linux-kernel - Re: [PATCH v6 14/26] drm/bridge: add support for refcounted DRM bridges

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Message-ID: <20250210181244.0e3e9189@booty>
Date: Mon, 10 Feb 2025 18:12:44 +0100
From: Luca Ceresoli <luca.ceresoli@...tlin.com>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>, Maxime Ripard
 <mripard@...nel.org>
Cc: Simona Vetter <simona@...ll.ch>, Inki Dae <inki.dae@...sung.com>, Jagan
 Teki <jagan@...rulasolutions.com>, Marek Szyprowski
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 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>,
 Sam Ravnborg <sam@...nborg.org>, Boris Brezillon <bbrezillon@...nel.org>,
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 <alexandre.belloni@...tlin.com>, Claudiu Beznea <claudiu.beznea@...on.dev>,
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 <thomas.petazzoni@...tlin.com>, linux-kernel@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, linux-doc@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, Paul Kocialkowski
 <paul.kocialkowski@...tlin.com>
Subject: Re: [PATCH v6 14/26] drm/bridge: add support for refcounted DRM
 bridges

Hi Maxime, Dmitry

On Fri, 7 Feb 2025 21:54:06 +0200
Dmitry Baryshkov <dmitry.baryshkov@...aro.org> wrote:

> > > +/* Internal function (for refcounted bridges) */
> > > +void __drm_bridge_free(struct kref *kref)
> > > +{
> > > +	struct drm_bridge *bridge = container_of(kref, struct drm_bridge, refcount);
> > > +	void *container = ((void *)bridge) - bridge->container_offset;
> > > +
> > > +	DRM_DEBUG("bridge=%p, container=%p FREE\n", bridge, container);  
> > 
> > Pointers are not really useful to track here, since they are obfuscated
> > most of the time. Using the bridge device name would probably be better
> > (or removing the SHOUTING DEBUG entirely :))  
> 
> bridge device name or bridge funcs (I opted for the latter for the
> debugfs file)

These DRM_DEBUG()s proved extremely useful exactly because of the
pointer. This is because when using hotplug one normally has the same
device added and removed multiple times, and so the device name or
bridge funcs is always the same, preventing from understanding which
instance is leaking, or being freed, get, put, etc.

Do you think this is a sufficient motivation to keep it?

Luca

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