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

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Date:   Mon, 14 Mar 2022 23:13:47 +0100
From:   Tobias Waldekranz <tobias@...dekranz.com>
To:     Vladimir Oltean <olteanv@...il.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 22:20, Vladimir Oltean <olteanv@...il.com> wrote:
> 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.

Agreed

>> 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?

I think that would mostly work. It would have to be positioned higher up
in the file though, so that it can be called from _bridge_join. Unless
we add a forward for it of course, but that seems to break with existing
conventions.