netdev - Re: [PATCH net-next] net: dsa: mv88e6xxx: Allow dynamic reconfiguration of tag protocol

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Date:   Wed, 24 Mar 2021 17:08:07 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Tobias Waldekranz <tobias@...dekranz.com>
Cc:     davem@...emloft.net, kuba@...nel.org, andrew@...n.ch,
        vivien.didelot@...il.com, f.fainelli@...il.com,
        netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: dsa: mv88e6xxx: Allow dynamic
 reconfiguration of tag protocol

On Wed, Mar 24, 2021 at 04:02:52PM +0100, Tobias Waldekranz wrote:
> On Wed, Mar 24, 2021 at 16:03, Vladimir Oltean <olteanv@...il.com> wrote:
> > On Tue, Mar 23, 2021 at 10:17:30PM +0100, Tobias Waldekranz wrote:
> >> > I don't see any place in the network stack that recalculates the FCS if
> >> > NETIF_F_RXALL is set. Additionally, without NETIF_F_RXFCS, I don't even
> >> > know how could the stack even tell a packet with bad FCS apart from one
> >> > with good FCS. If NETIF_F_RXALL is set, then once a packet is received,
> >> > it's taken for granted as good.
> >> 
> >> Right, but there is a difference between a user explicitly enabling it
> >> on a device and us enabling it because we need it internally in the
> >> kernel.
> >> 
> >> In the first scenario, the user can hardly complain as they have
> >> explicitly requested to see all packets on that device. That would not
> >> be true in the second one because there would be no way for the user to
> >> turn it off. It feels like you would end up in a similar situation as
> >> with the user- vs. kernel- promiscuous setting.
> >> 
> >> It seems to me if we enable it, we are responsible for not letting crap
> >> through to the port netdevs.
> >
> > I think there exists an intermediate approach between processing the
> > frames on the RX queue and installing a soft parser.
> >
> > The BMI of FMan RX ports has a configurable pipeline through Next
> > Invoked Actions (NIA). Through the FMBM_RFNE register (Rx Frame Next
> > Engine), it is possible to change the Next Invoked Action from the
> > default value (which is the hardware parser). You can choose to make the
> > Buffer Manager Interface enqueue the packet directly to the Queue
> > Manager Interface (QMI). This will effectively bypass the hardware
> > parser, so DSA frames will never be sent to the error queue if they have
> > an invalid EtherType/Length field.
> >
> > Additionally, frames with a bad FCS should still be discarded, as that
> > is done by the MAC (an earlier stage compared to the BMI).
> 
> Yeah this sounds like the perfect middle ground. I guess that would then
> be activated with an `if (netdev_uses_dsa(dev))`-guard in the driver,
> like how Florian solved it for stmmac? Since it is not quite "rx-all".

I think this would have to be guarded by netdev_uses_dsa for now, yes.
Also, it is far from being a "perfect" middle ground, because if you
disable the hardware parser, you also lose the ability to do frame
classification and hashing/flow steering to multiple RX queues on that
port, I think.