linux-kernel - Re: [PATCH v5 04/10] drm/bridge: add documentation of refcounted bridges

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Message-ID: <20250108162434.79268de1@booty>
Date: Wed, 8 Jan 2025 16:24:34 +0100
From: Luca Ceresoli <luca.ceresoli@...tlin.com>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc: Maxime Ripard <mripard@...nel.org>, Simona Vetter <simona@...ll.ch>,
 Inki Dae <inki.dae@...sung.com>, Jagan Teki <jagan@...rulasolutions.com>,
 Marek Szyprowski <m.szyprowski@...sung.com>, Catalin Marinas
 <catalin.marinas@....com>, Will Deacon <will@...nel.org>, Shawn Guo
 <shawnguo@...nel.org>, Sascha Hauer <s.hauer@...gutronix.de>, Pengutronix
 Kernel Team <kernel@...gutronix.de>, Fabio Estevam <festevam@...il.com>,
 Daniel Thompson <danielt@...nel.org>, Andrzej Hajda
 <andrzej.hajda@...el.com>, Jonathan Corbet <corbet@....net>, Paul
 Kocialkowski <contact@...lk.fr>, 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>, Maarten
 Lankhorst <maarten.lankhorst@...ux.intel.com>, Thomas Zimmermann
 <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Hervé Codina <herve.codina@...tlin.com>, Thomas Petazzoni
 <thomas.petazzoni@...tlin.com>, linux-kernel@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, linux-doc@...r.kernel.org, Paul
 Kocialkowski <paul.kocialkowski@...tlin.com>
Subject: Re: [PATCH v5 04/10] drm/bridge: add documentation of refcounted
 bridges

On Tue, 7 Jan 2025 12:35:15 +0200
Dmitry Baryshkov <dmitry.baryshkov@...aro.org> wrote:

> On Mon, Jan 06, 2025 at 03:49:48PM +0100, Maxime Ripard wrote:
> > On Mon, Jan 06, 2025 at 02:24:00PM +0200, Dmitry Baryshkov wrote:  
> > > On Mon, 6 Jan 2025 at 12:39, Maxime Ripard <mripard@...nel.org> wrote:  
> > > >
> > > > Hi,
> > > >
> > > > Most of these comments affect your earlier patches, but let's work on
> > > > the API-level view.
> > > >
> > > > On Tue, Dec 31, 2024 at 11:39:58AM +0100, Luca Ceresoli wrote:  
> 
> > > >         if (IS_ERR(priv))
> > > >            return ERR_PTR(priv);
> > > >         bridge = &priv->bridge;
> > > >
> > > >         ...
> > > >
> > > >         drm_bridge_add(bridge);
> > > > }
> > > >
> > > > Would work just as well.
> > > >
> > > > I also don't think we need explicit (at least for the common case)
> > > > drm_bridge_get and drm_bridge_put calls for bridge users.
> > > > drm_bridge_attach and drm_bridge_detach can get/put the reference
> > > > directly.  
> > > 
> > > As I wrote previously, I think drm_bridge_attach() might be too late for that.
> > > It sounds like drm_of_get_panel_or_bridge() and of_drm_find_bridge
> > > should increment the refcount, possibly adding a devres action to put
> > > the reference.  
> > 
> > We probably need both. drm_bridge_attach adds the bridge pointer to the
> > encoder bridge_chain list, so if we had something like
> > 
> > bridge = drm_of_find_bridge();
> > drm_bridge_attach(encoder, bridge);
> > drm_bridge_put(bridge);
> > 
> > We could have a dangling pointer.  
> 
> Yes... So, both drm_bridge_attach and drm_of_find_bridge() should take
> the refcount.

Exactly, just like drm_bridge_add/remove() do.

> Just as an idea, it might be nice to add refcounting to bridges_show(),
> so that we can easily verify that refcounting works correctly.

Good point, cheap and useful, adding that too!

Luca

-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com