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

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Date:   Mon, 14 Mar 2022 22:20:40 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Tobias Waldekranz <tobias@...dekranz.com>
Cc:     davem@...emloft.net, kuba@...nel.org, Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Ivan Vecera <ivecera@...hat.com>,
        Roopa Prabhu <roopa@...dia.com>,
        Nikolay Aleksandrov <razor@...ckwall.org>,
        Russell King <linux@...linux.org.uk>,
        Ido Schimmel <idosch@...dia.com>,
        Petr Machata <petrm@...dia.com>,
        Cooper Lees <me@...perlees.com>,
        Matt Johnston <matt@...econstruct.com.au>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        bridge@...ts.linux-foundation.org
Subject: Re: [PATCH v3 net-next 09/14] net: dsa: Validate hardware support
 for MST

On Mon, Mar 14, 2022 at 09:01:12PM +0100, Tobias Waldekranz wrote:
> On Mon, Mar 14, 2022 at 19:55, Vladimir Oltean <olteanv@...il.com> 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?

Yes, but we can leave that for another day :)

...ergo

> Should the unwinding patch still be part of this series then?

no.

> Still, I agree that _validate is a better name, and then _bridge_join
> seems like a more reasonable placement.
> 
> 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?

What you can do is follow the model of dsa_switch_supports_uc_filtering(),
and create a dsa_switch_supports_mst() which is called inside an
"if br_mst_enabled(br)" check, and returns bool. When false, you could
return -EINVAL or -EOPNOTSUPP, as appropriate.

This is mostly fine, except for the pesky dsa_port_can_configure_learning(dp)
check :) So while you could name it dsa_port_supports_mst() and pass it
a dsa_port, the problem is that you can't put the implementation of this
new dsa_port_supports_mst() next to dsa_switch_supports_uc_filtering()
where it would be nice to sit for symmetry, because the latter is static
inline and we're missing the definition of dsa_port_can_configure_learning().
So.. the second best thing is to keep dsa_port_supports_mst() in the
same place where dsa_port_mst_enable() currently is.

What do you think?