Re: [PATCH] usb/gadget/function/u_ether.c: Allow jumbo frames

From: Mike Looijmans
Date: Thu Sep 03 2015 - 10:24:06 EST

ïI'd like to bring this to your attention again please.

If there is something wrong about this patch, please tell me so.

And also note that this patch does not enable jumbo frames on itself, it just removes the artificial limits in the kernel prohibiting mtus above 1500. The MTU can be set from user space, and the impact on performance is quite impressive.

On 05-08-15 08:54, Mike Looijmans wrote:
USB network adapters support Jumbo frames. The only thing blocking
that feature is the code in the gadget driver that disposes of
packets larger than 1518 bytes, and the limit on the ioctl to set
the mtu.

This patch relaxes these limits, and allows up to 15k frames sizes.
The 15k value was chosen because 16k does not work on all platforms,
and usingclose to 16k will result in allocating 5 or 8 4k pages to
store the skb, wasting pages at no measurable performance gain.

On a topic-miami board (Zynq-7000), iperf3 performance reports:
MTU= 1500, PC-to-gadget: 139 Mbps, Gadget-to-PC: 116 Mbps
MTU=15000, PC-to-gadget: 239 Mbps, Gadget-to-PC: 361 Mbps

On boards with slower CPUs the performance improvement will be
relatively much larger, e.g. an OMAP-L138 increased from 40 to
220 Mbps using a similar patch on an 2.6.37 kernel.

Signed-off-by: Mike Looijmans <mike.looijmans@xxxxxxxx>
drivers/usb/gadget/function/u_ether.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/gadget/function/u_ether.c b/drivers/usb/gadget/function/u_ether.c
index f1fd777..6828ea2 100644
--- a/drivers/usb/gadget/function/u_ether.c
+++ b/drivers/usb/gadget/function/u_ether.c
@@ -48,6 +48,11 @@

#define UETH__VERSION "29-May-2008"

+/* Experiments show that both Linux and Windows hosts allow up to 16k
+ * frame sizes. Set the max size to 15k+52 to prevent allocating 32k
+ * blocks and still have efficient handling. */
struct eth_dev {
/* lock is held while accessing port_usb
@@ -146,7 +151,7 @@ static int ueth_change_mtu(struct net_device *net, int new_mtu)
spin_lock_irqsave(&dev->lock, flags);
if (dev->port_usb)
status = -EBUSY;
- else if (new_mtu <= ETH_HLEN || new_mtu > ETH_FRAME_LEN)
+ else if (new_mtu <= ETH_HLEN || new_mtu > GETHER_MAX_ETH_FRAME_LEN)
status = -ERANGE;
net->mtu = new_mtu;
@@ -294,7 +299,7 @@ static void rx_complete(struct usb_ep *ep, struct usb_request *req)
while (skb2) {
if (status < 0
|| ETH_HLEN > skb2->len
- || skb2->len > VLAN_ETH_FRAME_LEN) {
+ || skb2->len > GETHER_MAX_ETH_FRAME_LEN) {
DBG(dev, "rx length %d\n", skb2->len);

Kind regards,

Mike Looijmans
System Expert

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E-mail: mike.looijmans@xxxxxxxxxxxxxxxxx

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