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

[Philipp Stanner <phasta@kernel.org>]

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@kernel.org>
---
 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.
 	 */
 	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.
 	 */
 	void (*release)(struct dma_fence *fence);
 

base-commit: 2302669bb4b52d581cc2589b90b3af7bb3783a28
-- 
2.55.0