[PATCH RFC 1/3] media: videobuf2: add dma_resv release-fence helper
Markus Fritsche
mfritsche at reauktion.de
Wed Apr 29 12:53:04 PDT 2026
Add an opt-in API that lets vb2 producers populate a dma_resv
exclusive write fence on the dmabufs they export to userspace,
signalled when the buffer transitions to VB2_BUF_STATE_DONE.
V4L2 producers historically don't propagate buffer-state-done into
the dmabuf's dma_resv exclusive fence. Userspace consumers that
import V4L2-produced dmabufs and wait on the dmabuf's implicit-sync
fence (poll(POLLIN), DMA_BUF_IOCTL_EXPORT_SYNC_FILE) currently see
either zero fences or a stub fence from dma_fence_get_stub(). This
is correct by accident for the common case (clients call DQBUF
before importing) but represents a contract gap.
Drivers opt in by calling vb2_buffer_attach_release_fence(vb) from
their buf_queue callback. The helper allocates a dma_fence on the
queue's fence context (set up at vb2_core_queue_init), attaches it
as DMA_RESV_USAGE_WRITE on each plane's dmabuf->resv, and stashes
it in vb->release_fence. vb2_buffer_done signals + puts the fence
as part of its state transition.
For drivers that don't opt in, vb->release_fence stays NULL and
the signal path is a no-op.
Skips planes whose vb2_plane.dbuf is NULL — buffers never exported
via VIDIOC_EXPBUF (or imported via V4L2_MEMORY_DMABUF) have no
dmabuf for userspace to wait on.
Signed-off-by: Markus Fritsche <mfritsche at reauktion.de>
---
.../media/common/videobuf2/videobuf2-core.c | 95 +++++++++++++++++++
include/media/videobuf2-core.h | 29 ++++++
2 files changed, 124 insertions(+)
diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
index b0523fc23..ee766aae0 100644
--- a/drivers/media/common/videobuf2/videobuf2-core.c
+++ b/drivers/media/common/videobuf2/videobuf2-core.c
@@ -26,6 +26,9 @@
#include <linux/freezer.h>
#include <linux/kthread.h>
+#include <linux/dma-fence.h>
+#include <linux/dma-resv.h>
+#include <linux/dma-buf.h>
#include <media/videobuf2-core.h>
#include <media/v4l2-mc.h>
@@ -1179,6 +1182,86 @@ void *vb2_plane_cookie(struct vb2_buffer *vb, unsigned int plane_no)
}
EXPORT_SYMBOL_GPL(vb2_plane_cookie);
+/*
+ * dma_resv release-fence integration.
+ *
+ * V4L2 producers historically don't propagate buffer-state-done into
+ * the dmabuf's dma_resv exclusive fence. Userspace consumers that
+ * wait on that fence (e.g. wayland compositors via poll(POLLIN) or
+ * DMA_BUF_IOCTL_EXPORT_SYNC_FILE) currently see either no fences or
+ * a stub fence from dma_fence_get_stub(). The opt-in API below lets
+ * a driver attach a real producer fence at QBUF time and have it
+ * signalled by vb2_buffer_done().
+ */
+
+static const char *vb2_dma_resv_get_driver_name(struct dma_fence *fence)
+{
+ return "videobuf2";
+}
+
+static const char *vb2_dma_resv_get_timeline_name(struct dma_fence *fence)
+{
+ return "vb2-release-fence";
+}
+
+static const struct dma_fence_ops vb2_dma_resv_fence_ops = {
+ .get_driver_name = vb2_dma_resv_get_driver_name,
+ .get_timeline_name = vb2_dma_resv_get_timeline_name,
+};
+
+int vb2_buffer_attach_release_fence(struct vb2_buffer *vb)
+{
+ struct vb2_queue *q = vb->vb2_queue;
+ struct dma_fence *fence;
+ unsigned int plane;
+
+ if (WARN_ON(vb->release_fence))
+ return -EINVAL;
+
+ fence = kzalloc(sizeof(*fence), GFP_KERNEL);
+ if (!fence)
+ return -ENOMEM;
+
+ dma_fence_init(fence, &vb2_dma_resv_fence_ops, &q->dma_resv_fence_lock,
+ q->dma_resv_fence_context,
+ atomic64_inc_return(&q->dma_resv_fence_seqno));
+
+ for (plane = 0; plane < vb->num_planes; plane++) {
+ struct dma_buf *dbuf = vb->planes[plane].dbuf;
+
+ if (!dbuf)
+ continue;
+
+ dma_resv_lock(dbuf->resv, NULL);
