openwrt/package/mac80211/patches/606-rt2x00_no_realign.patch

[RFC] rt2x00: For drivers that only need L2 padding don't realign frames

Signed-off-by: Helmut Schaa <helmut.schaa@...>
---

Ivo, Gertjan, do you remeber by any chance why this alignment stuff was added
in the first place? Was it because of DMA restrictions?

While doing some profiling on the rt3052 SoC I noticed that 30-40% time was
spent in memmove calls. And the culprit is the memmove aligning the payload
to a 4byte boundary since that has to move a whole bunch of data.

Interesstingly the legacy drivers insert an l2pad between the header and the
payload but doesn't realign the payload itself to a 4-byte boundary. Hence,
I came up with this patch and indeed CPU usage improves impressively.

Only tested on rt2800pci!

Thanks,
Helmut

 drivers/net/wireless/rt2x00/rt2x00queue.c |   30 +++-------------------------
 1 files changed, 4 insertions(+), 26 deletions(-)

--- a/drivers/net/wireless/rt2x00/rt2x00queue.c
+++ b/drivers/net/wireless/rt2x00/rt2x00queue.c
@@ -163,36 +163,14 @@ void rt2x00queue_align_frame(struct sk_b
 void rt2x00queue_insert_l2pad(struct sk_buff *skb, unsigned int header_length)
 {
 	unsigned int payload_length = skb->len - header_length;
-	unsigned int header_align = ALIGN_SIZE(skb, 0);
-	unsigned int payload_align = ALIGN_SIZE(skb, header_length);
 	unsigned int l2pad = payload_length ? L2PAD_SIZE(header_length) : 0;
 
-	/*
-	 * Adjust the header alignment if the payload needs to be moved more
-	 * than the header.
-	 */
-	if (payload_align > header_align)
-		header_align += 4;
-
-	/* There is nothing to do if no alignment is needed */
-	if (!header_align)
+	if (!l2pad)
 		return;
 
-	/* Reserve the amount of space needed in front of the frame */
-	skb_push(skb, header_align);
-
-	/*
-	 * Move the header.
-	 */
-	memmove(skb->data, skb->data + header_align, header_length);
-
-	/* Move the payload, if present and if required */
-	if (payload_length && payload_align)
-		memmove(skb->data + header_length + l2pad,
-			skb->data + header_length + l2pad + payload_align,
-			payload_length);
-
-	/* Trim the skb to the correct size */
+	/* insert l2pad -> Move header */
+	skb_push(skb, l2pad);
+	memmove(skb->data, skb->data + l2pad, header_length);
 	skb_trim(skb, header_length + l2pad + payload_length);
 }