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

Felix Fietkau
2022-04-12T07:15:42Z

On 11.04.22 15:00, Andrew Lunn wrote:
> On Thu, Apr 07, 2022 at 08:21:43PM +0200, Felix Fietkau wrote:
>>
>> 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.
>
> What about the bridge? In Linux, it is the software bridge which
> controls all this at L2, and it should be offloading the flows, via
> switchdev. The egress port you derive here is from the software bridge
> FDB?
My code uses netlink to fetch and monitor the bridge configuration,
including fdb, port state, vlans, etc. and it uses that for the offload
path - no extra configuration needed.

>> > 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.
>
> O.K. What about IGMP and multicast? Does the accelerate match on IGMP
> and forwards it to the CPU, rather than follow the flow rules? Can you
> set multiple egress destinations for multicast so that it can go both
> to the switch and the host, when the host has a local interest in the
> traffic?
IGMP/multicast isn't handled yet at the moment. I still need to do some
research on what can be offloaded and how. The offload only handles
unicast and everything else is going through the CPU.

- Felix