netdev - Re: [PATCH v2 14/14] net: ethernet: mtk_eth_soc: support creating mac address based offload entries

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Date:   Thu, 7 Apr 2022 20:21:43 +0200
From:   Felix Fietkau <nbd@....name>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     netdev@...r.kernel.org, John Crispin <john@...ozen.org>,
        Sean Wang <sean.wang@...iatek.com>,
        Mark Lee <Mark-MC.Lee@...iatek.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 14/14] net: ethernet: mtk_eth_soc: support creating mac
 address based offload entries


On 07.04.22 20:10, Andrew Lunn wrote:
> On Tue, Apr 05, 2022 at 09:57:55PM +0200, Felix Fietkau wrote:
>> This will be used to implement a limited form of bridge offloading.
>> Since the hardware does not support flow table entries with just source
>> and destination MAC address, the driver has to emulate it.
>> 
>> The hardware automatically creates entries entries for incoming flows, even
>> when they are bridged instead of routed, and reports when packets for these
>> flows have reached the minimum PPS rate for offloading.
>> 
>> After this happens, we look up the L2 flow offload entry based on the MAC
>> header and fill in the output routing information in the flow table.
>> The dynamically created per-flow entries are automatically removed when
>> either the hardware flowtable entry expires, is replaced, or if the offload
>> rule they belong to is removed
> 
>> +
>> +	if (found)
>> +		goto out;
>> +
>> +	eh = eth_hdr(skb);
>> +	ether_addr_copy(key.dest_mac, eh->h_dest);
>> +	ether_addr_copy(key.src_mac, eh->h_source);
>> +	tag = skb->data - 2;
>> +	key.vlan = 0;
>> +	switch (skb->protocol) {
>> +#if IS_ENABLED(CONFIG_NET_DSA)
>> +	case htons(ETH_P_XDSA):
>> +		if (!netdev_uses_dsa(skb->dev) ||
>> +		    skb->dev->dsa_ptr->tag_ops->proto != DSA_TAG_PROTO_MTK)
>> +			goto out;
>> +
>> +		tag += 4;
>> +		if (get_unaligned_be16(tag) != ETH_P_8021Q)
>> +			break;
>> +
>> +		fallthrough;
>> +#endif
>> +	case htons(ETH_P_8021Q):
>> +		key.vlan = get_unaligned_be16(tag + 2) & VLAN_VID_MASK;
>> +		break;
>> +	default:
>> +		break;
>> +	}
> 
> I'm trying to understand the architecture here.
> 
> We have an Ethernet interface and a Wireless interface. The slow path
> is that frames ingress from one of these interfaces, Linux decides
> what to do with them, either L2 or L3, and they then egress probably
> out the other interface.
> 
> The hardware will look at the frames and try to spot flows? It will
> then report any it finds. You can then add an offload, telling it for
> a flow it needs to perform L2 or L3 processing, and egress out a
> specific port? Linux then no longer sees the frame, the hardware
> handles it, until the flow times out?
Yes, the hw handles it until either the flow times out, or the 
corresponding offload entry is removed.

For OpenWrt I also wrote a daemon that uses tc classifier BPF to 
accelerate the software bridge and create hardware offload entries as 
well via hardware TC flower rules: https://github.com/nbd168/bridger
It works in combination with these changes.

> So i'm wondering what is going on here. So is this a frame which has
> ingressed, either from the WiFi, or another switch port, gone to the
> software bridge, bridges to a DSA slave interface, the DSA tagger has
> added a tag and now it is in the master interface? Can you accelerate
> such frames? What is adding the DSA tag on the fast path? And in the
> opposite direction, frames which egress the switch which have a DSA
> tag and are heading to the WiFi, what is removing the tag? Does the
> accelerator also understand the tag and know what to do with it?WiFi -> Ethernet is not supported by MT7622, but will be added for newer 
SoCs like MT7986. The PPE supports both parsing and inserting MT7530 
compatible DSA tags. For Ethernet->WiFi flows, the PPE will also add 
required metadata that is parsed by the MT7915 WiFi Firmware in order to 
figure out what vif/station the packets were meant for.

- Felix