Re: [PATCH net-next v2 2/5] net: dsa: add out-of-band tagging protocol

[Maxime Chevallier <maxime.chevallier@bootlin.com>]

Hi Vlad,

On Sat, 14 May 2022 22:40:03 +0000
Vladimir Oltean <vladimir.oltean@nxp.com> wrote:

> On Sat, May 14, 2022 at 05:06:53PM +0200, 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.
> > 
> > This out-of-band tagging protocol is using the very beggining of
> > the skb headroom to store the tag. The drawback of this approch is
> > that the headroom isn't initialized upon allocating it, therefore
> > we have a chance that the garbage data that lies there at
> > allocation time actually ressembles a valid oob tag. This is only
> > problematic if we are sending/receiving traffic on the master port,
> > which isn't a valid DSA use-case from the beggining. When dealing
> > from traffic to/from a slave port, then the oob tag will be
> > initialized properly by the tagger or the mac driver through the
> > use of the dsa_oob_tag_push() call.
> > 
> > Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
> > ---  
> 
> Why put the DSA pseudo-header at skb->head rather than push it using
> skb_push()? I thought you were going to check for the presence of a
> DSA header using something like skb->mac_len == ETH_HLEN + tag len,
> but right now it sounds like treating garbage in the headroom as a
> valid DSA tag is indeed a potential problem. If you can't sort that
> out using information from the header offsets alone, maybe an skb
> extension is required?

Indeed, I thought of that, the main reason is that pushing/poping in
itself is not enough, you also have to move the whole mac_header to
leave room for the tag, and then re-set it in it's original location.
There's nothing wrong with this, but it looked a bit cumbersome just to
insert a dummy tag that gets removed rightaway. Does that make sense ?

But yes I would really like to get a way to know wether the tag is
there or not, I'll dig a bit more to see if I can find a way to get
this info from the various skb offsets in a reliable way.

Thanks,

Maxime