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

Luca Ceresoli
2025-02-26T15:22:39Z

Hi Maxime,

On Tue, 11 Feb 2025 14:10:50 +0100
Maxime Ripard <mripard@kernel.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@kernel.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