep it stored and used even after the bridge has > > > > been deallocated. > > > > > > > > The overall goal is supporting Linux devices with a DRM pipeline > > > > whose final components can be hot-plugged and hot-unplugged, > > > > including one or more bridges. For more details see the big picture > > > > [0]. > > > > > > > > DRM bridge drivers will have to be adapted to the new API, which is > > > > pretty simple for most cases. Refcounting will have to be adopted > > > > on the two sides: all functions returning a bridge pointer and all > > > > code obtaining such a pointer. This series has just an overview of > > > > some of those conversions, because for now the main goal is to > > > > agree on the API. > > > > > > > > Series layout: > > > > > > > > 1. Add the new API and refcounting: > > > > > > > > drm/bridge: add devm_drm_bridge_alloc() > > > > drm/bridge: add support for refcounting > > > > > > > > 2. get/put the reference in basic operations in the bridge core: > > > > > > > > drm/bridge: get/put the bridge reference in > > > > drm_bridge_add/remove() drm/bridge: get/put the bridge reference in > > > > drm_bridge_attach/detach() > > > > > > > > 3. as an example of changes for bridge consumers, get a reference > > > > for the bridge returned by drm_bridge_chain_get_first_bridge(), > > > > have it put by all callers (all users will be covered later on > > > > separately): > > > > > > > > drm/bridge: add a cleanup action for scope-based > > > > drm_bridge_put() invocation drm/bridge: get the bridge returned by > > > > drm_bridge_chain_get_first_bridge() drm/mxsfb: put the bridge > > > > returned by drm_bridge_chain_get_first_bridge() drm/atomic-helper: > > > > put the bridge returned by drm_bridge_chain_get_first_bridge() > > > > drm/probe-helper: put the bridge returned by > > > > drm_bridge_chain_get_first_bridge() > > > > > > > > 4. convert a few bridge drivers (bridge providers) to the new API: > > > > > > > > drm/bridge: ti-sn65dsi83: use dynamic lifetime management > > > > drm/bridge: samsung-dsim: use dynamic lifetime management > > > > > > > > This work was formerly a part of my v6 DRM bridge hotplug > > > > series[0], now split as a standalone series with many improvements, > > > > hence the "v7" version number. > > > > > > Except for one patch where I had comments, I think the series is in > > > excellent shape. We're still missing a couple of things to close this > > > topic though: > > > > > > - Converting the other bridge iterators/accessors to take / put the > > > references > > > > I sent a couple in this series as you had asked, to show how conversion > > looks like. But I have a large part of this conversion partially done > > already, and it is the largest part of the refcounting work in terms of > > touched files due to the large number of drivers using the iterators > > and accessors. Here are the functions to convert: > > > > A) drm_bridge_chain_get_first_bridge > > B) drm_bridge_get_prev_bridge > > C) drm_bridge_get_next_bridge > > D) drm_for_each_bridge_in_chain > > E) drm_bridge_connector_init > > F) of_drm_find_bridge > > > > A) is present in this series as an example but I don't think it should > > be applied until all bridge drivers are converted to > > drm_bridge_alloc(). Otherwise for not-yet-converted bridge drivers we'd > > have drm_bridge_get/put() operating on an uninitialized kref, and > > __drm_bridge_free() called on non-refcounted bridges, so I think we'd > > see fireworks. > > > > In the previous iteration I used drm_bridge_is_refcounted() in > > drm_bridge_get/put() to allow a progressive migration, but if we want > > to convert everything in a single run we need to first convert all > > bridges to drm_bridge_alloc() and then convert all accessors. > > > > The same reasoning applies to patches 3-4 which add get/put to > > drm_bridge_add/remove() and _attach/detach(). > > Agreed. > > > B) to E) are ready in my work branch, about 20 patches in total. > > Indeed item E) is a special case but it is handled there too. > > > > Item F) is the beast, because of the reverse call graph of > > of_drm_find_bridge() which includes drm_of_find_panel_or_bridge() and > > then *_of_get_bridge(), each having a few dozen callers, and leading > > to the panel_bridge topic. I have converted maybe half of the users of > > accessors in F), it's 35 patches but it's the easy half and I still need > > to tackle to hardest ones. > > One thing to keep in mind is that, while it's not correct, if the bridge > has been allocated with devm_drm_bridge_alloc, it's not worse either. If > you're not getting a reference to your pointer, the point is buggy, > sure, but it's just as buggy as before, and in the same situations. > > So we can make that gradually if it's more convenient. I see your point, right. And definitely doing it in gradually will help. > One way to solve it would be that, for example, of_drm_find_bridge is > oddly named according to our convention and it would make more sense to > be called drm_of_find_bridge(). > > So maybe we can just create drm_of_find_bridge() that takes a reference, > make of_drm_find_bridge() deprecated in favour of drm_of_find_bridge(), > add a TODO, and call it a day. People will gradually switch to the new > API over time. Thanks for the suggestion, I will certainly try this way when I get back to the accessor conversion work. While increasing the number of deprecated functions is not great, I'm starting to suspect a conversion of _all_ the of_drm_find_bridge() users at once is not realistic. > > > - Documenting somewhere (possibly in drm_bridge_init?) that it > > > really shouldn't be used anymore > > > > I'm afraid there is no drm_bridge_init(), bridge drivers just do > > [devm_]kzalloc and set fields explicitly. So I don't think there is a > > place to document this. > > Oh, right. > > Then, drm_bridge_add() would be a good candidate too to mention that > bridges must be allocated using devm_drm_bridge_alloc(). Sure. I'll add such a comment. > > Overall, I think this could be the path forward, let me know if > > youthink it should be done differently: > > > > A. have patches 1 and 2 of this series applied > > (why not, even patches 10-11) > > I had some comments on patch 2, but it's ok for me on principle. Sorry, my wording was not clear. I really meant "have patches 1 and 2 of this series applied after the improvements proposed". So hopefully the v8 I'm sending today-ish. I'm going to send only patches 1, 2, 10 and 11. The other patches are not meant to be merged now, so I'm resending them when appropriate. Luca -- Luca Ceresoli, Bootlin Embedded Linux and Kernel engineering https://bootlin.com