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

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Message-ID: <20250210181252.5ee028d4@booty>
Date: Mon, 10 Feb 2025 18:12:52 +0100
From: Luca Ceresoli <luca.ceresoli@...tlin.com>
To: Maxime Ripard <mripard@...nel.org>
Cc: Simona Vetter <simona@...ll.ch>, Inki Dae <inki.dae@...sung.com>, Jagan
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Subject: Re: [PATCH v6 14/26] drm/bridge: add support for refcounted DRM
 bridges

Hello Maxime,

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?

It can be named drm_bridge_object maybe, as it is somewhat resembling
struct drm_mode_object?

> > +	/**
> > +	 * @refcount: reference count for bridges with dynamic lifetime
> > +	 * (see drm_bridge_init)
> > +	 */
> > +	struct kref refcount;
> > +	/** @is_refcounted: this bridge has dynamic lifetime management */
> > +	bool is_refcounted;
> > +  
> 
> I'm not sure we want to treat both paths separately too. It'll require
> to update most of/all the drivers, but it's not too hard with
> coccinelle:
> 
> virtual patch
> 
> @@
> identifier f;
> identifier b, c, d;
> expression bf;
> type T;
> @@
> 
>  f(...)
>  {
> 	...
> -	T *c;
> +	T *c;
> 	...
> -	c = devm_kzalloc(d, ...);
> +	c = devm_drm_bridge_alloc(d, T, b, bf);
> 	...
> -	c->b.funcs = bf;
> 	...
> 	drm_bridge_add(&c->b);
> 	...
>  }
> 
> We need to add a bit more variations (like kzalloc vs devm_kzalloc,
> drm_bridge_add vs devm_drm_bridge_add, etc.), but it should be a good
> first approximation

Sure, this can be useful, thanks.

Luca

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