Re: [PATCH v5 04/10] drm/bridge: add documentation of refcounted bridges

[Luca Ceresoli <luca.ceresoli@bootlin.com>]

On Tue, 7 Jan 2025 12:35:15 +0200
Dmitry Baryshkov <dmitry.baryshkov@linaro.org> wrote:

> On Mon, Jan 06, 2025 at 03:49:48PM +0100, Maxime Ripard wrote:
> > On Mon, Jan 06, 2025 at 02:24:00PM +0200, Dmitry Baryshkov wrote:  
> > > On Mon, 6 Jan 2025 at 12:39, Maxime Ripard <mripard@kernel.org> wrote:  
> > > >
> > > > Hi,
> > > >
> > > > Most of these comments affect your earlier patches, but let's work on
> > > > the API-level view.
> > > >
> > > > On Tue, Dec 31, 2024 at 11:39:58AM +0100, Luca Ceresoli wrote:  
> 
> > > >         if (IS_ERR(priv))
> > > >            return ERR_PTR(priv);
> > > >         bridge = &priv->bridge;
> > > >
> > > >         ...
> > > >
> > > >         drm_bridge_add(bridge);
> > > > }
> > > >
> > > > Would work just as well.
> > > >
> > > > I also don't think we need explicit (at least for the common case)
> > > > drm_bridge_get and drm_bridge_put calls for bridge users.
> > > > drm_bridge_attach and drm_bridge_detach can get/put the reference
> > > > directly.  
> > > 
> > > As I wrote previously, I think drm_bridge_attach() might be too late for that.
> > > It sounds like drm_of_get_panel_or_bridge() and of_drm_find_bridge
> > > should increment the refcount, possibly adding a devres action to put
> > > the reference.  
> > 
> > We probably need both. drm_bridge_attach adds the bridge pointer to the
> > encoder bridge_chain list, so if we had something like
> > 
> > bridge = drm_of_find_bridge();
> > drm_bridge_attach(encoder, bridge);
> > drm_bridge_put(bridge);
> > 
> > We could have a dangling pointer.  
> 
> Yes... So, both drm_bridge_attach and drm_of_find_bridge() should take
> the refcount.

Exactly, just like drm_bridge_add/remove() do.

> Just as an idea, it might be nice to add refcounting to bridges_show(),
> so that we can easily verify that refcounting works correctly.

Good point, cheap and useful, adding that too!

Luca

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