Re: [PATCH v7 1/4] drm: Add drm_work_fence helper

[Matthew Brost <matthew.brost@intel.com>]

On Wed, Sep 02, 2026 at 08:50:46PM +0530, Srinivasan Shanmugam wrote:
> GPU drivers often need to queue work when a dma-fence signals
> because certain operations (copy_to_user, eventfd_signal, memory
> allocation) cannot run in IRQ context. This pattern is currently
> open-coded in multiple drivers.
> 
> Introduce drm_work_fence — an embeddable base structure that handles the
> dma-fence-callback-to-workqueue pattern in one place. Drivers embed this
> in their own structure and implement ops->writeback() for the deferred
> work and ops->destroy() for cleanup.
> 
> The helper manages:
>  - kref lifetime
>  - dma-fence callback registration
>  - workqueue dispatch on fence signal
>  - safe cancellation before driver teardown
> 
> For work that additionally requires borrowing the process MM via
> kthread_use_mm(), see drm_user_fence which builds on top of this.
> 
> Suggested-by: Matthew Brost <matthew.brost@intel.com>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: Christian König <christian.koenig@amd.com>
> Cc: dri-devel@lists.freedesktop.org
> Cc: intel-xe@lists.freedesktop.org
> Cc: amd-gfx@lists.freedesktop.org
> Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
> ---
>  drivers/gpu/drm/Makefile         |   1 +
>  drivers/gpu/drm/drm_work_fence.c | 184 +++++++++++++++++++++++++++++++
>  include/drm/drm_work_fence.h     |  69 ++++++++++++
>  3 files changed, 254 insertions(+)
>  create mode 100644 drivers/gpu/drm/drm_work_fence.c
>  create mode 100644 include/drm/drm_work_fence.h
> 
> diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
> index e97faabcd783..c5be8e80d0c8 100644
> --- a/drivers/gpu/drm/Makefile
> +++ b/drivers/gpu/drm/Makefile
> @@ -72,6 +72,7 @@ drm-y := \
>  	drm_vblank.o \
>  	drm_vblank_work.o \
>  	drm_vma_manager.o \
> +	drm_work_fence.o \
>  	drm_writeback.o
>  drm-$(CONFIG_DRM_CLIENT) += \
>  	drm_client.o \
> diff --git a/drivers/gpu/drm/drm_work_fence.c b/drivers/gpu/drm/drm_work_fence.c
> new file mode 100644
> index 000000000000..482eb3d4475f
> --- /dev/null
> +++ b/drivers/gpu/drm/drm_work_fence.c
> @@ -0,0 +1,184 @@
> +// SPDX-License-Identifier: MIT
> +/*
> + * Copyright © 2024 The Linux Foundation

ianal but this copywrite looks wrong for fresh code.

> + *
> + * Common DRM work fence helper.
> + *
> + * When a GPU dma-fence signals, drivers often need to perform work that
> + * cannot run in IRQ context (e.g., memory allocation, copy_to_user,
> + * eventfd_signal). This helper queues a work item when a dma-fence
> + * signals, allowing that work to run safely in a workqueue context.
> + *
> + * NOTE: This helper consumes dma_fences but CANNOT implement
> + * dma_fence_ops. Work items queued here may sleep; dma_fence_ops
> + * callbacks are called under the fence spinlock and must not sleep.
> + *
> + * For work that additionally requires accessing userspace memory via
> + * kthread_use_mm(), see drm_user_fence which builds on top of this.
> + */
> +
> +#include <linux/workqueue.h>
> +
> +#include <drm/drm_work_fence.h>
> +
> +static void drm_work_fence_destroy(struct kref *kref)
> +{
> +	struct drm_work_fence *wfence =
> +		container_of(kref, struct drm_work_fence, refcount);
> +	struct dma_fence *fence = wfence->fence;
> +
> +	wfence->ops->destroy(wfence);
> +	dma_fence_put(fence);	/* NULL-safe */
> +}
> +
> +/**
> + * drm_work_fence_get - Acquire a reference to a work fence
> + * @wfence: work fence
> + */
> +void drm_work_fence_get(struct drm_work_fence *wfence)
> +{
> +	kref_get(&wfence->refcount);
> +}
> +EXPORT_SYMBOL_GPL(drm_work_fence_get);

EXPORT_SYMBOL I think for MIT licenced code, unless the kernel compile
complains which I don't think will be the case for anything in this
file - I could be wrong though.

