netdev - Re: [Query]: DSA Understanding

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Date:   Mon, 13 Aug 2018 12:06:52 +0100
From:   "Lad, Prabhakar" <prabhakar.csengg@...il.com>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Andrew Lunn <andrew@...n.ch>, netdev <netdev@...r.kernel.org>
Subject: Re: [Query]: DSA Understanding

Hi Florian,

On Fri, Aug 10, 2018 at 6:36 PM Florian Fainelli <f.fainelli@...il.com> wrote:
>
> On 08/10/2018 04:26 AM, Lad, Prabhakar wrote:
> > 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.
>
> net/dsa/tag_brcm.c ended up doing the padding because that was a more
> generic and central location:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next.git/tree/net/dsa/tag_brcm.c#n73
>
> >
> >> You probably need something similar for the cpsw.
> >>
> > looking at the tag_ksz.c in xmit function this is taken care of
>
> I agree, this should be padding packets correctly, can you still
> instrument cpsw to make sure that what comes to its ndo_start_xmit() is
> ETH_ZLEN + tag_len or more?
>
Yes I can confirm the skb->len is always >= 62 (ETH_ZLEN + 2)

Cheers,
--Prabhakar