netdev - Re: [PATCH V2 2/3] net-next: dsa: add multi cpu port support

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Date:   Wed, 31 May 2017 02:52:42 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     John Crispin <john@...ozen.org>,
        Vivien Didelot <vivien.didelot@...oirfairelinux.com>,
        "David S . Miller" <davem@...emloft.net>,
        Sean Wang <sean.wang@...iatek.com>, jiri@...nulli.us,
        idosch@...lanox.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 2/3] net-next: dsa: add multi cpu port support

On Tue, May 30, 2017 at 05:16:27PM -0700, Florian Fainelli wrote:
> On 05/30/2017 05:06 PM, Andrew Lunn wrote:
> >> - past the initial setup, if we start creating bridge devices and so on,
> >> we have no way to tell: group Ports 0-3 together and send traffic to CPU
> >> port 0, then let Port 5 alone and send traffic to CPU port 1, that's a
> >> DSA-only problem though, because we still have the CPU port(s) as
> >> independent network interfaces.
> > 
> > What is the problem here? Frames come out the master interface, get
> > untagged and passed to the slave interface and go upto the bridge. It
> > should all just work. Same in the reverse direction.
> 
> The problem is really that is you have multiple CPU ports, how do you
> define which one gets all the traffic by default? Ascending order of
> port number? Descending order?

I would probably default to round robin when allocating user ports to
CPU ports. That probably gives you the best default.

> I actually tend to think that most use cases our there are in the order
> of dedicating one CPU port to one corresponding switch port (user
> facing, or internal) in order to provided guaranteed bandwidth for that
> port.

Which is generally a waste of bandwidth. Best case, i get 40Mbps
Internet access. Meaning 960Mbps of a dedicated cpu port would be
wasted.

>  But as an user, I want to choose how the grouping is going to
> work, and right now, I cannot, unless this is hardcoded in Device Tree,
> which sounds both wrong and inadequate.

So how about round-robin default, and then devlink to move a user port
to a specific cpu port?

We also need to watch out for asymmetry. I think newer marvell chips
don't support egress to multiple CPU ports. Ingress to the switch i
think is unlimited. The older chips are more flexible in this
respect. So we need some degree of flexibility here.
 
	Andrew