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

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Date:   Tue, 7 Dec 2021 20:44:33 +0100
From:   Ansuel Smith <ansuelsmth@...il.com>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Vladimir Oltean <olteanv@...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 Tue, Dec 07, 2021 at 10:49:43AM -0800, Florian Fainelli wrote:
> On 12/7/21 7:15 AM, Andrew Lunn wrote:
> > On Tue, Dec 07, 2021 at 03:59:36PM +0100, Ansuel Smith wrote:
> >> Hi, this is still WIP and currently has some problem but I would love if
> >> someone can give this a superficial review and answer to some problem
> >> with this.
> >>
> >> The main reason for this is that we notice some routing problem in the
> >> switch and it seems assisted learning is needed. Considering mdio is
> >> quite slow due to the indirect write using this Ethernet alternative way
> >> seems to be quicker.
> >>
> >> The qca8k switch supports a special way to pass mdio read/write request
> >> using specially crafted Ethernet packet.
> > 
> > Oh! Cool! Marvell has this as well, and i suspect a few others. It is
> > something i've wanted to work on for a long long time, but never had
> > the opportunity.
> > 
> > This also means that, even if you are focusing on qca8k, please try to
> > think what could be generic, and what should specific to the
> > qca8k. The idea of sending an Ethernet frame and sometime later
> > receiving a reply should be generic and usable for other DSA
> > drivers. The contents of those frames needs to be driver specific.
> > How we hook this into MDIO might also be generic, maybe.
> > 
> > I will look at your questions later, but soon.
> 
> There was a priori attempt from Vivien to add support for mv88e6xxx over
> RMU frames:
> 
> https://www.mail-archive.com/netdev@vger.kernel.org/msg298317.html
> 
> This gets interesting because the switch's control path moves from MDIO
> to Ethernet and there is not really an "ethernet bus" though we could
> certainly come up with one. We have mdio-i2c, so maybe we should have
> mdio-ethernet?
> -- 
> Florian

I checked that series and I notice that the proposed implementation used
a workqueue. The current implementation here use completion and mutex so
the transaction is really one command at time and wait for response.
Considering most of the time we do read and write operation is seems a
bit overkill to use a queue... Also to track the response.
Using a single queue simplify the implementation and should be just
good. (btw qca8k supports a way to queue packet using a seq int but we
don't use it to keep things simple)

Is that acceptable? Also I notice in that series mru have some
limitation and can be used only for some kind of data...
Should we add a way to blacklist some particular reg and use the legacy
mdio way?

-- 
	Ansuel