> +
> +/**
> + * drm_work_fence_put - Release a reference to a work fence
> + * @wfence: work fence
> + */
> +void drm_work_fence_put(struct drm_work_fence *wfence)
> +{
> +	kref_put(&wfence->refcount, drm_work_fence_destroy);
> +}
> +EXPORT_SYMBOL_GPL(drm_work_fence_put);
> +
> +static void drm_work_fence_work(struct work_struct *w)
> +{
> +	struct drm_work_fence *wfence =
> +		container_of(w, struct drm_work_fence, work);
> +
> +	wfence->ops->writeback(wfence);

I think you inverted the naming vfuncs for drm_work_fence vs drm_user_fence.

I'd expect drm_work_fence.worker and drm_user_fence.writeback.

Matt

> +	drm_work_fence_put(wfence);
> +}
> +
> +static void drm_work_fence_queue(struct drm_work_fence *wfence)
> +{
> +	queue_work(wfence->wq, &wfence->work);
> +}
> +
> +static void drm_work_fence_cb(struct dma_fence *fence, struct dma_fence_cb *cb)
> +{
> +	struct drm_work_fence *wfence =
> +		container_of(cb, struct drm_work_fence, cb);
> +
> +	drm_work_fence_queue(wfence);
> +	/* Single ref: wfence->fence released in drm_work_fence_destroy(). */
> +}
> +
> +/**
> + * drm_work_fence_init - Initialize a work fence
> + * @wfence: work fence to initialize
> + * @wq: workqueue to run the worker on (must be ordered if sequencing matters)
> + * @ops: driver operations
> + */
> +void drm_work_fence_init(struct drm_work_fence *wfence,
> +			 struct workqueue_struct *wq,
> +			 const struct drm_work_fence_ops *ops)
> +{
> +	kref_init(&wfence->refcount);
> +	wfence->wq = wq;
> +	wfence->ops = ops;
> +	wfence->fence = NULL;
> +	INIT_WORK(&wfence->work, drm_work_fence_work);
> +}
> +EXPORT_SYMBOL_GPL(drm_work_fence_init);
> +
> +/**
> + * drm_work_fence_add_callback - Attach a work fence to a dma-fence
> + * @wfence: work fence; caller retains their reference and must release
> + *          it via drm_work_fence_put() when no longer needed
> + * @fence: dma-fence to watch; one reference is consumed on any return value
> + *
> + * When @fence signals, a work item is queued that calls ops->writeback().
> + * If @fence has already signaled, the work item is queued immediately.
> + *
> + * Return: 0 on success, negative errno on error.
> + */
> +int drm_work_fence_add_callback(struct drm_work_fence *wfence,
> +				struct dma_fence *fence)
> +{
> +	int err;
> +
> +	drm_work_fence_get(wfence);
> +	wfence->fence = fence;	/* transfer caller's ref — single ref, no get */
> +
> +	err = dma_fence_add_callback(fence, &wfence->cb, drm_work_fence_cb);
> +	if (err == -ENOENT) {
> +		drm_work_fence_queue(wfence);
> +		err = 0;
> +	} else if (err) {
> +		wfence->fence = NULL;
> +		dma_fence_put(fence);
> +		drm_work_fence_put(wfence);
> +	}
> +
> +	return err;
> +}
> +EXPORT_SYMBOL_GPL(drm_work_fence_add_callback);
> +
> +/**
> + * drm_work_fence_cancel - Cancel a pending work fence callback
> + * @wfence: work fence
> + *
> + * Attempts to remove the pending callback before driver context teardown.
> + * The caller must hold a reference to @wfence across this call.
> + *
> + * If the callback has already fired this returns false and all cleanup
> + * has been handled internally.
> + *
> + * If removal succeeds the callback reference is released internally.
> + * The caller must still release its own reference via drm_work_fence_put().
> + *
> + * This function is safe to call from atomic context as it only acquires
> + * the dma-fence spinlock internally. If the caller also needs to wait
> + * for the worker to finish, use drm_work_fence_cancel_sync() instead,
> + * which may sleep.
> + *
> + * Return: true if callback was removed, false if it had already fired.
> + */
> +bool drm_work_fence_cancel(struct drm_work_fence *wfence)
> +{
