linux-kernel - Re: [net-next RFC PATCH 0/6] Add support for qca8k mdio rw in Ethernet packet

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Date:   Wed, 8 Dec 2021 01:20:20 +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 0/6] Add support for qca8k mdio rw in
 Ethernet packet

On Wed, Dec 08, 2021 at 12:05:11AM +0100, Ansuel Smith wrote:
> Hm. Interesting idea. So qca8k would provide the way to parse the packet
> and made the request. The tagger would just detect the packet and
> execute the dedicated function.
> About mib considering the driver autocast counter for every port and
> every packet have the relevant port to it (set in the qca tag), the
> idea was to put a big array and directly write the data. The ethtool
> function will then just read the data and report it. (or even work
> directly on the ethtool data array).

Apart from the fact that you'd be running inside the priv->rw_reg_ack_handler()
which runs in softirq context (so you need spinlocks to serialize with
the code that runs in process and/or workqueue context), you have access
to all the data structures from the switch driver that you're used to.
So you could copy from the void *buf into something owned by struct
qca8k_priv *priv, sure.

> >   My current idea is maybe not ideal and a bit fuzzy, because the switch
> >   driver would need to be aware of the fact that the tagger private data
> >   is in dp->priv, and some code in one folder needs to be in sync with
> >   some code in another folder. But at least it should be safer this way,
> >   because we are in more control over the exact connection that's being
> >   made.
> > 
> > - to avoid leaking memory, we also need to patch dsa_tree_put() to issue
> >   a disconnect event on unbind.
> > 
> > - the tagging protocol driver would always need to NULL-check the
> >   function pointer before dereferencing it, because it may connect to a
> >   switch driver that doesn't set them up (dsa_loop):
> > 
> > 	struct qca8k_tagger_private *priv = dp->priv;
> > 
> > 	if (priv->rw_reg_ack_handler)
> > 		priv->rw_reg_ack_handler(dp, skb_mac_header(skb));
> 
> Ok so your idea is to make the driver the one controlling ""everything""
> and keep the tagger as dummy as possible. That would also remove all the
> need to put stuff in the global include dir. Looks complex but handy. We
> still need to understand the state part. Any hint about that?
> 
> In the mean time I will try implement this.

What do you mean exactly by understanding the state?