When a GPU dma-fence signals, drivers often need to access userspace memory from a kthread context — either to write a fence completion value to a userspace VA (XE) or to signal a per-queue eventfd (AMDGPU). Both require borrowing the process MM via kthread_use_mm(), and both use the same kref-managed dma-fence-callback-to-workqueue pattern.
Extracting this pattern into a shared DRM helper allows both XE and AMDGPU to use it instead of maintaining independent open-coded implementations. This series does exactly that: Patch 1 introduces drm_user_fence — an embeddable base structure with driver-supplied worker/destroy callbacks. The common code handles: - process MM grab at init (mmgrab) - dma-fence callback registration - workqueue dispatch on fence signal - mmget_not_zero/kthread_use_mm/mmput in the worker - kref lifetime management - cancellation via drm_user_fence_cancel() for safe driver teardown Patch 2 converts XE to use the new helper. struct xe_user_fence embeds struct drm_user_fence as its base. XE-specific fields (xe_device pointer for ufence_wq wake-up, userspace VA, expected value, signalled flag) remain in the wrapper. No behavioral change is intended. A follow-on patch (not in this series) will wire AMDGPU's render-node EOP eventfd signaling path to the same helper. v2: - Move INIT_WORK() to drm_user_fence_init() so the work_struct is initialized once at object creation, fixing lockdep class divergence caused by two different INIT_WORK() call sites. (Sashiko review) - drm_user_fence_add_callback() now consumes the fence reference in all paths: transferred to the callback on success, put immediately on the already-signaled and error paths. Callers must not put the fence after this call. (Sashiko review) - Add drm_user_fence_cancel() to allow drivers to safely detach a pending callback before context teardown, preventing use-after-free when a foreign dma-fence signals after driver unload. (Sashiko review) - Fix missing newline at end of new files. Srinivasan Shanmugam (2): drm: Add common drm_user_fence helper drm/xe: Convert xe_user_fence to drm_user_fence drivers/gpu/drm/Makefile | 1 + drivers/gpu/drm/drm_user_fence.c | 164 +++++++++++++++++++++++++++++ drivers/gpu/drm/xe/xe_sync.c | 112 ++++++++------------ drivers/gpu/drm/xe/xe_sync_types.h | 1 - include/drm/drm_user_fence.h | 71 +++++++++++++ 5 files changed, 280 insertions(+), 69 deletions(-) create mode 100644 drivers/gpu/drm/drm_user_fence.c create mode 100644 include/drm/drm_user_fence.h -- 2.34.1
