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> --- Hello, 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: https://lkml.org/lkml/2016/7/20/338 Best regards, Javier 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); -- 2.5.5 -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
