netdev - Re: [PATCH v2 net-next 4/4] net: dsa: tag_dsa: Support reception of packets from LAG devices

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Date:   Tue, 01 Dec 2020 23:31:02 +0100
From:   Tobias Waldekranz <tobias@...dekranz.com>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     davem@...emloft.net, kuba@...nel.org, andrew@...n.ch,
        vivien.didelot@...il.com, f.fainelli@...il.com,
        j.vosburgh@...il.com, vfalico@...il.com, andy@...yhouse.net,
        netdev@...r.kernel.org
Subject: Re: [PATCH v2 net-next 4/4] net: dsa: tag_dsa: Support reception of packets from LAG devices

On Tue, Dec 01, 2020 at 23:24, Vladimir Oltean <olteanv@...il.com> wrote:
> On Mon, Nov 30, 2020 at 03:06:10PM +0100, Tobias Waldekranz wrote:
>> Packets ingressing on a LAG that egress on the CPU port, which are not
>> classified as management, will have a FORWARD tag that does not
>> contain the normal source device/port tuple. Instead the trunk bit
>> will be set, and the port field holds the LAG id.
>> 
>> Since the exact source port information is not available in the tag,
>> frames are injected directly on the LAG interface and thus do never
>> pass through any DSA port interface on ingress.
>> 
>> Management frames (TO_CPU) are not affected and will pass through the
>> DSA port interface as usual.
>> 
>> Signed-off-by: Tobias Waldekranz <tobias@...dekranz.com>
>> ---
>>  net/dsa/dsa.c     | 12 +++++++++++-
>>  net/dsa/tag_dsa.c | 17 ++++++++++++++++-
>>  2 files changed, 27 insertions(+), 2 deletions(-)
>> 
>> diff --git a/net/dsa/dsa.c b/net/dsa/dsa.c
>> index a1b1dc8a4d87..7325bf4608e9 100644
>> --- a/net/dsa/dsa.c
>> +++ b/net/dsa/dsa.c
>> @@ -219,11 +219,21 @@ static int dsa_switch_rcv(struct sk_buff *skb, struct net_device *dev,
>>  	}
>>  
>>  	skb = nskb;
>> -	p = netdev_priv(skb->dev);
>>  	skb_push(skb, ETH_HLEN);
>>  	skb->pkt_type = PACKET_HOST;
>>  	skb->protocol = eth_type_trans(skb, skb->dev);
>>  
>> +	if (unlikely(!dsa_slave_dev_check(skb->dev))) {
>> +		/* Packet is to be injected directly on an upper
>> +		 * device, e.g. a team/bond, so skip all DSA-port
>> +		 * specific actions.
>> +		 */
>> +		netif_rx(skb);
>> +		return 0;
>
> netif_rx returns an int code, it seems odd to ignore it.

This is exactly the same treatment that the return code from
gro_cells_receive gets just a few lines down. They return the same set
of codes (NET_RX_{SUCCESS,DROP}).

Looking through the source base, there are a few callers that look at
the return value (the overwhelming majority ignore it). Actions vary
from printing warnings (without rate-limit, yikes), setting variables
that are otherwise unused, or bumping a counter (the only reasonable
thing I have seen).

But looking through enqueue_to_backlog, it seems like there already is a
counter for this that is accessible from /proc/net/softnet_data.

>> +	}
>> +
>> +	p = netdev_priv(skb->dev);
>> +
>>  	if (unlikely(cpu_dp->ds->untag_bridge_pvid)) {
>>  		nskb = dsa_untag_bridge_pvid(skb);
>>  		if (!nskb) {