> +	struct dma_fence *fence = wfence->fence;
> +
> +	if (!fence)
> +		return false;
> +
> +	if (dma_fence_remove_callback(fence, &wfence->cb)) {
> +		drm_work_fence_put(wfence);	/* drop ref from add_callback */
> +		return true;
> +	}
> +
> +	return false;
> +}
> +EXPORT_SYMBOL_GPL(drm_work_fence_cancel);
> +
> +/**
> + * drm_work_fence_cancel_sync - Cancel callback and wait for worker to finish
> + * @wfence: work fence
> + *
> + * Calls drm_work_fence_cancel() then cancel_work_sync() to guarantee
> + * the worker has fully completed before returning.
> + *
> + * This function may sleep. Must not be called from atomic or interrupt
> + * context. Use drm_work_fence_cancel() instead when sleeping is not allowed.
> + *
> + * Drivers must call this during teardown before freeing any resources
> + * accessed by ops->writeback().
> + */
> +void drm_work_fence_cancel_sync(struct drm_work_fence *wfence)
> +{
> +	if (drm_work_fence_cancel(wfence))
> +		return;
> +	if (cancel_work_sync(&wfence->work))
> +		drm_work_fence_put(wfence);
> +}
> +EXPORT_SYMBOL_GPL(drm_work_fence_cancel_sync);
> diff --git a/include/drm/drm_work_fence.h b/include/drm/drm_work_fence.h
> new file mode 100644
> index 000000000000..c8e3c5b9f0c4
> --- /dev/null
> +++ b/include/drm/drm_work_fence.h
> @@ -0,0 +1,69 @@
> +/* SPDX-License-Identifier: MIT */
> +/*
> + * Copyright © 2024 The Linux Foundation
> + */
> +
> +#ifndef __DRM_WORK_FENCE_H__
> +#define __DRM_WORK_FENCE_H__
> +
> +#include <linux/dma-fence.h>
> +#include <linux/kref.h>
> +#include <linux/workqueue.h>
> +
> +struct drm_work_fence;
> +
> +/**
> + * struct drm_work_fence_ops - driver callbacks for a DRM work fence
> + */
> +struct drm_work_fence_ops {
> +	/**
> +	 * @writeback: Called from workqueue context when the dma-fence signals.
> +	 *
> +	 * Perform the deferred work here (copy_to_user, eventfd_signal, etc.).
> +	 * May sleep. Must not requeue the fence.
> +	 */
> +	void (*writeback)(struct drm_work_fence *wfence);
> +
> +	/**
> +	 * @destroy: Called when the last reference is dropped.
> +	 * Free the containing structure here.
> +	 */
> +	void (*destroy)(struct drm_work_fence *wfence);
> +};
> +
> +/**
> + * struct drm_work_fence - DRM dma-fence-to-workqueue helper
> + *
> + * Embeddable base structure that queues a work item when a dma-fence signals.
> + * Drivers embed this in their own structure and implement ops->writeback()
> + * for the deferred work and ops->destroy() for cleanup.
> + *
> + * Call drm_work_fence_init() at creation and drm_work_fence_add_callback()
> + * to arm on a dma-fence. Call drm_work_fence_cancel_sync() before teardown.
> + */
> +struct drm_work_fence {
> +	/** @refcount: Reference count. */
> +	struct kref refcount;
> +	/** @wq: Workqueue on which to run the worker. */
> +	struct workqueue_struct *wq;
> +	/** @ops: Driver operations. */
> +	const struct drm_work_fence_ops *ops;
> +	/** @fence: The watched dma-fence; holds a single reference. */
> +	struct dma_fence *fence;
> +	/** @work: Work item queued when the fence signals. */
> +	struct work_struct work;
> +	/** @cb: Callback registered on the dma-fence. */
> +	struct dma_fence_cb cb;
> +};
> +
> +void drm_work_fence_init(struct drm_work_fence *wfence,
> +			 struct workqueue_struct *wq,
> +			 const struct drm_work_fence_ops *ops);
> +void drm_work_fence_get(struct drm_work_fence *wfence);
> +void drm_work_fence_put(struct drm_work_fence *wfence);
> +int  drm_work_fence_add_callback(struct drm_work_fence *wfence,
> +				 struct dma_fence *fence);
> +bool drm_work_fence_cancel(struct drm_work_fence *wfence);
> +void drm_work_fence_cancel_sync(struct drm_work_fence *wfence);
> +
> +#endif /* __DRM_WORK_FENCE_H__ */
> -- 
> 2.34.1
>