On 9/23/26 17:03, Philipp Stanner wrote:
> dma_fence's backend_ops functions ops->wait() and ops->release() are
> problematic because they prevent fence producers from unloading.
> Moreover, they can easily be implemented through simpler means more
> aligned with the dma_fence contract.
>
> Since the declared goal of dma_fence is to move towards an
> implementation that supports driver-unload in all circumstances, we must
> prevent more parties from implementing those callbacks.
>
> Deprecate ops->release() and ops->wait() and document what users should
> do instead.
>
> Signed-off-by: Philipp Stanner <[email protected]>
> ---
> include/linux/dma-fence.h | 14 ++++++++++++++
> 1 file changed, 14 insertions(+)
>
> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
> index dd07d128adc3..59af0159cd25 100644
> --- a/include/linux/dma-fence.h
> +++ b/include/linux/dma-fence.h
> @@ -220,6 +220,8 @@ struct dma_fence_ops {
> /**
> * @wait:
> *
> + * DEPRECATED!
> + *
> * Custom wait implementation, defaults to dma_fence_default_wait() if
> * not set.
> *
> @@ -236,6 +238,10 @@ struct dma_fence_ops {
> * Implementing this callback prevents the fence from detaching after
> * signaling and so it is necessary for the module providing the
> * dma_fence_ops to stay loaded as long as the dma_fence exists.
> + *
> + * Deprecated for the reason mentioned above. No new users must be
> + * implemented.
> Consumers of a fence can instead notify themselves by
> + * registering a callback on the fence.
Mhm, the wait callback is transparent to consumers it's just that
implementations used it for quite a number of different hacks.
I would just drop that sentence.
> */
> signed long (*wait)(struct dma_fence *fence,
> bool intr, signed long timeout);
> @@ -243,6 +249,8 @@ struct dma_fence_ops {
> /**
> * @release:
> *
> + * DEPRECATED!
> + *
> * Called on destruction of fence to release additional resources.
> * Can be called from irq context. This callback is optional. If it is
> * NULL, then dma_fence_free() is instead called as the default
> @@ -254,6 +262,12 @@ struct dma_fence_ops {
> *
> * If the callback is implemented the memory backing the dma_fence
> * object must be freed RCU safe.
> + *
> + * Deprecated because it prevents the producer of a fence from
> + * unloading. No new users must be implemented. Parties with a
> + * hypothetical need for this callback can instead simply and directly
> + * perform their custom release operations one RCU grace period after
> + * they have signaled the fence.
Yeah that is a bit problematic.
We need my patch set to explicit signal fences instead of returning true/false
from callback for that so that a backend can properly implement this.
It's on my TODO list, but not the highest priority at the moment.
Regards,
Christian.
> */
> void (*release)(struct dma_fence *fence);
>
>
> base-commit: 2302669bb4b52d581cc2589b90b3af7bb3783a28
