[PATCH 2/3] virtio: Introduce virtio_max_dma_size()

[Joerg Roedel]

From: Joerg Roedel <jroedel@xxxxxxx>

This function returns the maximum segment size for a single
dma transaction of a virtio device. The possible limit comes
from the SWIOTLB implementation in the Linux kernel, that
has an upper limit of (currently) 256kb of contiguous
memory it can map.

The functions return the lower limie when SWIOTLB is used for DMA
transactions of the device and -1U otherwise.

Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
---
 drivers/virtio/virtio_ring.c | 11 +++++++++++
 include/linux/virtio.h       |  2 ++
 2 files changed, 13 insertions(+)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index cd7e755484e3..c8d229da203e 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -266,6 +266,17 @@ static bool vring_use_dma_api(struct virtio_device *vdev)
 	return false;
 }
 
+size_t virtio_max_dma_size(struct virtio_device *vdev)
+{
+	const struct dma_map_ops *ops = get_dma_ops(&vdev->dev);
+	size_t max_segment_size = -1U;
+
+	if (vring_use_dma_api(vdev) && dma_is_direct(ops))
+		max_segment_size = swiotlb_max_alloc_size();
+
+	return max_segment_size;
+}
+
 static void *vring_alloc_queue(struct virtio_device *vdev, size_t size,
 			      dma_addr_t *dma_handle, gfp_t flag)
 {
diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index fa1b5da2804e..673fe3ef3607 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -157,6 +157,8 @@ int virtio_device_freeze(struct virtio_device *dev);
 int virtio_device_restore(struct virtio_device *dev);
 #endif
 
+size_t virtio_max_dma_size(struct virtio_device *vdev);
+
 #define virtio_device_for_each_vq(vdev, vq) \
 	list_for_each_entry(vq, &vdev->vqs, list)
 
-- 
2.17.1