netdev - Re: [PATCH net-next 4/6] net: dsa: calculate the largest

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Date:   Sun, 20 Jun 2021 07:23:37 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Cc:     Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Vladimir Oltean <vladimir.oltean@....com>
Subject: Re: [PATCH net-next 4/6] net: dsa: calculate the largest_mtu across
 all ports in the tree



On 6/18/2021 11:30 AM, Vladimir Oltean wrote:
> From: Vladimir Oltean <vladimir.oltean@....com>
> 
> If we have a cross-chip topology like this:
> 
>    sw0p0     sw0p1     sw0p2     sw0p3     sw0p4
> [  cpu  ] [  user ] [  user ] [  dsa  ] [  user ]
>                                   |
>                                   +---------+
>                                             |
>    sw1p0     sw1p1     sw1p2     sw1p3     sw1p4
> [  user ] [  user ] [  user ] [  dsa  ] [  dsa  ]
> 
> and we issue the following commands:
> 
> 1. ip link set sw0p1 mtu 1700
> 2. ip link set sw1p1 mtu 1600
> 
> we notice the following happening:
> 
> Command 1. emits a non-targeted MTU notifier for the CPU port (sw0p0)
> with the largest_mtu calculated across switch 0, of 1700. This matches
> sw0p0, sw0p3 and sw1p4 (all CPU ports and DSA links).
> Then, it emits a targeted MTU notifier for the user port (sw0p1), again
> with MTU 1700 (this doesn't matter).
> 
> Command 2. emits a non-targeted MTU notifier for the CPU port (sw0p0)
> with the largest_mtu calculated across switch 1, of 1600. This matches
> the same group of ports as above, and decreases the MTU for the CPU port
> and the DSA links from 1700 to 1600.
> 
> As a result, the sw0p1 user port can no longer communicate with its CPU
> port at MTU 1700.
> 
> To address this, we should calculate the largest_mtu across all switches
> that may share a CPU port, and only emit MTU notifiers with that value.
> 
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>

Reviewed-by: Florian Fainelli <f.fainelli@...il.com>
-- 
Florian