AMD General
> -----Original Message-----
> From: Matthew Brost <[email protected]>
> Sent: Tuesday, September 1, 2026 1:52 AM
> To: SHANMUGAM, SRINIVASAN <[email protected]>
> Cc: Thomas Hellström <[email protected]>; dri-
> [email protected]; [email protected]; Koenig, Christian
> <[email protected]>; Deucher, Alexander
> <[email protected]>; [email protected]; Maarten
> Lankhorst <[email protected]>
> Subject: Re: [PATCH v6 1/4] drm: Add drm_work_fence helper
>
> On Mon, Aug 31, 2026 at 07:15:36PM +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->work() 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 <[email protected]>
> > Cc: Maarten Lankhorst <[email protected]>
> > Cc: Christian König <[email protected]>
> > Cc: [email protected]
> > Cc: [email protected]
> > Cc: [email protected]
> > Signed-off-by: Srinivasan Shanmugam <[email protected]>
> > ---
> > drivers/gpu/drm/Makefile | 1 +
> > drivers/gpu/drm/drm_work_fence.c | 195
> +++++++++++++++++++++++++++++++
> > include/drm/drm_work_fence.h | 76 ++++++++++++
> > 3 files changed, 272 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..9f6b779d0fe9
> > --- /dev/null
> > +++ b/drivers/gpu/drm/drm_work_fence.c
> > @@ -0,0 +1,195 @@
> > +// SPDX-License-Identifier: MIT
> > +/*
> > + * Copyright © 2024 The Linux Foundation
> > + *
> > + * 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);
> > +
> > + if (wfence->fence)
> > + dma_fence_put(wfence->fence);
> > +
> > + wfence->ops->destroy(wfence);
>
> I'd invert these for safety in case destroy wants to looks at the fence,
> admittedly
> that would be an odd use case.
>
> So...
>
> 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); /* this has a NULL check */
>
>
> > +}
> > +
> > +/**
> > + * 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);
> > +
> > +/**
> > + * 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->work(wfence);
> > + drm_work_fence_put(wfence);
> > +}
> > +
> > +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);
> > +
> > + queue_work(wfence->wq, &wfence->work);
> > + /*
> > + * Put the transferred reference from add_callback. The stored
> > + * reference in wfence->fence is released in drm_work_fence_destroy().
> > + */
> > + dma_fence_put(fence);
> > +}
> > +
> > +/**
> > + * 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
> > + * @fence: dma-fence to watch; ownership of this reference is transferred
> > + * to the callback — caller must NOT put it afterward.
>
> This isn't right. It is perfectly reasonable for caller to hold more than 1
> reference to
> @fence, thus put it again. It consumes a single reference @fence on success or
> failure - that is it.
>
> > + *
> > + * When @fence signals, a work item is queued that calls ops->work().
> > + * If @fence has already signaled, the work item is queued immediately.
> > + *
> > + * An additional reference to @fence is stored internally in @wfence
> > + to
> > + * allow drm_work_fence_cancel() to be called safely without the
> > + caller
> > + * needing to hold a separate fence reference.
> > + *
>
> Ideally get rid of double ref count on @fence. I don't think above reasoning
> justifies
> the needed for a double ref on the fence. I'd tie exactly one refernece
> @fence which
> is attached to lifetime of @wfence (i.e., drop the dma_fence_put in
> drm_work_fence_cb).
>
> > + * On any return value the caller's fence reference is consumed.
> > + *
>
> I'd mention regardless of success or fail, a reference to drm_work_fence is
> consumed too.
>
> > + * 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 = dma_fence_get(fence);
> > +
> > + err = dma_fence_add_callback(fence, &wfence->cb, drm_work_fence_cb);
> > + if (err == -ENOENT) {
> > + queue_work(wfence->wq, &wfence->work);
> > + dma_fence_put(fence);
>
> Keep the implementation in one place?
>
> drm_work_fence_work(&wfence->work);
Hi Matt,
Thanks for your feedbacks once again!,
For the ENOENT path — I'm planning to extract a small shared helper:
static void drm_work_fence_queue(struct drm_work_fence *wfence)
{
queue_work(wfence->wq, &wfence->work);
}
and call it from both drm_work_fence_cb() and the ENOENT path in
add_callback(). This keeps the implementation in one place while
preserving async execution.
May I kno pls, is that what you had in mind, or did you mean something
different?
Thanks,
Srini
