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@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.