netdev - Re: [Query]: DSA Understanding

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Date:   Fri, 10 Aug 2018 12:26:36 +0100
From:   "Lad, Prabhakar" <prabhakar.csengg@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     netdev <netdev@...r.kernel.org>
Subject: Re: [Query]: DSA Understanding

Hi Andrew,

On Thu, Aug 9, 2018 at 6:23 PM Andrew Lunn <andrew@...n.ch> wrote:
>
> > Its coming from the switch lan4 I have attached the png, where
> > C4:F3:12:08:FE:7F is
> > the mac of lan4, which is broadcast to ff:ff:ff:ff:ff:ff, which is
> > causing rx counter on
> > PC to go up.
>
> So, big packets are making it from the switch to the PC. But the small
> ARP packets are not.
>
> This is what Florian was suggesting.
>
> ARP packets are smaller than 64 bytes, which is the minimum packet
> size for Ethernet. Any packets smaller than 64 bytes are called runt
> packets. They have to be padded upto 64 bytes in order to make them
> valid. Otherwise the destination, or any switch along the path, might
> throw them away.
>
> What could be happening is that the CSPW driver or hardware is padding
> the packet to 64 bytes. But that packet has a DSA header in it. The
> switch removes the header, recalculate the checksum and sends the
> packet. It is now either 4 or 8 bytes smaller, depending on what DSA
> header was used. It then becomes a runt packet.
>
Thank you for the clarification, this really helped me out.

> Florian had to fix this problem recently.
>
> http://patchwork.ozlabs.org/patch/836534/
>
But seems like this patch was never accepted, instead
brcm_tag_xmit_ll() does it if I am understanding it correctly.
similarly to this ksz_xmit() is taking care of padding.

> You probably need something similar for the cpsw.
>
looking at the tag_ksz.c in xmit function this is taken care of

/* For Ingress (Host -> KSZ), 2 bytes are added before FCS.
 * ---------------------------------------------------------------------------
 * DA(6bytes)|SA(6bytes)|....|Data(nbytes)|tag0(1byte)|tag1(1byte)|FCS(4bytes)
 * ---------------------------------------------------------------------------
 * tag0 : Prioritization (not used now)
 * tag1 : each bit represents port (eg, 0x01=port1, 0x02=port2, 0x10=port5)
 *
 * For Egress (KSZ -> Host), 1 byte is added before FCS.
 * ---------------------------------------------------------------------------
 * DA(6bytes)|SA(6bytes)|....|Data(nbytes)|tag0(1byte)|FCS(4bytes)
 * ---------------------------------------------------------------------------
 * tag0 : zero-based value represents port
 *      (eg, 0x00=port1, 0x02=port3, 0x06=port7)
 */

#define    KSZ_INGRESS_TAG_LEN    2
#define    KSZ_EGRESS_TAG_LEN    1

static struct sk_buff *ksz_xmit(struct sk_buff *skb, struct net_device *dev)
{
    struct dsa_slave_priv *p = netdev_priv(dev);
    struct sk_buff *nskb;
    int padlen;
    u8 *tag;

    padlen = (skb->len >= ETH_ZLEN) ? 0 : ETH_ZLEN - skb->len;

    if (skb_tailroom(skb) >= padlen + KSZ_INGRESS_TAG_LEN) {
        /* Let dsa_slave_xmit() free skb */
        if (__skb_put_padto(skb, skb->len + padlen, false))
            return NULL;

        nskb = skb;
    } else {
        nskb = alloc_skb(NET_IP_ALIGN + skb->len +
                 padlen + KSZ_INGRESS_TAG_LEN, GFP_ATOMIC);
        if (!nskb)
            return NULL;
        skb_reserve(nskb, NET_IP_ALIGN);

        skb_reset_mac_header(nskb);
        skb_set_network_header(nskb,
                       skb_network_header(skb) - skb->head);
        skb_set_transport_header(nskb,
                     skb_transport_header(skb) - skb->head);
        skb_copy_and_csum_dev(skb, skb_put(nskb, skb->len));

        /* Let skb_put_padto() free nskb, and let dsa_slave_xmit() free
         * skb
         */
        if (skb_put_padto(nskb, nskb->len + padlen))
            return NULL;

        consume_skb(skb);
    }

    tag = skb_put(nskb, KSZ_INGRESS_TAG_LEN);
    tag[0] = 0;
    tag[1] = 1 << p->dp->index; /* destination port */

    return nskb;
}

Cheers,
--Prabhakar Lad