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

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Date:   Mon, 7 Nov 2022 09:39:50 +0100
From:   Maxime Chevallier <maxime.chevallier@...tlin.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     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,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        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,
        Vladimir Oltean <vladimir.oltean@....com>,
        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

Hello Jakub,

On Fri, 4 Nov 2022 20:05:30 -0700
Jakub Kicinski <kuba@...nel.org> wrote:

> On Fri,  4 Nov 2022 18:41:49 +0100 Maxime Chevallier wrote:
> > This tagging protocol is designed for the situation where the link
> > between the MAC and the Switch is designed such that the Destination
> > Port, which is usually embedded in some part of the Ethernet
> > Header, is sent out-of-band, and isn't present at all in the
> > Ethernet frame.
> > 
> > This can happen when the MAC and Switch are tightly integrated on an
> > SoC, as is the case with the Qualcomm IPQ4019 for example, where
> > the DSA tag is inserted directly into the DMA descriptors. In that
> > case, the MAC driver is responsible for sending the tag to the
> > switch using the out-of-band medium. To do so, the MAC driver needs
> > to have the information of the destination port for that skb.
> > 
> > Add a new tagging protocol based on SKB extensions to convey the
> > information about the destination port to the MAC driver  
> 
> This is what METADATA_HW_PORT_MUX is for, you shouldn't have 
> to allocate a piece of memory for every single packet.

Does this work with DSA ? The information conveyed in the extension is
the DSA port identifier. I'm not familiar at all with
METADATA_HW_PORT_MUX, should we extend that mechanism to convey the
DSA port id ?

I also agree that allocating data isn't the best way to go, but from
the history of this series, we've tried 3 approaches so far :

 - Adding a new field to struct sk_buff, which isn't a good idea
 - Using the skb headroom, but then we can't know for sure is the skb
   contains a DSA tag or not
 - Using skb extensions, that comes with the cost of this memory
   allocation. Is this approach also incorrect then ?

> Also the series doesn't build.

Can you elaborate more ? I can't reproduce the build failure on my
side, and I didn't get any reports from the kbuild bot, are you using a
specific config file ?

Thanks,

Maxime