rt2x00: do not align payload on modern H/W

PATCH Superseded Stanislaw Gruszka 1 participant 3 messages Nov 2, 2014 – Nov 11, 2014 Networking drivers (wireless)

lore

Share & tools

lore

Stanislaw Gruszka

author

2014-11-02 12:41 UTC #0 lore raw

PATCH 1/1 rt2x00: do not align payload on modern H/W

RT2800 and newer hardware require padding between header and payload if header length is not multiple of 4.

For historical reasons we also align payload to to 4 bytes boundary, but such alignment is not needed on modern H/W.

Patch improve performance on embedded CPUs and _possibly_ fixes skb_under_panic problems reported from time to time:

https://bugzilla.kernel.org/show_bug.cgi?id=84911
https://bugzilla.kernel.org/show_bug.cgi?id=72471
http://marc.info/?l=linux-wireless&m=139108549530402&w=2
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1087591

But we can not explain or otherwise confirm the patch fixes this panic issue for sure.

Originally-From: Helmut Schaa <[email protected]>

Signed-off-by Stanislaw Gruszka <[email protected]>

---
 drivers/net/wireless/rt2x00/rt2x00queue.c | 50 ++++++++-----------------------
 1 file changed, 12 insertions(+), 38 deletions(-)

drivers/net/wireless/rt2x00/rt2x00queue.c +12 ~~-38~~

index 5642ccc..22d49d5 100644
@@ -158,55 +158,29 @@ void rt2x00queue_align_frame(struct sk_buff *skb)
 	skb_trim(skb, frame_length);
 }
 
-void rt2x00queue_insert_l2pad(struct sk_buff *skb, unsigned int header_length)
+/*
+ * H/W needs L2 padding between the header and the paylod if header size
+ * is not 4 bytes aligned.
+ */
+void rt2x00queue_insert_l2pad(struct sk_buff *skb, unsigned int hdr_len)
 {
-	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;
+	unsigned int l2pad = (skb->len > hdr_len) ? L2PAD_SIZE(hdr_len) : 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 */
-	skb_trim(skb, header_length + l2pad + payload_length);
+	skb_push(skb, l2pad);
+	memmove(skb->data, skb->data + l2pad, hdr_len);
 }
 
-void rt2x00queue_remove_l2pad(struct sk_buff *skb, unsigned int header_length)
+void rt2x00queue_remove_l2pad(struct sk_buff *skb, unsigned int hdr_len)
 {
-	/*
-	 * L2 padding is only present if the skb contains more than just the
-	 * IEEE 802.11 header.
-	 */
-	unsigned int l2pad = (skb->len > header_length) ?
-				L2PAD_SIZE(header_length) : 0;
+	unsigned int l2pad = (skb->len > hdr_len) ? L2PAD_SIZE(hdr_len) : 0;
 
 	if (!l2pad)
 		return;
 
-	memmove(skb->data + l2pad, skb->data, header_length);
+	memmove(skb->data + l2pad, skb->data, hdr_len);
 	skb_pull(skb, l2pad);
 }

Signature · 1 line

1.8.3.1

Stanislaw Gruszka

author

2014-11-11 13:09 UTC #1 lore raw

Quoted 9 lines · Stanislaw Gruszka

RT2800 and newer hardware require padding between header and payload if header length is not multiple of 4.
For historical reasons we also align payload to to 4 bytes boundary, but such alignment is not needed on modern H/W.
Patch improve performance on embedded CPUs and _possibly_ fixes skb_under_panic problems reported from time to time:

Quoted 7 lines

https://bugzilla.kernel.org/show_bug.cgi?id=84911
https://bugzilla.kernel.org/show_bug.cgi?id=72471
http://marc.info/?l=linux-wireless&m=139108549530402&w=2
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1087591
But we can not explain or otherwise confirm the patch fixes this panic issue for sure.

Antti on https://bugzilla.kernel.org/show_bug.cgi?id=72471 tested patch and confirm it fixes the panic, he also provided traces showing that we really eat headroom on each retransmission in some special case - no payload and header length not being multiple of 4.

Taking that and since this patch was not yet applied I'll repost it with updated changelog and direct to 3.18 stream and -stable.

Stanislaw

Stanislaw Gruszka

author

2014-11-11 13:31 UTC #2 lore raw

PATCH v2 1/1 rt2x00: do not align payload on modern H/W

RT2800 and newer hardware require padding between header and payload if header length is not multiple of 4.

For historical reasons we also align payload to to 4 bytes boundary, but such alignment is not needed on modern H/W.

Patch fixes skb_under_panic problems reported from time to time:

Panic happened because we eat 4 bytes of skb headroom on each (re)transmission when sending frame without the payload and the header length not being multiple of 4 (i.e. QoS header has 26 bytes). On such case because paylad_aling=2 is bigger than header_align=0 we increase header_align by 4 bytes. To prevent that we could change the check to:

	if (payload_length && payload_align > header_align)
		header_align += 4;

but not aligning payload at all is more effective and alignment is not really needed by H/W (that has been tested on OpenWrt project for few years now).

Reported-and-tested-by Antti S. Lankila <[email protected]>

Debugged-by Antti S. Lankila <[email protected]>

Reported-by Henrik Asp <[email protected]>

Originally-From: Helmut Schaa <[email protected]>

Cc [email protected]

Signed-off-by Stanislaw Gruszka <[email protected]>

---
v1 -> v2: update the changelog.

 drivers/net/wireless/rt2x00/rt2x00queue.c | 50 ++++++++-----------------------
 1 file changed, 12 insertions(+), 38 deletions(-)

drivers/net/wireless/rt2x00/rt2x00queue.c +12 ~~-38~~

index 5642ccc..22d49d5 100644
@@ -158,55 +158,29 @@ void rt2x00queue_align_frame(struct sk_buff *skb)
 	skb_trim(skb, frame_length);
 }
 
-void rt2x00queue_insert_l2pad(struct sk_buff *skb, unsigned int header_length)
+/*
+ * H/W needs L2 padding between the header and the paylod if header size
+ * is not 4 bytes aligned.
+ */
+void rt2x00queue_insert_l2pad(struct sk_buff *skb, unsigned int hdr_len)
 {
-	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;
+	unsigned int l2pad = (skb->len > hdr_len) ? L2PAD_SIZE(hdr_len) : 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 */
-	skb_trim(skb, header_length + l2pad + payload_length);
+	skb_push(skb, l2pad);
+	memmove(skb->data, skb->data + l2pad, hdr_len);
 }
 
-void rt2x00queue_remove_l2pad(struct sk_buff *skb, unsigned int header_length)
+void rt2x00queue_remove_l2pad(struct sk_buff *skb, unsigned int hdr_len)
 {
-	/*
-	 * L2 padding is only present if the skb contains more than just the
-	 * IEEE 802.11 header.
-	 */
-	unsigned int l2pad = (skb->len > header_length) ?
-				L2PAD_SIZE(header_length) : 0;
+	unsigned int l2pad = (skb->len > hdr_len) ? L2PAD_SIZE(hdr_len) : 0;
 
 	if (!l2pad)
 		return;
 
-	memmove(skb->data + l2pad, skb->data, header_length);
+	memmove(skb->data + l2pad, skb->data, hdr_len);
 	skb_pull(skb, l2pad);
 }

Signature · 6 lines

1.8.3.1

--
To unsubscribe from this list: send the line "unsubscribe linux-wireless" in
the body of a message to [email protected]
More majordomo info at  http://vger.kernel.org/majordomo-info.html