[PATCH v4 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.

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.

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).
And vhci_map_urb_for_dma() prevents isoc pipe from using SG as
hcd_map_urb_for_dma() does.

Signed-off-by: Suwan Kim <suwan.kim027@gmail.com>
---
v3 - v4:
- Replace WARN_ON() with pr_err() in the error path.

v2 - v3
- Move setting URB_DMA_MAP_SG flag to the patch 2.
- Prevent isoc pipe from using SG buffer.

v1 - v2
- Add setting URB_DMA_MAP_SG flag in urb->transfer_flags to tell
stub driver to use SG buffer.
---
 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..429e4e989f38 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)
+{
+	if (usb_endpoint_xfer_isoc(&urb->ep->desc) && urb->num_sgs) {
+		pr_err("SG is not supported for isochronous transfer\n");
+		return -EINVAL;
+	}
+
+	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