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

Felix Fietkau
2022-04-07T18:23:05Z

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