netdev - Re: [PATCH v2 2/2] drivers: net: dsa: qca8k: add support for multiple cpu port

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Date:   Tue, 6 Apr 2021 07:16:08 +0200
From:   Ansuel Smith <ansuelsmth@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] drivers: net: dsa: qca8k: add support for
 multiple cpu port

On Wed, Apr 07, 2021 at 02:41:02AM +0200, Andrew Lunn wrote:
> On Tue, Apr 06, 2021 at 06:50:40AM +0200, Ansuel Smith wrote:
> > qca8k 83xx switch have 2 cpu ports. Rework the driver to support
> > multiple cpu port. All ports can access both cpu ports by default as
> > they support the same features.
> 
> Do you have more information about how this actually works. How does
> the switch decide which port to use when sending a frame towards the
> CPU? Is there some sort of load balancing?
> 
> How does Linux decide which CPU port to use towards the switch?
> 
>     Andrew

I could be very wrong, but in the current dsa code, only the very first
cpu port is used and linux use only that to send data.
In theory the switch send the frame to both CPU, I'm currently testing a
multi-cpu patch for dsa and I can confirm that with the proposed code
the packets are transmitted correctly and the 2 cpu ports are used.
(The original code has one cpu dedicated to LAN ports and one cpu
dedicated to the unique WAN port.) Anyway in the current implementation
nothing will change. DSA code still supports one cpu and this change
would only allow packet to be received and trasmitted from the second
cpu.