Hello Maxime,
thanks for the very prompt feedback!
On Fri, 14 Mar 2025 19:04:40 +0100
Maxime Ripard <mripard@xxxxxxxxxx> wrote:
> On Fri, Mar 14, 2025 at 11:31:15AM +0100, Luca Ceresoli wrote:
> > DRM bridges are currently considered as a fixed element of a DRM card, and
> > thus their lifetime is assumed to extend for as long as the card
> > exists. New use cases, such as hot-pluggable hardware with video bridges,
> > require DRM bridges to be added to and removed from a DRM card without
> > tearing the card down. This is possible for connectors already (used by DP
> > MST), it is now needed for DRM bridges as well.
> >
> > As a first preliminary step, make bridges reference-counted to allow a
> > struct drm_bridge (along with the private driver structure embedding it) to
> > stay allocated even after the driver has been removed, until the last
> > reference is put.
> >
> > Signed-off-by: Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx>
> >
> > ---
> >
> > Changes in v7:
> > - export drm_bridge_put_void
> > - struct drm_bridge: use container pointer instead of container_offset
> > - remove drm_bridge_is_refcounted()
> > - remove all DRM_DEBUG()s
> > - drm_bridge_get/put: accept NULL pointer and return the bridge pointer to
> > allow pass-through calls
> > - extract to separate patches:
> > - the addition of drm_bridge_alloc
> > - the addition of drm_bridge_get/put() to drm_bridge_add/remove()
> > - the addition of drm_bridge_get/put() to drm_bridge_attach/detach()
> > - fix a typo, slightly improve kerneldoc
> >
> > Changes in v6:
> > - use drm_warn, not WARN_ON (Jani Nikula)
> > - Add devm_drm_bridge_alloc() to replace drm_bridge_init() (similar to
> > drmm_encoder_alloc)
> > - Remove .destroy func: deallocation is done via the struct offset
> > computed by the devm_drm_bridge_alloc() macro
> > - use fixed free callback, as the same callback is used in all cases
> > anyway (remove free_cb, add bool is_refcounted)
> > - add drm_bridge_get/put() to drm_bridge_attach/detach() (add the bridge
> > to a list)
> > - make some DRM_DEBUG() strings more informative
> >
> > This patch was added in v5.
> > ---
> > drivers/gpu/drm/drm_bridge.c | 33 +++++++++++++++-
> > include/drm/drm_bridge.h | 91 ++++++++++++++++++++++++++++++++++++++++++++
> > 2 files changed, 123 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
> > index 96df717b2caeb41d45346ded576eaeb2806fd051..2ba0dac9bfc2dfd709d5e2457d69067c7324972c 100644
> > --- a/drivers/gpu/drm/drm_bridge.c
> > +++ b/drivers/gpu/drm/drm_bridge.c
> > @@ -199,23 +199,54 @@
> > static DEFINE_MUTEX(bridge_lock);
> > static LIST_HEAD(bridge_list);
> >
> > +void __drm_bridge_free(struct kref *kref)
> > +{
> > + struct drm_bridge *bridge = container_of(kref, struct drm_bridge, refcount);
> > +
> > + kfree(bridge->container);
> > +}
> > +EXPORT_SYMBOL(__drm_bridge_free);
> > +
> > +/**
> > + * drm_bridge_put_void - wrapper to drm_bridge_put() taking a void pointer
> > + *
> > + * @data: pointer to @struct drm_bridge, cast to a void pointer
> > + *
> > + * Wrapper of drm_bridge_put() to be used when a function taking a void
> > + * pointer is needed, for example as a devm action.
> > + */
> > +void drm_bridge_put_void(void *data)
> > +{
> > + struct drm_bridge *bridge = (struct drm_bridge *)data;
> > +
> > + drm_bridge_put(bridge);
> > +}
> > +EXPORT_SYMBOL(drm_bridge_put_void);
> > +
> > void *__devm_drm_bridge_alloc(struct device *dev, size_t size, size_t offset,
> > const struct drm_bridge_funcs *funcs)
> > {
> > void *container;
> > struct drm_bridge *bridge;
> > + int err;
> >
> > if (!funcs) {
> > dev_warn(dev, "Missing funcs pointer\n");
> > return ERR_PTR(-EINVAL);
> > }
> >
> > - container = devm_kzalloc(dev, size, GFP_KERNEL);
> > + container = kzalloc(size, GFP_KERNEL);
> > if (!container)
> > return ERR_PTR(-ENOMEM);
> >
> > bridge = container + offset;
> > + bridge->container = container;
> > bridge->funcs = funcs;
> > + kref_init(&bridge->refcount);
> > +
> > + err = devm_add_action_or_reset(dev, drm_bridge_put_void, bridge);
> > + if (err)
> > + return ERR_PTR(err);
> >
> > return container;
> > }
> > diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
> > index dae463b30542d586a595b67f7bdf5a5e898e9572..5c1e2b9cafb12eb429d1f5d3ef312e6cf9b54f47 100644
> > --- a/include/drm/drm_bridge.h
> > +++ b/include/drm/drm_bridge.h
> > @@ -840,6 +840,17 @@ struct drm_bridge {
> > const struct drm_bridge_timings *timings;
> > /** @funcs: control functions */
> > const struct drm_bridge_funcs *funcs;
> > +
> > + /**
> > + * @container: Pointer to the private driver struct embedding this
> > + * @struct drm_bridge.
