netdev - Re: [PATCH net-next] net: dsa: mv88e6xxx: Allow dynamic reconfiguration of tag protocol

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Message-ID: <20210325013432.muugsogq4mzmalpd@skbuf>
Date:   Thu, 25 Mar 2021 03:34:32 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Tobias Waldekranz <tobias@...dekranz.com>
Cc:     davem@...emloft.net, kuba@...nel.org, andrew@...n.ch,
        vivien.didelot@...il.com, f.fainelli@...il.com,
        netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: dsa: mv88e6xxx: Allow dynamic
 reconfiguration of tag protocol

On Wed, Mar 24, 2021 at 05:07:09PM +0100, Tobias Waldekranz wrote:
> But even if the parser was enabled, it would never get anywhere since
> the Ethertype would look like random garbage. Unless we have the soft
> parser, but then it is not the middle ground anymore :)

Garbage, true, but garbage with enough entropy to allow for some sort of
RFS (ideally you can get the source port field from the DSA tag into the
area covered by the n-tuple on which the master performs hashing). This
is the way in which the switches inside NXP LS1028A and T1040 work.

> I suppose you would like to test for netdev_uses_dsa_and_violates_8023,
> that way you could still do RSS on DSA devices using regular 1Q-tags for
> example. Do we want to add this property to the taggers so that we do
> not degrade performance for any existing users?

Yes, so T1040 is one such example of device that would be negatively
affected by this change. There isn't a good solution to solve all
problems: there will be some Marvell switches which can't operate in
EDSA mode, and there will be some DSA masters that can't parse Marvell
DSA tags. Eventually all possible combinations of workarounds will have
to be implemented. But for now, I think I prefer to see the simplest
one, which has just become the one based on device tree.