Re: Do I need to skb_put() Ethernet frames to a minimum of 60 bytes?

On 2012-12-17 14:43, Nicolas Ferre wrote:

On 08/21/2012 07:34 PM, Arvid Brodin :

quoted

On 2012-08-14 22:35, Ben Hutchings wrote:

quoted

On Tue, 2012-08-14 at 18:53 +0000, Arvid Brodin wrote:

quoted

Hi,

If I create an sk_buff with a payload of less than 28 bytes (ethheader + data),
and send it using the cadence/macb (Ethernet) driver, I get

eth0: TX underrun, resetting buffers

Now I know the minimum Ethernet frame size is 64 bytes (including the 4-byte
FCS), but whose responsibility is it to pad the frame to this size if necessary?
Mine or the driver's - i.e. should I just skb_put() to the minimum size or
should I report the underrun as a driver bug?

If the hardware doesn't pad frames automatically then it's the driver's
reponsibility to do so.

Nicolas, can you take a look at this? At the moment I'm using the following change
in macb.c to avoid TX underruns on short packages:

--- a/drivers/net/ethernet/cadence/macb.c	2012-05-04 19:14:41.927719667 +0200
+++ b/drivers/net/ethernet/cadence/macb.c	2012-08-21 19:22:40.063739049 +0200

@@ -618,6 +618,7 @@ static void macb_poll_controller(struct
 }
 #endif

+#define MIN_ETHFRAME_LEN	60
 static int macb_start_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct macb *bp = netdev_priv(dev);

@@ -635,6 +636,12 @@ static int macb_start_xmit(struct sk_buf
 	printk("\n");
 #endif

+	if (skb->len < MIN_ETHFRAME_LEN) {
+		/* Pad skb to minium Ethernet frame size */
+		if (skb_tailroom(skb) >= MIN_ETHFRAME_LEN - skb->len)
+			memset(skb_put(skb, MIN_ETHFRAME_LEN - skb->len), 0,
+						MIN_ETHFRAME_LEN - skb->len);
+	}
 	len = skb->len;
 	spin_lock_irqsave(&bp->lock, flags);

... but as you can see this is limited to linear skbs which has been allocated with
enough tailroom. Perhaps there are better ways to fix the problem? (Maybe the hardware
is actually doing the padding already and the problem has to do with the way the DMA
transfer is set up?)

I come back to this issue. It seems to me that the macb Cadence IP is
padding automatically a too little packet. It is the usual behavior
unless you specify otherwise in the CTRL register embedded in the tx
descriptor. I also verified this with wireshark on both ICMP and UDP
packets.

The error that you are experiencing is on at91sam9260 or at91sam9263
SoCs, am I right?

No, this was on an AVR32 AP7000 board.

I believe this is what I did to solve the issue (patch for linux-2.6.37):

diff -Nurp linux-2.6.37-001-bsa400/drivers/net//macb.c
linux-2.6.37-macb-hsr/drivers/net//macb.c

--- linux-2.6.37-orig/drivers/net//macb.c	2012-09-16 22:41:02.746754672 +0200
+++ linux-2.6.37-macb/drivers/net//macb.c	2012-09-17 00:34:35.161389720 +0200

@@ -376,8 +379,9 @@ static void macb_tx(struct macb *bp)

 			rmb();

-			dma_unmap_single(&bp->pdev->dev, rp->mapping, skb->len,
-							 DMA_TO_DEVICE);
+			dma_unmap_single(&bp->pdev->dev, rp->mapping,
+					 max(skb->len, (unsigned int) ETH_ZLEN),
+					 DMA_TO_DEVICE);
 			rp->skb = NULL;
 			dev_kfree_skb_irq(skb);
 		}

@@ -413,7 +417,8 @@ static void macb_tx(struct macb *bp)

 		dev_dbg(&bp->pdev->dev, "skb %u (data %p) TX complete\n",
 			tail, skb->data);
-		dma_unmap_single(&bp->pdev->dev, rp->mapping, skb->len,
+		dma_unmap_single(&bp->pdev->dev, rp->mapping,
+				 max(skb->len, (unsigned int) ETH_ZLEN),
 				 DMA_TO_DEVICE);
 		bp->stats.tx_packets++;
 		bp->stats.tx_bytes += skb->len;

@@ -675,7 +680,10 @@ static int macb_start_xmit(struct sk_buf
 	printk("\n");
 #endif

-	len = skb->len;
+	if (skb_padto(skb, ETH_ZLEN) != 0)
+		return NETDEV_TX_OK; /* There is no NETDEV_TX_FAIL... */
+
+	len = max(skb->len, (unsigned int) ETH_ZLEN);
 	spin_lock_irqsave(&bp->lock, flags);

 	/* This is a hard error, log it. */

-- 
Arvid Brodin | Consultant (Linux)
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