Message-ID: <CAA8EJppyDiAeH8bm-rdCUsGoyVOrp=4AvrYLJ-=5BTLtmnP11g@mail.gmail.com>
Date: Mon, 10 Feb 2025 16:23:44 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Maxime Ripard <mripard@...nel.org>
Cc: Luca Ceresoli <luca.ceresoli@...tlin.com>, 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>, 
	Sam Ravnborg <sam@...nborg.org>, Boris Brezillon <bbrezillon@...nel.org>, 
	Nicolas Ferre <nicolas.ferre@...rochip.com>, 
	Alexandre Belloni <alexandre.belloni@...tlin.com>, Claudiu Beznea <claudiu.beznea@...on.dev>, 
	Jessica Zhang <quic_jesszhan@...cinc.com>, 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, 
	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

On Mon, 10 Feb 2025 at 14:31, Maxime Ripard <mripard@...nel.org> wrote:
>
> On Fri, Feb 07, 2025 at 09:54:06PM +0200, Dmitry Baryshkov wrote:
> > On Fri, Feb 07, 2025 at 12:47:51PM +0100, Maxime Ripard wrote:
> > > Hi,
> > >
> > > On Thu, Feb 06, 2025 at 07:14:29PM +0100, Luca Ceresoli wrote:
> > > > DRM bridges are currently considered as a fixed element of a DRM card, and
> > > > thus their lifetime is assumed to extend for as long as the card
> > > > exists. New use cases, such as hot-pluggable hardware with video bridges,
> > > > require DRM bridges to be added and removed to a DRM card without tearing
> > > > the card down. This is possible for connectors already (used by DP MST), so
> > > > add this possibility to DRM bridges as well.
> > > >
> > > > Implementation is based on drm_connector_init() as far as it makes sense,
> > > > and differs when it doesn't. A difference is that bridges are not exposed
> > > > to userspace, hence struct drm_bridge does not embed a struct
> > > > drm_mode_object which would provide the refcount. Instead we add to struct
> > > > drm_bridge a refcount field (we don't need other struct drm_mode_object
> > > > fields here) and instead of using the drm_mode_object_*() functions we
> > > > reimplement from those functions the few lines that drm_bridge needs for
> > > > refcounting.
> > > >
> > > > Also add a new devm_drm_bridge_alloc() macro to allocate a new refcounted
> > > > bridge.
> > > >
> > > > Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>
> > >
> > > So, a couple of general comments:
> > >
> > > - I've said it a couple of times already, but I really think you're
> > >   making it harder than necessary for you here. This (and only this!)
> > >   should be the very first series you should be pushing. The rest can
> > >   only ever work if that work goes through, and it's already hard enough
> > >   as it is. So, split that patch into a series of its own, get that
> > >   merged, and then we will be able to deal with panels conversion and
> > >   whatever. That's even more true with panels since there's ongoing work
> > >   that will make it easier for you too. So the best thing here is
> > >   probably to wait.
> >
> > Luca and I had a quick chat on this during FOSDEM. I really think that
> > panel (part of the) series can go in first as it fixes a very well known
> > bug _and_ allows a pretty good cleanup to a whole set of drivers.
>
> I don't necessarily disagree on principle, but if you state that it can
> get first, and fixes a known problem (which one?), then it should be a
> separate, standalone, series.

A problem of panel bridges having the wrong lifetime because of devm_
attachment to a wrong device and so either being kept for too long or
being destroyed too early.

>
> Ever-expanding features are bad for both the reviewers and the
> contributors, even more so when the discussion happens off-list.
>
> > With all those panel / bridge wrappers gone we should be able to see a
> > clearer picture of what individual drivers are doing. In other words,
> > which memory and which code actually hosts and uses internal
> > 'next_bridge' reference.
> >
> > > - This patch really needs to be split into several patches, something
> > >   along the lines of:
> > >
> > >   + Creating devm_drm_bridge_alloc()
> > >   + Adding refcounting
> > >   + Taking the references in all the needed places
> > >   + Converting a bunch of drivers
> >
> > The last two parts seem troublematic to me, but, I must admit, I didn't
> > spend so much time reviewing all drm_bridge usage patterns.
>
> Why? the third one is already done by that patch, the fourth can
> relatively easily be done using coccinelle.

I have doubts about cocci. It doesn't have a way to know, what is the
lifetime of the references to the reference-holding memory. Maybe I'm
missing a point there.

-- 
With best wishes
Dmitry

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