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

On 2012-08-14 22:35, Ben Hutchings wrote:
> On Tue, 2012-08-14 at 18:53 +0000, Arvid Brodin wrote:

>> 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:


... 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?)


-- 
Arvid Brodin | Consultant (Linux)
XDIN AB | Jan Stenbecks Torg 17 | SE-164 40 Kista | Sweden | xdin.com

On Tue, 2012-08-21 at 17:34 +0000, Arvid Brodin wrote:

> 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?)
> 

other net drivers use skb_padto() for this ...




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On Tue, 2012-08-21 at 17:34 +0000, Arvid Brodin wrote:
> On 2012-08-14 22:35, Ben Hutchings wrote:
> > On Tue, 2012-08-14 at 18:53 +0000, Arvid Brodin wrote:
> >> 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

<linux/etherdevice.h> already names this as ETH_ZLEN, by the way.

>  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?

skb_padto() should be all you need.  Note that it frees the skb on
failure, so you must just return NETDEV_TX_OK then.

Ben.

> (Maybe the hardware
> is actually doing the padding already and the problem has to do with the way the DMA
> transfer is set up?)

On 08/21/2012 07:34 PM, Arvid Brodin :
> On 2012-08-14 22:35, Ben Hutchings wrote:
>> On Tue, 2012-08-14 at 18:53 +0000, Arvid Brodin wrote:
>>> 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?

Best regards,