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

[Maxime Ripard <mripard@kernel.org>]

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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:
> > > + * When using refcounted mode, the driver should allocate ``struct
> > > + * my_bridge`` using regular allocation (as opposed to ``devm_`` or
> > > + * ``drmm_`` allocation), call drm_bridge_init() immediately afterwards to
> > > + * transfer lifecycle management to the DRM bridge core, and implement a
> > > + * ``.destroy`` function to deallocate the ``struct my_bridge``, as in this
> > > + * example::
> > > + *
> > > + *     static void my_bridge_destroy(struct drm_bridge *bridge)
> > > + *     {
> > > + *         kfree(container_of(bridge, struct my_bridge, bridge));
> > > + *     }
> > > + *
> > > + *     static const struct drm_bridge_funcs my_bridge_funcs = {
> > > + *         .destroy = my_bridge_destroy,
> > > + *         ...
> > > + *     };
> > > + *
> > > + *     static int my_bridge_probe(...)
> > > + *     {
> > > + *         struct my_bridge *mybr;
> > > + *         int err;
> > > + *
> > > + *         mybr = kzalloc(sizeof(*mybr), GFP_KERNEL);
> > > + *         if (!mybr)
> > > + *             return -ENOMEM;
> > > + *
> > > + *         err = drm_bridge_init(dev, &mybr->bridge, &my_bridge_funcs);
> > > + *         if (err)
> > > + *             return err;
> > > + *
> > > + *         ...
> > > + *         drm_bridge_add();
> > > + *         ...
> > > + *     }
> > > + *
> > > + *     static void my_bridge_remove()
> > > + *     {
> > > + *         struct my_bridge *mybr = ...;
> > > + *         drm_bridge_remove(&mybr->bridge);
> > > + *         // ... NO kfree here!
> > > + *     }
> >
> > I'm a bit worried there, since that API is pretty difficult to get
> > right, and we don't have anything to catch bad patterns.
> >
> > Let's take a step back. What we're trying to solve here is:
> >
> >   1) We want to avoid any dangling pointers to a bridge if the bridge
> >      device is removed.
> >
> >   2) To do so, we need to switch to reference counted allocations and
> >      pointers.
> >
> >   3) Most bridges structures are allocated through devm_kzalloc, and they
> >      one that aren't are freed at remove time anyway, so the allocated
> >      structure will be gone when the device is removed.
> >
> >   4) To properly track users, each user that will use a drm_bridge needs
> >      to take a reference.
> 
> 5) Handle the disappearing next_bridge problem: probe() function gets
> a pointer to the next bridge, but then for some reasons (e.g. because
> of the other device being removed or because of some probe deferral)
> the next_bridge driver gets unbdound and the next_bridge becomes
> unusable before a call to drm_bridge_attach().

Oh, right. We need to plumb it in drm_of_find_bridge somehow too.

> > AFAIU, the destroy introduction and the on-purpose omission of kfree in
> > remove is to solve 3.
> >
> > Introducing a function that allocates the drm_bridge container struct
> > (like drmm_encoder_alloc for example), take a reference, register a devm
> > kfree action, and return the pointer to the driver structure would solve
> > that too pretty nicely.
> >
> > So, something like:
> >
> >
> > struct driver_priv {
> >        struct drm_bridge bridge;
> >
> >        ...
> > }
> >
> > static int driver_probe(...)
> > {
> >         struct driver_priv *priv;
> >         struct drm_bridge *bridge;
> >
> >         ....
> >
> >         priv = devm_drm_bridge_alloc(dev, struct driver_priv, bridge);
> 
> Ah... And devm-cleanup will just drop a reference to that data,
> freeing it when all refs are cleaned? Nice idea.

Yup.

> >         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.

Maxime

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