+ dma_resv_add_fence(dbuf->resv, fence, DMA_RESV_USAGE_WRITE);
+ dma_resv_unlock(dbuf->resv);
+ }
+
+ /* One reference for the eventual signal in vb2_buffer_done. */
+ vb->release_fence = dma_fence_get(fence);
+
+ /* The dma_resv held its own reference per plane. Drop ours. */
+ dma_fence_put(fence);
+
+ return 0;
+}
+EXPORT_SYMBOL_GPL(vb2_buffer_attach_release_fence);
+
+static void vb2_buffer_signal_release_fence(struct vb2_buffer *vb,
+ enum vb2_buffer_state state)
+{
+ struct dma_fence *fence = vb->release_fence;
+
+ if (!fence)
+ return;
+
+ if (state == VB2_BUF_STATE_ERROR)
+ dma_fence_set_error(fence, -EIO);
+ dma_fence_signal(fence);
+ dma_fence_put(fence);
+ vb->release_fence = NULL;
+}
+
void vb2_buffer_done(struct vb2_buffer *vb, enum vb2_buffer_state state)
{
struct vb2_queue *q = vb->vb2_queue;
@@ -1205,6 +1288,9 @@ void vb2_buffer_done(struct vb2_buffer *vb, enum vb2_buffer_state state)
if (state != VB2_BUF_STATE_QUEUED)
__vb2_buf_mem_finish(vb);
+ if (state != VB2_BUF_STATE_QUEUED)
+ vb2_buffer_signal_release_fence(vb, state);
+
spin_lock_irqsave(&q->done_lock, flags);
if (state == VB2_BUF_STATE_QUEUED) {
vb->state = VB2_BUF_STATE_QUEUED;
@@ -2652,6 +2738,15 @@ int vb2_core_queue_init(struct vb2_queue *q)
mutex_init(&q->mmap_lock);
init_waitqueue_head(&q->done_wq);
+ /*
+ * Per-queue dma_resv release-fence context. Drivers opt-in via
+ * vb2_buffer_attach_release_fence(); other drivers pay only the
+ * cost of the unused fields.
+ */
+ q->dma_resv_fence_context = dma_fence_context_alloc(1);
+ atomic64_set(&q->dma_resv_fence_seqno, 0);
+ spin_lock_init(&q->dma_resv_fence_lock);
+
q->memory = VB2_MEMORY_UNKNOWN;
if (q->buf_struct_size == 0)
diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
index 9b02aeba4..2bf3272d4 100644
--- a/include/media/videobuf2-core.h
+++ b/include/media/videobuf2-core.h
@@ -288,6 +288,12 @@ struct vb2_buffer {
unsigned int skip_cache_sync_on_finish:1;
struct vb2_plane planes[VB2_MAX_PLANES];
+ /*
+ * dma_resv release fence — set by vb2_buffer_attach_release_fence()
+ * (driver opt-in from buf_queue), signalled and put by
+ * vb2_buffer_done(). NULL for drivers that don't opt in.
+ */
+ struct dma_fence *release_fence;
struct list_head queued_entry;
struct list_head done_entry;
#ifdef CONFIG_VIDEO_ADV_DEBUG
@@ -658,6 +664,15 @@ struct vb2_queue {
spinlock_t done_lock;
wait_queue_head_t done_wq;
+ /*
+ * Per-queue dma_resv release-fence context. Drivers that opt
+ * into vb2_buffer_attach_release_fence() use these to allocate
+ * fences on a single per-queue timeline.
+ */
+ u64 dma_resv_fence_context;
+ atomic64_t dma_resv_fence_seqno;
+ spinlock_t dma_resv_fence_lock;
+
unsigned int streaming:1;
unsigned int start_streaming_called:1;
unsigned int error:1;
@@ -747,6 +762,20 @@ void *vb2_plane_cookie(struct vb2_buffer *vb, unsigned int plane_no);
*/
void vb2_buffer_done(struct vb2_buffer *vb, enum vb2_buffer_state state);
+/**
+ * vb2_buffer_attach_release_fence() - opt-in dma_resv release fence.
+ * @vb: the buffer being queued to the producer.
+ *
+ * Drivers call this from their buf_queue callback to attach an
+ * exclusive write fence to each plane's dmabuf->resv. The fence
+ * is signalled and put by vb2_buffer_done() when the buffer
+ * transitions to VB2_BUF_STATE_DONE / _ERROR. Skips planes whose
+ * dbuf is NULL.
+ *
+ * Returns 0 on success, negative errno on allocation failure.
+ */
+int vb2_buffer_attach_release_fence(struct vb2_buffer *vb);
+
/**
* vb2_discard_done() - discard all buffers marked as DONE.
* @q: pointer to &struct vb2_queue with videobuf2 queue.
--
2.47.3
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