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

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Message-ID: <20250226152813.4a1ad218@booty>
Date: Wed, 26 Feb 2025 15:28:13 +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

Hi Maxime,

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:
> > 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?  
> 
> 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.

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;
}

?

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

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.

Additionally, the consuming action for that struct just needs a
drm_bridge pointer:

     static void drm_bridge_put_void(void *data)
     {
	struct drm_bridge *bridge = (struct drm_bridge *)data;

	drm_bridge_put(bridge);
     }

Can you clarify this? I'd love to have this cleanup in the next
iteration.

Luca

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