Re: [PATCH] dma-buf/dma-fence: Mark two callbacks as deprecated

[Christian König]

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 <phasta@xxxxxxxxxx>
> ---
>  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