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Date:   Wed, 8 Apr 2020 22:51:55 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     netdev <netdev@...r.kernel.org>, Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Ido Schimmel <idosch@...sch.org>,
        Jiri Pirko <jiri@...nulli.us>, Jakub Kicinski <kuba@...nel.org>
Subject: Re: Changing devlink port flavor dynamically for DSA

Hi Florian,

On Sun, 5 Apr 2020 at 23:42, Florian Fainelli <f.fainelli@...il.com> wrote:
>
> Hi all,
>
> On a BCM7278 system, we have two ports of the switch: 5 and 8, that
> connect to separate Ethernet MACs that the host/CPU can control. In
> premise they are both interchangeable because the switch supports
> configuring the management port to be either 5 or 8 and the Ethernet
> MACs are two identical instances.
>
> The Ethernet MACs are scheduled differently across the memory controller
> (they have different bandwidth and priority allocations) so it is
> desirable to select an Ethernet MAC capable of sustaining bandwidth and
> latency for host networking. Our current (in the downstream kernel) use
> case is to expose port 5 solely as a control end-point to the user and
> leave it to the user how they wish to use the Ethernet MAC behind port
> 5. Some customers use it to bridge Wi-Fi traffic, some simply keep it
> disabled. Port 5 of that switch does not make use of Broadcom tags in
> that case, since ARL-based forwarding works just fine.
>
> The current Device Tree representation that we have for that system
> makes it possible for either port to be elected as the CPU port from a
> DSA perspective as they both have an "ethernet" phandle property that
> points to the appropriate Ethernet MAC node, because of that the DSA
> framework treats them as CPU ports.
>
> My current line of thinking is to permit a port to be configured as
> either "cpu" or "user" flavor and do that through devlink. This can
> create some challenges but hopefully this also paves the way for finally
> supporting "multi-CPU port" configurations. I am thinking something like
> this would be how I would like it to be configured:
>
> # First configure port 8 as the new CPU port
> devlink port set pci/0000:01:00.0/8 type cpu
> # Now unmap port 5 from being a CPU port
> devlink port set pci/0000:01:00.0/1 type eth
>
> and this would do a simple "swap" of all user ports being now associated
> with port 8, and no longer with port 5, thus permitting port 5 from
> becoming a standard user port. Or maybe, we need to do this as an atomic
> operation in order to avoid a switch being configured with no CPU port
> anymore, so something like this instead:
>
> devlink port set pci/0000:01:00.0/5 type eth mgmt pci/0000:01:00.0/8
>
> The latter could also be used to define groups of ports within a switch
> that has multiple CPU ports, e.g.:
>
> # Ports 1 through 4 "bound" to CPU port 5:
>
> for i in $(seq 0 3)
> do
>         devlink port set pci/0000:01:00.0/$i type eth mgmt pci/0000:01:00.0/5
> done
>
> # Ports 7 bound to CPU port 8:
>
> devlink port set pci/0000:01:00.0/1 type eth mgmt pci/0000:01:00.0/8
>
> Let me know what you think!
>
> Thanks
> --
> Florian

What is missing from your argumentation is what would the new devlink
mechanism of changing the CPU port bring for your particular use case.
I mean you can already remove the "ethernet" device tree property from
port 5 and end up exactly with the configuration that you want, no?

Regards,
-Vladimir

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