Message-ID: <20250210-spirited-premium-rabbit-34e1cc@houat>
Date: Mon, 10 Feb 2025 19:16:40 +0100
From: Maxime Ripard <mripard@...nel.org>
To: Luca Ceresoli <luca.ceresoli@...tlin.com>
Cc: Dmitry Baryshkov <dmitry.baryshkov@...aro.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>, 
	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, Feb 10, 2025 at 06:12:03PM +0100, Luca Ceresoli wrote:
> Hello Maxime, Dmitry,
> 
> On Mon, 10 Feb 2025 16:23:44 +0200
> Dmitry Baryshkov <dmitry.baryshkov@...aro.org> wrote:
> 
> > 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.  
> 
> The idea you proposed was to handle the issues current panel bridge
> code adds to the hotplug work by adding a .destroy callback and some
> more devm magic. I explored the idea but even after some clarifications
> from you it still didn't appear clearly doable and correct to me. And
> even in the case it were perfectly correct and doable, it is based on
> adding more complexity and "magic" on top of a topic that is already
> hard to understand: panel_bridge lifetime.

Not really, no. I told you several time that you shouldn't deal with
panels yet.

> So I opted for the other way: rework panel_bridge code so its lifetime
> is clear and as one would expect (panel_bridge lifetime == panel
> lifetime).
> 
> Possibly more work for me, but now it's done and it's in these patches
> so why waiting?

No, it's not done. You have the same issue with panels than you are
trying to fix with bridges: it's allocated through devm so they'll get
destroyed too soon. The panel_bridge might work fine now, but the panel
won't.

So it's more work, more scope-creep, and more discussions. For example,
I'm really not convinced on moving the drm_panel code under bridge.

Splitting it up will allow you to at least merge the parts that are
somewhat agreed upon. But do however you want :)

Maxime

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