Date:   Thu, 19 Sep 2019 10:44:16 +0200
From:   Sascha Hauer <s.hauer@...gutronix.de>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Vladimir Oltean <olteanv@...il.com>,
        netdev <netdev@...r.kernel.org>, Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...oirfairelinux.com>,
        kernel@...gutronix.de
Subject: Re: dsa traffic priorization

Hi Florian,

On Wed, Sep 18, 2019 at 10:41:58AM -0700, Florian Fainelli wrote:
> On 9/18/19 7:36 AM, Vladimir Oltean wrote:
> > Hi Sascha,
> > 
> > On Wed, 18 Sep 2019 at 17:03, Sascha Hauer <s.hauer@...gutronix.de> wrote:
> >>
> >> Hi All,
> >>
> >> We have a customer using a Marvell 88e6240 switch with Ethercat on one port and
> >> regular network traffic on another port. The customer wants to configure two things
> >> on the switch: First Ethercat traffic shall be priorized over other network traffic
> >> (effectively prioritizing traffic based on port). Second the ethernet controller
> >> in the CPU is not able to handle full bandwidth traffic, so the traffic to the CPU
> >> port shall be rate limited.
> >>
> > 
> > You probably already know this, but egress shaping will not drop
> > frames, just let them accumulate in the egress queue until something
> > else happens (e.g. queue occupancy threshold triggers pause frames, or
> > tail dropping is enabled, etc). Is this what you want? It sounds a bit
> > strange to me to configure egress shaping on the CPU port of a DSA
> > switch. That literally means you are buffering frames inside the
> > system. What about ingress policing?
> 
> Indeed, but I suppose that depending on the switch architecture and/or
> nomenclature, configuring egress shaping amounts to determining ingress
> for the ports where the frame is going to be forwarded to.
> 
> For instance Broadcom switches rarely if at all mention ingress because
> the frames have to originate from somewhere and be forwarded to other
> port(s), therefore, they will egress their original port (which for all
> practical purposes is the direct continuation of the ingress stage),
> where shaping happens, which immediately influences the ingress shaping
> of the destination port, which will egress the frame eventually because
> packets have to be delivered to the final port's egress queue anyway.
> 
> > 
> >> For reference the patch below configures the switch to their needs. Now the question
> >> is how this can be implemented in a way suitable for mainline. It looks like the per
> >> port priority mapping for VLAN tagged packets could be done via ip link add link ...
> >> ingress-qos-map QOS-MAP. How the default priority would be set is unclear to me.
> >>
> > 
> > Technically, configuring a match-all rxnfc rule with ethtool would
> > count as 'default priority' - I have proposed that before. Now I'm not
> > entirely sure how intuitive it is, but I'm also interested in being
> > able to configure this.
> 
> That does not sound too crazy from my perspective.
> 
> > 
> >> The other part of the problem seems to be that the CPU port has no network device
> >> representation in Linux, so there's no interface to configure the egress limits via tc.
> >> This has been discussed before, but it seems there hasn't been any consensous regarding how
> >> we want to proceed?
> 
> You have the DSA master network device which is on the other side of the
> switch,

You mean the (in my case) i.MX FEC? Everything I do on this device ends
up in the FEC itself and not on the switch, right?

Sascha

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