[PATCH v4 1/2] drm: Add common drm_user_fence helper

[Srinivasan Shanmugam]

Introduce a common DRM user fence helper providing the kref-managed,
MM-borrowing dma-fence-callback-to-workqueue pattern used by drivers
that must access userspace memory from a kthread context when a GPU
fence signals.

XE uses this pattern (xe_sync.c) to write a fence completion value
to a userspace VA. AMDGPU will use the same pattern to signal a
per-queue eventfd from a user-queue EOP fence callback.

The helper provides:
 - struct drm_user_fence: embeddable base structure
 - struct drm_user_fence_ops: worker/destroy callbacks
 - drm_user_fence_init(): initialize and grab the process MM
 - drm_user_fence_get/put(): reference counting
 - drm_user_fence_add_callback(): attach to a dma-fence

The worker callback receives a bool indicating whether the process
MM was successfully obtained, allowing drivers to handle the
unavailable-MM case (log, skip the userspace write, etc.) without
duplicating the mmget/kthread_use_mm/mmput boilerplate.

Suggested-by: Christian König <christian.koenig@xxxxxxx>
Cc: Alex Deucher <alexander.deucher@xxxxxxx>
Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
Cc: Maxime Ripard <mripard@xxxxxxxxxx>
Cc: Thomas Zimmermann <tzimmermann@xxxxxxx>
Cc: David Airlie <airlied@xxxxxxxxx>
Cc: Simona Vetter <simona@xxxxxxxx>
Cc: Sumit Semwal <sumit.semwal@xxxxxxxxxx>
Cc: Matthew Brost <matthew.brost@xxxxxxxxx>
Cc: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: intel-xe@xxxxxxxxxxxxxxxxxxxxx
Cc: linux-media@xxxxxxxxxxxxxxx
Cc: linaro-mm-sig@xxxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: amd-gfx@xxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@xxxxxxx>
---
v4:
 - Check cancel_work_sync() return value in drm_user_fence_cancel_sync()
   and call drm_user_fence_put() if work was dequeued, fixing a memory
   leak of the drm_user_fence, mm_struct and stored dma_fence when a
   pending work item is cancelled. (Sashiko review)

 drivers/gpu/drm/Makefile         |   1 +
 drivers/gpu/drm/drm_user_fence.c | 223 +++++++++++++++++++++++++++++++
 include/drm/drm_user_fence.h     |  76 +++++++++++
 3 files changed, 300 insertions(+)
 create mode 100644 drivers/gpu/drm/drm_user_fence.c
 create mode 100644 include/drm/drm_user_fence.h

diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
index e97faabcd783..52de1f474535 100644
--- a/drivers/gpu/drm/Makefile
+++ b/drivers/gpu/drm/Makefile
@@ -69,6 +69,7 @@ drm-y := \
 	drm_syncobj.o \
 	drm_sysfs.o \
 	drm_trace_points.o \
+	drm_user_fence.o \
 	drm_vblank.o \
 	drm_vblank_work.o \
 	drm_vma_manager.o \
diff --git a/drivers/gpu/drm/drm_user_fence.c b/drivers/gpu/drm/drm_user_fence.c
new file mode 100644
index 000000000000..cdc47d092cbb
--- /dev/null
+++ b/drivers/gpu/drm/drm_user_fence.c
@@ -0,0 +1,223 @@
+// SPDX-License-Identifier: MIT
+/*
+ * Copyright © 2024 The Linux Foundation
+ *
+ * Common DRM user fence helper.
+ *
+ * When a GPU dma-fence signals, drivers often need to write a value to a
+ * userspace VA or notify userspace via an eventfd. Both operations require
+ * a valid process MM, which is not available in IRQ context.
+ *
+ * This helper queues a work item on fence signal. The work item borrows the
+ * process MM via kthread_use_mm() and calls ops->worker(), which the driver
+ * implements to perform the actual userspace access.
+ */
+
+#include <linux/kthread.h>
+#include <linux/sched/mm.h>
+#include <linux/workqueue.h>
+
+#include <drm/drm_user_fence.h>
+
+static void drm_user_fence_destroy(struct kref *kref)
+{
+	struct drm_user_fence *ufence =
+		container_of(kref, struct drm_user_fence, refcount);
+
+	/* Release the extra reference stored for cancel() */
+	if (ufence->fence)
+		dma_fence_put(ufence->fence);
+
+	mmdrop(ufence->mm);
+	ufence->ops->destroy(ufence);
+}
+
+/**
+ * drm_user_fence_get - Acquire a reference to a user fence
+ * @ufence: user fence
+ */
+void drm_user_fence_get(struct drm_user_fence *ufence)
+{
+	kref_get(&ufence->refcount);
+}
+EXPORT_SYMBOL_GPL(drm_user_fence_get);
+
+/**
+ * drm_user_fence_put - Release a reference to a user fence
+ * @ufence: user fence
+ */
+void drm_user_fence_put(struct drm_user_fence *ufence)
+{
+	kref_put(&ufence->refcount, drm_user_fence_destroy);
+}
+EXPORT_SYMBOL_GPL(drm_user_fence_put);
+
+static void drm_user_fence_work(struct work_struct *w)
+{
+	struct drm_user_fence *ufence =
+		container_of(w, struct drm_user_fence, work);
+	bool mm_ok = false;
+
+	if (mmget_not_zero(ufence->mm)) {
+		kthread_use_mm(ufence->mm);
+		mm_ok = true;
+	}
+
+	ufence->ops->worker(ufence, mm_ok);
+
+	if (mm_ok) {
+		kthread_unuse_mm(ufence->mm);
+		mmput(ufence->mm);
+	}
+
+	drm_user_fence_put(ufence);
+}
+
+static void drm_user_fence_cb(struct dma_fence *fence, struct dma_fence_cb *cb)
+{
+	struct drm_user_fence *ufence =
+		container_of(cb, struct drm_user_fence, cb);
+
+	queue_work(ufence->wq, &ufence->work);
+	/*
+	 * Put the transferred reference from add_callback. The stored
+	 * reference in ufence->fence is released in drm_user_fence_destroy().
+	 */
+	dma_fence_put(fence);
+}
+
+/**
+ * drm_user_fence_init - Initialize a user fence
+ * @ufence: user fence to initialize
+ * @wq: workqueue to run the worker on (must be ordered if sequencing matters)
+ * @ops: driver operations
+ *
+ * Must be called from process context with a valid current->mm.
+ * Grabs a reference to current->mm via mmgrab().
+ */
+void drm_user_fence_init(struct drm_user_fence *ufence,
+			 struct workqueue_struct *wq,
+			 const struct drm_user_fence_ops *ops)
+{
+	kref_init(&ufence->refcount);
+	ufence->mm = current->mm;
+	mmgrab(ufence->mm);
+	ufence->wq = wq;
+	ufence->ops = ops;
+	ufence->fence = NULL;
+	INIT_WORK(&ufence->work, drm_user_fence_work);
+}
+EXPORT_SYMBOL_GPL(drm_user_fence_init);
+
+/**
+ * drm_user_fence_add_callback - Attach a user fence to a dma-fence
+ * @ufence: user fence
+ * @fence: dma-fence to watch; ownership of this reference is transferred
+ *         to the callback — caller must NOT put it afterward.
+ *
+ * When @fence signals, a work item is queued that calls ops->worker() with
+ * the process MM active. If @fence has already signaled the work item is
+ * queued immediately.
+ *
+ * An additional reference to @fence is stored internally in @ufence to
+ * allow drm_user_fence_cancel() to be called safely without the caller
+ * needing to hold a separate fence reference.
+ *
+ * On any return value the caller's fence reference is consumed.
+ *
+ * Return: 0 on success, negative errno on error.
+ */
+int drm_user_fence_add_callback(struct drm_user_fence *ufence,
+				struct dma_fence *fence)
+{
+	int err;
+
+	drm_user_fence_get(ufence);
+
+	/* Extra ref stored for cancel() — lives until drm_user_fence_destroy() */
+	ufence->fence = dma_fence_get(fence);
+
+	err = dma_fence_add_callback(fence, &ufence->cb, drm_user_fence_cb);
+	if (err == -ENOENT) {
+		/* fence already signaled — queue work and release transferred ref */
+		queue_work(ufence->wq, &ufence->work);
+		dma_fence_put(fence);
+		err = 0;
+	} else if (err) {
+		dma_fence_put(ufence->fence);
+		ufence->fence = NULL;
