netdev - Re: [PATCH net-next 4/4] net: dsa: implement and use a generic procedure for the flow dissector

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Message-ID: <CA+h21hpXpakJ06BufXRzWeAVG5-av2zTzVLjcz9NUvnWpAB7Bg@mail.gmail.com>
Date:   Tue, 12 May 2020 04:11:11 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 4/4] net: dsa: implement and use a generic
 procedure for the flow dissector

On Tue, 12 May 2020 at 02:15, Florian Fainelli <f.fainelli@...il.com> wrote:
>
>
>
> On 5/11/2020 1:20 PM, Vladimir Oltean wrote:
> > From: Vladimir Oltean <vladimir.oltean@....com>
> >
> > For all DSA formats that don't use tail tags, it looks like behind the
> > obscure number crunching they're all doing the same thing: locating the
> > real EtherType behind the DSA tag. Nonetheless, this is not immediately
> > obvious, so create a generic helper for those DSA taggers that put the
> > header before the EtherType.
> >
> > Another assumption for the generic function is that the DSA tags are of
> > equal length on RX and on TX. Prior to the previous patch, this was not
> > true for ocelot and for gswip. The problem was resolved for ocelot, but
> > for gswip it still remains, so that hasn't been converted.
> >
> > Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>
>
> Given that __skb_flow_dissect() already de-references dsa_device_ops
> from skb->dev->dsa_ptr, maybe we can go one step further and just have
> dsa_tag_generic_flow_dissect obtain the overhead from the SKB directly
> since this will already have touched the cache lines involved. This then
> makes it unnecessary for the various taggers to specify a custom
> function and instead, dsa_tag_generic_flow_dissect() can be assigned
> where the dsa_device_ops are declared for the various tags. Did I miss
> something?
>
> It also looks like tag_ocelot.c and tag_sja1105.c should have their
> dsa_device_ops structures const, as a separate patch certainly.
> --
> Florian

Actually I wrote this patch in such a way that the assembly code
generated would remain the same, since dsa_tag_generic_flow_dissect is
inline, the arithmetic would be done at compile time, which it
wouldn't be if this function were to look at the tagger .overhead.
I don't know, I can do either way.