netdev - Re: [net-next RFC PATCH v2 0/8] Add support for qca8k mdio rw in Ethernet packet

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Date:   Wed, 8 Dec 2021 16:53:41 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Ansuel Smith <ansuelsmth@...il.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [net-next RFC PATCH v2 0/8] Add support for qca8k mdio rw in
 Ethernet packet

On Wed, Dec 08, 2021 at 03:33:27PM +0100, Ansuel Smith wrote:
> > But there are some problems with offering a "master_going_up/master_going_down"
> > set of callbacks. Specifically, we could easily hook into the NETDEV_PRE_UP/
> > NETDEV_GOING_DOWN netdev notifiers and transform these into DSA switch
> > API calls. The goal would be for the qca8k tagger to mark the
> > Ethernet-based register access method as available/unavailable, and in
> > the regmap implementation, to use that or the other. DSA would then also
> > be responsible for calling "master_going_up" when the switch ports and
> > master are sufficiently initialized that traffic should be possible.
> > But that first "master_going_up" notification is in fact the most
> > problematic one, because we may not receive a NETDEV_PRE_UP event,
> > because the DSA master may already be up when we probe our switch tree.
> > This would be a bit finicky to get right. We may, for instance, hold
> > rtnl_lock for the entirety of dsa_tree_setup_master(). This will block
> > potentially concurrent netdevice notifiers handled by dsa_slave_nb.
> > And while holding rtnl_lock() and immediately after each dsa_master_setup(),
> > we may check whether master->flags & IFF_UP is true, and if it is,
> > synthesize a call to ds->ops->master_going_up(). We also need to do the
> > reverse in dsa_tree_teardown_master().
> 
> Should we care about holding the lock for that much time? Will do some
> test hoping the IFF_UP is sufficient to make the Ethernet mdio work.

I'm certainly not smart enough to optimize things, so I'd rather hold
the rtnl_lock for as long as I'm comfortable is enough to avoid races.
The reason why we must hold rtnl_lock is because during
dsa_master_setup(), the value of netdev_uses_dsa(dp->master) changes
from false to true.
The idea is that if IFF_UP isn't set right now, no problem, release the
lock and we'll catch the NETDEV_UP notifier when that will appear.
But we want to
(a) replay the master up state if it was already up while it wasn't a
    DSA master
(b) avoid a potential race where the master does go up, we receive that
    notification, but netdev_uses_dsa() doesn't yet return true for it.

The model would be similar to what we have for the NETDEV_GOING_DOWN
handler.

Please wait for me to finish the sja1105 conversion. There are some
issues I've noticed in your connect/disconnect implementation that I
haven't had a chance to comment on, yet. I've tested ocelot-8021q plus
the tagging protocol change and these appear fine.
I'd like to post the changes I have, to make sure that what works for me
works for you, and what works for you works for me. I may also have some
patches laying around that track the master up/down state (I needed
those for some RFC DSA master change patches). I'll build a mini patch
series and post it soon-ish.