[PATCH v2 1/2] usbip: Skip DMA mapping and unmapping for urb at vhci

[Suwan Kim <suwan.kim027@gmail.com>]

vhci doesn’t do DMA for remote device. Actually, the real DMA
operation is done by network card driver. vhci just passes virtual
address of the buffer to the network stack, so vhci doesn’t use and
need dma address of the buffer of the URB.

When it comes to supporting SG for vhci, it is useful to use native
SG list (urb->num_sgs) instead of mapped SG list because DMA mapping
fnuction can adjust the number of SG list (urb->num_mapped_sgs).

But HCD provides DMA mapping and unmapping function by default.
Moreover, it causes unnecessary DMA mapping and unmapping which
will be done again at the NIC driver and it wastes CPU cycles.
So, implement map_urb_for_dma and unmap_urb_for_dma function for
vhci in order to skip the DMA mapping and unmapping procedure.

To support SG, vhci_map_urb_for_dma() sets URB_DMA_MAP_SG flag in
urb->transfer_flags if URB has SG list and this flag will tell the
stub driver to use SG list.

Signed-off-by: Suwan Kim <suwan.kim027@gmail.com>
---
 drivers/usb/usbip/vhci_hcd.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/usb/usbip/vhci_hcd.c b/drivers/usb/usbip/vhci_hcd.c
index 000ab7225717..14fc6d9f4e6a 100644
--- a/drivers/usb/usbip/vhci_hcd.c
+++ b/drivers/usb/usbip/vhci_hcd.c
@@ -1288,6 +1288,22 @@ static int vhci_free_streams(struct usb_hcd *hcd, struct usb_device *udev,
 	return 0;
 }
 
+static int vhci_map_urb_for_dma(struct usb_hcd *hcd, struct urb *urb,
+			gfp_t mem_flags)
+{
+	dev_dbg(hcd->self.controller, "vhci does not map urb for dma\n");
+
+	if (urb->num_sgs)
+		urb->transfer_flags |= URB_DMA_MAP_SG;
+
+       return 0;
+}
+
+static void vhci_unmap_urb_for_dma(struct usb_hcd *hcd, struct urb *urb)
+{
+       dev_dbg(hcd->self.controller, "vhci does not unmap urb for dma\n");
+}
+
 static const struct hc_driver vhci_hc_driver = {
 	.description	= driver_name,
 	.product_desc	= driver_desc,
@@ -1304,6 +1320,9 @@ static const struct hc_driver vhci_hc_driver = {
 
 	.get_frame_number = vhci_get_frame_number,
 
+	.map_urb_for_dma = vhci_map_urb_for_dma,
+	.unmap_urb_for_dma = vhci_unmap_urb_for_dma,
+
 	.hub_status_data = vhci_hub_status,
 	.hub_control    = vhci_hub_control,
 	.bus_suspend	= vhci_bus_suspend,
-- 
2.20.1