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

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Message-ID: <20250227-savvy-monumental-toucan-edffe2@houat>
Date: Thu, 27 Feb 2025 10:32:20 +0100
From: Maxime Ripard <mripard@...nel.org>
To: Luca Ceresoli <luca.ceresoli@...tlin.com>
Cc: 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>, Dmitry Baryshkov <dmitry.baryshkov@...aro.org>, 
	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 Wed, Feb 26, 2025 at 03:28:13PM +0100, Luca Ceresoli wrote:
> On Tue, 11 Feb 2025 14:10:50 +0100
> Maxime Ripard <mripard@...nel.org> wrote:
> > On Mon, Feb 10, 2025 at 06:12:52PM +0100, Luca Ceresoli wrote:
> > > On Fri, 7 Feb 2025 12:47:51 +0100
> > > Maxime Ripard <mripard@...nel.org> wrote:
> > >   
> > > > > diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
> > > > > index ad7ba444a13e5ecf16f996de3742e4ac67dc21f1..43cef0f6ccd36034f64ad2babfebea62db1d9e43 100644
> > > > > --- a/include/drm/drm_bridge.h
> > > > > +++ b/include/drm/drm_bridge.h
> > > > > @@ -31,6 +31,7 @@
> > > > >  #include <drm/drm_encoder.h>
> > > > >  #include <drm/drm_mode_object.h>
> > > > >  #include <drm/drm_modes.h>
> > > > > +#include <drm/drm_print.h>
> > > > >  
> > > > >  struct device_node;
> > > > >  
> > > > > @@ -863,6 +864,22 @@ struct drm_bridge {
> > > > >  	const struct drm_bridge_timings *timings;
> > > > >  	/** @funcs: control functions */
> > > > >  	const struct drm_bridge_funcs *funcs;
> > > > > +
> > > > > +	/**
> > > > > +	 * @container_offset: Offset of this struct within the container
> > > > > +	 * struct embedding it. Used for refcounted bridges to free the
> > > > > +	 * embeddeing struct when the refcount drops to zero. Unused on
> > > > > +	 * legacy bridges.
> > > > > +	 */
> > > > > +	size_t container_offset;    
> > > > 
> > > > This shouldn't be in there. You can create an intermediate structure and
> > > > store both pointers for the action to consume.  
> > > 
> > > You mean to store container_offset + refcount + is_refcounted?  
> > 
> > No, I meant for the private structure pointer and the drm_bridge
> > pointer. refcount should be in drm_bridge, and I think is_refcounted
> > should be dropped.
> 
> Storing the container pointer instead of the offset is a good idea, it
> will allow to get rid of is_refcounted: drm_bridge_is_refcounted() can
> just return "container != NULL" instead of "bridge->is_refcounted". So
> far so good.

Again, I don't think the whole is_refcounted thing is a good idea. Once
we have the right API, we should convert all bridges to the new
allocation and assume that they are refcounted.

> I'm not sure about the intermediate struct you have in mind though.
> 
> Do you mean:
> 
> struct drm_bridge_pointers {
>     struct drm_bridge *bridge;
>     void              *container;
> }
> 
> ?

Yes

> If that's what you mean, should it be embedded in drm_struct or
> allocated separately?

Separately, but still as part of the bridge allocation function.

> If you mean to embed that struct in drm_bridge, then I the drm_bridge
> pointer inside the intermediate struct would be useless.
> 
> If instead you mean to embed it in drm_struct: I'm not sure I see much
> benefit except maybe not exposing the container pointer to drm_bridge
> users, but I see a drawbacks: at the last put we need to find the
> container pointer to free from a struct kref pointer, which can work
> only if the container pointer is in the same struct as struct kref.

Yeah, that's true. Storing the container pointer in drm_bridge makes
sense to solve this.

I'm still not sure why we need the container offset though: if we have a
bridge and container pointer, then the offset is bridge - container, so
there's no point in storing it, right?

Maxime

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