Patchwork Fix tx transfer padding for full-speed USB

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Submitter Ingo van Lil
Date April 20, 2012, 10 a.m.
Message ID <1334916021-9124-1-git-send-email-inguin@gmx.de>
Download mbox | patch
Permalink /patch/153995/
State Changes Requested
Delegated to: David Miller
Headers show

Comments

Ingo van Lil - April 20, 2012, 10 a.m.
The asix.c USB Ethernet driver avoids ending a tx transfer with a zero-
length packet by appending a four-byte padding to transfers whose length
is a multiple of maxpacket. However, the hard-coded 512 byte maxpacket
length is valid for high-speed USB only; full-speed USB uses 64 byte
packets.

Signed-off-by: Ingo van Lil <inguin@gmx.de>
---
 drivers/net/usb/asix.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
David Miller - April 24, 2012, 4:28 a.m.
From: Ingo van Lil <inguin@gmx.de>
Date: Fri, 20 Apr 2012 12:00:21 +0200

> -	padlen = ((skb->len + 4) % 512) ? 0 : 4;
> +	padlen = ((skb->len + 4) % dev->maxpacket) ? 0 : 4;

This is now an expensive modulus operation, instead of
a cheap mask.

Since you know that dev->maxpacket is always a power of two, tell the
compiler this so it can still optimize the expression by changing this
to "& (dev->maxpacket - 1)" and we'll thus avoid the costly modulus.
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Patch

diff --git a/drivers/net/usb/asix.c b/drivers/net/usb/asix.c
index 5ee032c..aeb2702 100644
--- a/drivers/net/usb/asix.c
+++ b/drivers/net/usb/asix.c
@@ -355,7 +355,7 @@  static struct sk_buff *asix_tx_fixup(struct usbnet *dev, struct sk_buff *skb,
 	u32 packet_len;
 	u32 padbytes = 0xffff0000;
 
-	padlen = ((skb->len + 4) % 512) ? 0 : 4;
+	padlen = ((skb->len + 4) % dev->maxpacket) ? 0 : 4;
 
 	if ((!skb_cloned(skb)) &&
 	    ((headroom + tailroom) >= (4 + padlen))) {
@@ -377,7 +377,7 @@  static struct sk_buff *asix_tx_fixup(struct usbnet *dev, struct sk_buff *skb,
 	cpu_to_le32s(&packet_len);
 	skb_copy_to_linear_data(skb, &packet_len, sizeof(packet_len));
 
-	if ((skb->len % 512) == 0) {
+	if (padlen) {
 		cpu_to_le32s(&padbytes);
 		memcpy(skb_tail_pointer(skb), &padbytes, sizeof(padbytes));
 		skb_put(skb, sizeof(padbytes));