[RFC LINUX PATCH 1/2] virtio_ring: Do not call dma_map_page if sg is already mapped.

[Wendy Liang <wendy.liang@xilinx.com>]

If sg is already dma mapped, do not call dma_map_page() in
vring_map_one_sg().

In case of rpmsg, rpmsg uses dma_alloc_coherent() to allocate
memory to share with the remote. There is no pages setup
in dma_alloc_coherent().

In this case, we cannot convert the virtual address back to the
physical address. In this case, we can setup the sg_dma_addr to
store the DMA address, and also mark the sg is already mapped.

In the vring, we can detect if the address is already mapped
by checking the sg_dma_addr. If yes, do not call dma_map_page().

Signed-off-by: Wendy Liang <jliang@xilinx.com>
---
 drivers/virtio/virtio_ring.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 489bfc6..9793e1f 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -180,6 +180,12 @@ static dma_addr_t vring_map_one_sg(const struct vring_virtqueue *vq,
 	if (!vring_use_dma_api(vq->vq.vdev))
 		return (dma_addr_t)sg_phys(sg);
 
+	/* If the sg is already mapped, return the DMA address */
+	if (sg_dma_address(sg)) {
+		sg->length = sg_dma_len(sg);
+		return sg_dma_address(sg);
+	}
+
 	/*
 	 * We can't use dma_map_sg, because we don't use scatterlists in
 	 * the way it expects (we don't guarantee that the scatterlist
-- 
1.9.1