[PATCH] [media] vb2: remove TODO comment for dma-buf in QBUF

From: Javier Martinez Canillas
Date: Wed Jul 20 2016 - 12:18:49 EST

There is a TODO comment about the dma-buf being mapped in VIDIOC_QBUF
instead of doing it closer to when the actual DMA is going to happen
when the buffers are queued in the driver (i.e: __enqueue_in_driver).

But there is a reason to do it earlier in QBUF, and is that userspace
has no way to know if a exported dma-buf can be imported successfully
and so relies on QBUF succeeding as indication that the dma-buf mapped.

If QBUF fails, the application can fallback to another streaming I/O
method. But moving the dma-buf mapping later when queueing the buffers
can be too late for userspace to recover, since it may had dropped the
buffer(s) already when it knows that the dma-buf mapping failed.

So remove the TODO instead and change the comment to explain this.

Suggested-by: Hans Verkuil <hverkuil@xxxxxxxxx>
Signed-off-by: Javier Martinez Canillas <javier@xxxxxxxxxxxxxxx>


This patch was suggested by Hans as a feedback in a previous patch
that attempted to get rid of the TODO by moving the dma-buf mapping:


Best regards,

drivers/media/v4l2-core/videobuf2-core.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/media/v4l2-core/videobuf2-core.c b/drivers/media/v4l2-core/videobuf2-core.c
index bbba50d6e1ad..7128b09810be 100644
--- a/drivers/media/v4l2-core/videobuf2-core.c
+++ b/drivers/media/v4l2-core/videobuf2-core.c
@@ -1272,9 +1272,10 @@ static int __qbuf_dmabuf(struct vb2_buffer *vb, const void *pb)
vb->planes[plane].mem_priv = mem_priv;

- /* TODO: This pins the buffer(s) with dma_buf_map_attachment()).. but
- * really we want to do this just before the DMA, not while queueing
- * the buffer(s)..
+ /*
+ * This pins the buffer(s) with dma_buf_map_attachment()). It's done
+ * here instead just before the DMA, while queueing the buffer(s) so
+ * userspace knows sooner rather than later if the dma-buf map fails.
for (plane = 0; plane < vb->num_planes; ++plane) {
ret = call_memop(vb, map_dmabuf, vb->planes[plane].mem_priv);