Subject: Re: [PATCH v3 net-next 09/14] net: dsa: Validate hardware support for MST

From: Tobias Waldekranz <[email-masked]>
Date: Mon, 14 Mar 2022 21:01:12 +0100

On Mon, Mar 14, 2022 at 19:55, Vladimir Oltean <[email-masked]> wrote:
> On Mon, Mar 14, 2022 at 06:56:49PM +0200, Vladimir Oltean wrote:
>> > diff --git a/net/dsa/port.c b/net/dsa/port.c
>> > index 58291df14cdb..1a17a0efa2fa 100644
>> > --- a/net/dsa/port.c
>> > +++ b/net/dsa/port.c
>> > @@ -240,6 +240,10 @@ static int dsa_port_switchdev_sync_attrs(struct dsa_port *dp,
>> >  	if (err && err != -EOPNOTSUPP)
>> >  		return err;
>> >  
>> > +	err = dsa_port_mst_enable(dp, br_mst_enabled(br), extack);
>> > +	if (err && err != -EOPNOTSUPP)
>> > +		return err;
>> 
>> Sadly this will break down because we don't have unwinding on error in
>> place (sorry). We'd end up with an unoffloaded bridge port with
>> partially synced bridge port attributes. Could you please add a patch
>> previous to this one that handles this, and unoffloads those on error?
>
> Actually I would rather rename the entire dsa_port_mst_enable() function
> to dsa_port_mst_validate() and move it to the beginning of dsa_port_bridge_join().
> This simplifies the unwinding that needs to take place quite a bit.

Well you still need to unwind vlan filtering if setting the ageing time
fails, which is the most complicated one, right? Still, I agree that
_validate is a better name, and then _bridge_join seems like a more
reasonable placement. Should the unwinding patch still be part of this
series then?

While we're here, I actually made this a hard error in both scenarios
(but forgot to update the log - will do that in v4, depending on what we
decide here). There's a dilemma:

- When reacting to the attribute event, i.e. changing the mode on a
  member we're apart of, we _can't_ return -EOPNOTSUPP as it will be
  ignored, which is why dsa_port_mst_validate (nee _enable) returns
  -EINVAL.

- When joining a bridge, we _must_ return -EOPNOTSUPP to trigger the
  software fallback.

Having something like this in dsa_port_bridge_join...

err = dsa_port_mst_validate(dp);
if (err == -EINVAL)
	return -EOPNOTSUPP;
else if (err)
	return err;

...works I suppose, but feels somewhat awkwark. Any better ideas?