netdev - Re: [PATCH net-next v8 3/5] net: dsa: add out-of-band tagging protocol

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Message-ID: <20221107170455.dqyf5lf55lot2hra@skbuf>
Date:   Mon, 7 Nov 2022 17:04:55 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Jakub Kicinski <kuba@...nel.org>
CC:     Maxime Chevallier <maxime.chevallier@...tlin.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "thomas.petazzoni@...tlin.com" <thomas.petazzoni@...tlin.com>,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Luka Perkov <luka.perkov@...tura.hr>,
        Robert Marko <robert.marko@...tura.hr>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...ainline.org>
Subject: Re: [PATCH net-next v8 3/5] net: dsa: add out-of-band tagging
 protocol

On Mon, Nov 07, 2022 at 08:49:34AM -0800, Jakub Kicinski wrote:
> On Mon, 7 Nov 2022 12:51:26 +0000 Vladimir Oltean wrote:
> > There is also another problem having to do with future extensibility of
> > METADATA_HW_PORT_MUX for DSA. I don't know how much of this is going to
> > be applicable for qca8k, but DSA tags might also carry such information
> > as trap reason (RX) or injection type (into forwarding plane or control
> > packet; the latter bypasses port STP state) and the FID to which the
> > packet should be classified by the hardware (TX). If we're going to
> > design a mechanism which only preallocates metadata dst's for ports,
> > it's going to be difficult to make that work for more information later on.
>
> The entire patch we're commenting on is 100 LoC. Seems like a small
> thing, which can be rewritten later as needed. I don't think hand wave-y
> arguments are sufficient to go with a much heavier solution from the
> start.

I don't think it's as hand wavey as you think. Maxime did not present
the switch-side changes or device tree in this patch set. If it's going
to be based on drivers/net/dsa/qca/qca8k-common.c as I suspect, then it
might have some obscure features which are already supported by 'normal'
QCA8K DSA switches, like register read/write over Ethernet, and MIB
autocasting. If these features exist in hardware (they aren't exposed by
this patch set for sure), you'd be hard-pressed to fit them into the
METADATA_HW_PORT_MUX model, since it's pure management traffic consumed
by the switch driver and not delivered to the network stack, as opposed
to packets sent/received on behalf of any switch port.