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Date:   Sat, 4 May 2019 05:23:23 +0300
From:   Vladimir Oltean <>
To:     Florian Fainelli <>
Cc:, Andrew Lunn <>,
        "David S. Miller" <>,
        netdev <>,
Subject: Re: [PATCH net-next 4/9] net: dsa: Keep private info in the skb->cb

On Sat, 4 May 2019 at 05:04, Florian Fainelli <> wrote:
> On 5/3/2019 6:18 PM, Vladimir Oltean wrote:
> > Map a DSA structure over the 48-byte control block that will hold
> > persistent skb info on transmit and receive.
> >
> On receive you cannot quite do that because you don't know if the DSA
> master network device calls netif_receive_skb() or napi_gro_receive().
> The latter arguably may not be able to aggregate flows at all because it
> does not know how to parse the SKB, but the point remains that skb->cb[]
> on receive may already be used, up to 48 bytes already. I tried to make
> use of it for storing the HW extracted Broadcom tag, but it blew the
> budge on 64-bit hosts:
> Not asking you to change anything here, just to be aware of it.
> > Also add a DSA_SKB_CB_PRIV() macro which retrieves a pointer to the
> > space up to 48 bytes that the DSA structure does not use. This space can
> > be used for drivers to add their own private info.
> >
> > One use is for the PTP timestamping code path. When cloning a skb,
> > annotate the original with a pointer to the clone, which the driver can
> > then find easily and place the timestamp to. This avoids the need of a
> > separate queue to hold clones and a way to match an original to a cloned
> > skb.
> >
> > Signed-off-by: Vladimir Oltean <>
> Reviewed-by: Florian Fainelli <>
> --
> Florian

Hi Florian,
This is good to know. So I'm understanding that you tried to pass data
in the skb->cb between the master netdev and DSA, and GRO got
in-between? That's kind of expected in a way. I'm proposing a more
minimalistic use even on receive, which should be ok. If you look at
07/09 you'll see I'm setting the frame type from within the
eth_type_trans callback (technically still not in the DSA packet_type
handler yet, but quite close) and using it in the actual rcv. Then for
timestamping I'm communicating between the rcv function and a
workqueue that I start from .port_rxtstamp.

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