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

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Date:   Tue, 23 Mar 2021 13:41:05 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Tobias Waldekranz <tobias@...dekranz.com>
Cc:     davem@...emloft.net, kuba@...nel.org, vivien.didelot@...il.com,
        f.fainelli@...il.com, olteanv@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: dsa: mv88e6xxx: Allow dynamic
 reconfiguration of tag protocol

On Tue, Mar 23, 2021 at 11:23:26AM +0100, Tobias Waldekranz wrote:
> All devices are capable of using regular DSA tags. Support for
> Ethertyped DSA tags sort into three categories:
> 
> 1. No support. Older chips fall into this category.
> 
> 2. Full support. Datasheet explicitly supports configuring the CPU
>    port to receive FORWARDs with a DSA tag.
> 
> 3. Undocumented support. Datasheet lists the configuration from
>    category 2 as "reserved for future use", but does empirically
>    behave like a category 2 device.
> 
> Because there are ethernet controllers that do not handle regular DSA
> tags in all cases, it is sometimes preferable to rely on the
> undocumented behavior, as the alternative is a very crippled
> system. But, in those cases, make sure to log the fact that an
> undocumented feature has been enabled.

Hi Tobias

I wonder if dynamic reconfiguration is the correct solution here. By
default it will be wrong for this board, and you need user space to
flip it.

Maybe a DT property would be better. Extend dsa_switch_parse_of() to
look for the optional property dsa,tag-protocol, a string containing
the name of the tag ops to be used.

    Andrew