> > + */
> > + void *container;
>
> newline here
>
> > + /**
> > + * @refcount: reference count of users referencing this bridge.
> > + */
> > + struct kref refcount;
> > +
> > /** @driver_private: pointer to the bridge driver's internal context */
> > void *driver_private;
> > /** @ops: bitmask of operations supported by the bridge */
> > @@ -941,6 +952,82 @@ drm_priv_to_bridge(struct drm_private_obj *priv)
> > return container_of(priv, struct drm_bridge, base);
> > }
> >
> > +void __drm_bridge_free(struct kref *kref);
> > +
> > +/**
> > + * drm_bridge_get - Acquire a bridge reference
> > + * @bridge: DRM bridge
> > + *
> > + * This function increments the bridge's refcount.
> > + *
> > + * Returns:
> > + * Pointer to @bridge.
> > + */
> > +static inline struct drm_bridge *drm_bridge_get(struct drm_bridge *bridge)
> > +{
> > + if (!bridge)
> > + return bridge;
> > +
> > + kref_get(&bridge->refcount);
> > +
> > + return bridge;
> > +}
> > +
> > +/**
> > + * drm_bridge_put - Release a bridge reference
> > + * @bridge: DRM bridge
> > + *
> > + * This function decrements the bridge's reference count and frees the
> > + * object if the reference count drops to zero.
> > + *
> > + * See also drm_bridge_put_and_clear() which is more handy in many cases.
> > + *
> > + * Returns:
> > + * Pointer to @bridge.
> > + */
> > +static inline struct drm_bridge *drm_bridge_put(struct drm_bridge *bridge)
> > +{
> > + if (!bridge)
> > + return bridge;
> > +
> > + kref_put(&bridge->refcount, __drm_bridge_free);
> > +
> > + return bridge;
> > +}
>
> I'm not sure we discussed it already, but why do you need to put both
> drm_bridge_get and drm_bridge_put into the header, and thus export
> __drm_bridge_free?
>
> I'd expect both to be in drm_bridge.c?
I don't think we had discussed this before. There is no strong reason,
just a few small ones, and one is related to the DRM_DEBUG() calls that
I have removed from this series, so it's fine to move back
drm_bridge_get/put() in the .c file, and make __drm_bridge_free()
private again.
> > +/**
> > + * drm_bridge_put_and_clear - Given a bridge pointer, clear the pointer
> > + * then put the bridge
> > + *
> > + * @bridge_pp: pointer to pointer to a struct drm_bridge
> > + *
> > + * Helper to put a DRM bridge (whose pointer is passed), but only after
> > + * setting its pointer to NULL. Useful for drivers having struct drm_bridge
> > + * pointers they need to dispose of, without leaving a use-after-free
> > + * window where the pointed bridge might have been freed while still
> > + * holding a pointer to it.
> > + *
> > + * For example a driver having this private struct::
> > + *
> > + * struct my_bridge {
> > + * struct drm_bridge *remote_bridge;
> > + * ...
> > + * };
> > + *
> > + * can dispose of remote_bridge using::
> > + *
> > + * drm_bridge_put_and_clear(&my_bridge->remote_bridge);
> > + */
> > +static inline void drm_bridge_put_and_clear(struct drm_bridge **bridge_pp)
> > +{
> > + struct drm_bridge *bridge = *bridge_pp;
> > +
> > + *bridge_pp = NULL;
> > + drm_bridge_put(bridge);
> > +}
>
> I'm not convinced we need that one for now, and it doesn't look like
> there's a user for it in your series, so I'd rather introduce it once we
> actually need it.
Indeed, that's a leftover I missed when extracting this series for the
branch with the entire work. I'm definitely removing it in the next
iteration.
> > void *__devm_drm_bridge_alloc(struct device *dev, size_t size, size_t offset,
> > const struct drm_bridge_funcs *funcs);
> >
> > @@ -951,6 +1038,10 @@ void *__devm_drm_bridge_alloc(struct device *dev, size_t size, size_t offset,
> > * @member: the name of the &drm_bridge within @type
> > * @funcs: callbacks for this bridge
> > *
> > + * The returned refcount is initialized to 1.
>
> I'm confused, there's no returned refcount here? Or did you mean the
> returned bridge refcount?
Indeed, good catch!
Luca
--
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com
