On Mon, 6 Jan 2025 at 12:39, Maxime Ripard <mripard@xxxxxxxxxx> 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(). > > 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. > 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. > And we'll also need some flag in drm_bridge to indicate that the device > is gone, similar to what drm_dev_enter()/drm_dev_exit() provides, > because now your bridge driver sticks around for much longer than your > device so the expectation that your device managed resources (clocks, > registers, etc.) are always going to be around. -- With best wishes Dmitry