+		drm_user_fence_put(ufence);
+		dma_fence_put(fence);
+	}
+	/* on success: transferred ref goes to drm_user_fence_cb */
+
+	return err;
+}
+EXPORT_SYMBOL_GPL(drm_user_fence_add_callback);
+
+/**
+ * drm_user_fence_cancel - Cancel a pending user fence callback
+ * @ufence: user fence
+ *
+ * Attempts to remove the pending callback before driver context teardown.
+ * Must be called before the driver tears down its workqueue or ops.
+ * The caller must hold a reference to @ufence across this call.
+ *
+ * If the callback has already fired this returns false and no additional
+ * action is needed — the callback handles its own reference.
+ *
+ * If removal succeeds the callback reference is released internally.
+ * The caller must still release its own separate reference via
+ * drm_user_fence_put() when done with the object.
+ *
+ * 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_user_fence_cancel_sync() instead,
+ * which may sleep.
+ *
+ * Return: true if callback was removed, false if it had already fired.
+ */
+bool drm_user_fence_cancel(struct drm_user_fence *ufence)
+{
+	struct dma_fence *fence = ufence->fence;
+
+	if (!fence)
+		return false;
+
+	if (dma_fence_remove_callback(fence, &ufence->cb)) {
+		/*
+		 * Callback will not fire — release the transferred reference
+		 * that would have been put by drm_user_fence_cb(). The stored
+		 * reference in ufence->fence is released in destroy().
+		 */
+		dma_fence_put(fence);
+		drm_user_fence_put(ufence);
+		return true;
+	}
+
+	/* Callback already fired — it handled its own cleanup */
+	return false;
+}
+EXPORT_SYMBOL_GPL(drm_user_fence_cancel);
+
+/**
+ * drm_user_fence_cancel_sync - Cancel callback and wait for worker to finish
+ * @ufence: user fence
+ *
+ * Calls drm_user_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_user_fence_cancel() instead when sleeping is not
+ * allowed.
+ *
+ * Drivers must call this during teardown before freeing any resources
+ * accessed by ops->worker().
+ */
+void drm_user_fence_cancel_sync(struct drm_user_fence *ufence)
+{
+	drm_user_fence_cancel(ufence);
+	if (cancel_work_sync(&ufence->work))
+		drm_user_fence_put(ufence);
+}
+EXPORT_SYMBOL_GPL(drm_user_fence_cancel_sync);
diff --git a/include/drm/drm_user_fence.h b/include/drm/drm_user_fence.h
new file mode 100644
index 000000000000..02a02266ab93
--- /dev/null
+++ b/include/drm/drm_user_fence.h
@@ -0,0 +1,75 @@
+/* SPDX-License-Identifier: MIT */
+/*
+ * Copyright © 2024 The Linux Foundation
+ */
+
+#ifndef __DRM_USER_FENCE_H__
+#define __DRM_USER_FENCE_H__
+
+#include <linux/dma-fence.h>
+#include <linux/kref.h>
+#include <linux/workqueue.h>
+
+struct drm_user_fence;
+
+/**
+ * struct drm_user_fence_ops - driver callbacks for a DRM user fence
+ */
+struct drm_user_fence_ops {
+	/**
+	 * @worker: Called from workqueue context.
+	 *
+	 * If @mm_ok is true, kthread_use_mm() is active and userspace memory
+	 * (copy_to_user, eventfd_signal, etc.) may be accessed safely.
+	 * If @mm_ok is false, the process MM was already gone; the driver
+	 * should log a warning and skip the userspace write.
+	 *
+	 * wake_up() or other post-signal housekeeping should also happen here.
+	 */
+	void (*worker)(struct drm_user_fence *ufence, bool mm_ok);
+
+	/**
+	 * @destroy: Called when the last reference is dropped.
+	 * Free the containing structure here.
+	 */
+	void (*destroy)(struct drm_user_fence *ufence);
+};
+
+/**
+ * struct drm_user_fence - embeddable DRM user fence
+ *
+ * Drivers embed this in their own structure and implement
+ * &drm_user_fence_ops. Call drm_user_fence_init() at creation and
+ * drm_user_fence_add_callback() to arm on a dma-fence.
+ * Call drm_user_fence_cancel_sync() before driver teardown.
+ */
+struct drm_user_fence {
+	/** @refcount: Reference count. */
+	struct kref refcount;
+	/** @mm: Process MM grabbed at init time. */
+	struct mm_struct *mm;
+	/** @work: Work item queued when the dma-fence signals. */
+	struct work_struct work;
+	/** @cb: dma-fence callback. */
+	struct dma_fence_cb cb;
+	/**
+	 * @fence: Extra reference held for safe cancel(). Set during
+	 * add_callback, released in destroy().
+	 */
+	struct dma_fence *fence;
+	/** @wq: Workqueue to run @work on. */
+	struct workqueue_struct *wq;
+	/** @ops: Driver operations. */
+	const struct drm_user_fence_ops *ops;
+};
+
+void drm_user_fence_init(struct drm_user_fence *ufence,
+			 struct workqueue_struct *wq,
+			 const struct drm_user_fence_ops *ops);
+void drm_user_fence_get(struct drm_user_fence *ufence);
+void drm_user_fence_put(struct drm_user_fence *ufence);
+int drm_user_fence_add_callback(struct drm_user_fence *ufence,
+				struct dma_fence *fence);
+bool drm_user_fence_cancel(struct drm_user_fence *ufence);
+void drm_user_fence_cancel_sync(struct drm_user_fence *ufence);
+
+#endif /* __DRM_USER_FENCE_H__ */
-- 
2.34.1