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

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Date:   Wed, 24 Mar 2021 01:44:15 +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

> This was my initial approach. It gets quite messy though. Since taggers
> can be modules, there is no way of knowing if a supplied protocol name
> is garbage ("asdf"), or just part of a module in an initrd that is not
> loaded yet when you are probing the tree.

Hi Tobias

I don't think that is an issue. We currently lookup the tagger in
dsa_port_parse_cpu(). If it does not exist, we return
-EPROBE_DEFER. Either it eventually gets loaded, or the driver core
gives up. I don't see why the same cannot be done for a DT
property. If dsa_find_tagger_by_name() does not find the tagger return
-EPROBE_DEFER. Garbage will result in the switch never loading, and
the DT writer will go find their typo.

> Even when the tagger is available, there is no way to verify if the
> driver is compatible with it.

I would of though, calling the switch drivers change_tag_protocol() op
will that for you. If it comes back with -EINVAL, or -EOPNOTSUPP, you
know it is not compatible.

So i guess i would keep all the code you are adding here to allow
dynamic setting of the protocol. And add more code in
dsa_switch_parse_of() to parse the optional tagging protocol name,
error out -EPROBE_DEFER if it is not known yet, otherwise store it
away in something like dst->tag_ops_name. And then probably in
dsa_switch_setup(), if dst->tag_ops_name is not NULL, invoke the
dynamic change code to perform the actual change.

	Andrew