netdev - Re: [RFC net-next 0/5] Add ETS and TBF Qdisc offload for Airoha EN7581 SoC

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Message-ID: <20241216194641.b7altsgtjjuloslx@skbuf>
Date: Mon, 16 Dec 2024 21:46:41 +0200
From: Vladimir Oltean <olteanv@...il.com>
To: Lorenzo Bianconi <lorenzo@...nel.org>
Cc: Oleksij Rempel <linux@...pel-privat.de>, netdev@...r.kernel.org,
	andrew@...n.ch, davem@...emloft.net, edumazet@...gle.com,
	kuba@...nel.org, pabeni@...hat.com, horms@...nel.org, nbd@....name,
	sean.wang@...iatek.com, Mark-MC.Lee@...iatek.com,
	lorenzo.bianconi83@...il.com
Subject: Re: [RFC net-next 0/5] Add ETS and TBF Qdisc offload for Airoha
 EN7581 SoC

On Mon, Dec 16, 2024 at 08:01:33PM +0100, Lorenzo Bianconi wrote:
> Considering patch [0], we are still offloading the Qdisc on the provided
> DSA switch port (e.g. LANx) via the port_setup_tc() callback available in
> dsa_user_setup_qdisc(), but we are introducing even the ndo_setup_tc_conduit()
> callback in order to use the hw Qdisc capabilities available on the mac chip
> (e.g. EN7581) for the routed traffic from WAN to LANx. We will still apply
> the Qdisc defined on LANx for L2 traffic from LANy to LANx. Agree?

Not quite, no.

ndo_setup_tc_conduit() does not have the same instruments to offload
what port_setup_tc() can offload. It is not involved in all data paths
that port_setup_tc() has to handle. Please ack this. So if port_setup_tc()
returns -EOPNOTSUPP, the entire dsa_user_setup_qdisc() should return
-EOPNOTSUPP, UNLESS you install packet traps on all other offloaded data
paths in the switch, such that all packets that egress the DSA user port
are handled by ndo_setup_tc_conduit()'s instruments.

> > How does the introduction of ndo_setup_tc_conduit() help, since the problem
> > is elsewhere? You are not making "tc qdisc add lanN root ets" work correctly.
> > It is simply not comparable to the way in which it is offloaded by
> > drivers/net/dsa/microchip/ksz_common.c, even though the user space
> > syntax is the same. Unless you're suggesting that for ksz it is not
> > offloaded correctly?
> 
> nope, I am not saying the current Qdisc DSA infrastructure is wrong, it just
> does not allow to exploit all hw capabilities available on EN7581 when the
> traffic is routed from the WAN port to a given DSA switch port.

And I don't believe it should, in this way.

> If we do:
> 
> $tc qdisc add dev lan0 root handle 1: ets strict 8 priomap ...
> 
> in the current upstream implementation we do:
>   dsa_user_setup_tc():
>      ...
>        -> ds->ops->port_setup_tc(ds, dp->index, type, type_data)
>           (it applies the Qdisc on lan0 configuring the hw switch)
> 
> adding the ndo_setup_tc_conduit() callback we have:
> 
>   dsa_user_setup_qdisc()
>     ...
>       -> conduit->netdev_ops->ndo_setup_tc_conduit(conduit, dp->index, type, type_data)
>          (it applies the Qdisc on EN7581 mac chip for the routed traffic destinated to lan0)
> 
>       -> ds->ops->port_setup_tc(ds, dp->index, type, type_data)
>          (it applies the Qdisc on lan0 configuring the hw switch)
> 
> > Also, flower is a classifier, not an action. It doesn't mangle packets
> > by the very definition of what a classifier is.
> 
> yes, but goal of the series is the Queue scheduler offloading, not
> classifier/action. Agree?

Classifiers + flowid are an instrument to direct packets to classes of a
classful egress Qdisc. They seem perfectly relevant to the discussion,
given the information I currently have.

> > Can't you do something like this to guide packets to the correct channels?
> > 
> > tc qdisc add dev eth0 clsact
> > tc qdisc add dev eth0 root handle 1: ets strict 8 priomap ...
> > tc filter add dev eth0 egress ${u32 or flower filter to match on DSA tagged packets} \
> > 	flowid 1:1
> 
> If we apply the Qdisc directly on the conduit port (eth0) we can just apply the
> queue scheduler on all the traffic egressing via the DSA switch while I would
> like to apply it on per DSA port basis (but using the mac chip hw capabilities),
> got my point?

We need something as the root Qdisc of the conduit which exposes its
hardware capabilities. I just assumed that would be a simple (and single)
ETS, you can correct me if I am wrong.

On conduit egress, what is the arbitration scheme between the traffic
destined towards each DSA user port (channel, as the driver calls them)?
How can this be best represented?

IIUC, in your patch set, you expose the conduit hardware QoS capabilities
as if they can be perfectly virtualized among DSA user ports, and as if
each DSA user port can have its own ETS root Qdisc, completely independent
of each other, as if the packets do not serialize on the conduit <-> CPU
port link, and as if that is not a bottleneck. Is that really the case?
If so (but please explain how), maybe you really need your own root Qdisc
driver, with one class per DSA user port, and those classes have ETS
attached to them.