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Thank you for the clarifications, Andrew.
I have an extra question, does the CPU ports use Priority flow control
(PFC) using the PRI field in the DSA tag ?
Shall a device sending DSA frames to another one react to pause frames and
so on ?
I think no, it doesn't make sense for me. But I can't find anything the
confirms or denies that.
Thanks.
On Wed, Jul 28, 2021 at 2:27 AM Andrew Lunn <and...@lunn.ch> wrote:
> On Sun, Jul 25, 2021 at 11:11:41PM +0200, Shady Atef via tcpdump-workers
> wrote:
> > Date: Sun, 25 Jul 2021 23:11:41 +0200
> > From: Shady Atef <shadyoa...@gmail.com>
> > To: tcpdump-workers@lists.tcpdump.org
> > Subject: Fwd: Wrong bytes for DSA To_CPU code.
> >
> > I think the following page on the website has something wrong.
> > It lists b17,b16 and b12 as the code field for the To_CPU frames. While
> in
> > the `print_dsa.c` or the Linux kernel implementation, b18, b17 and b12
> are
> > used.
>
> The kernel gets it correct.
>
> > Another question, I work in a team building verification tool for
> Ethernet
> > designs.
> > Is there a method to differentiate between the normal Ethernet frames and
> > DSA-tagged one ?
>
> By looking at the frame itself? You can detect EDSA frames, since they
> have an Ethertype value which indicates EDSA. But the value is not
> registered with IEEE as far as i know. And the switches allow the
> value to be configured, so a vendor crap driver could use a different
> value. DSA you cannot detect in a reliable way. Maybe using heuristics
> you can hunt around for something that looks like an IP header, see
> that all frames use the same offset, and imply there is some extra
> header in there. But i think it will be hard to say what that header
> is. If you have looked at the tag drivers in Linux, you will notice
> DSA is not the only tagger to use 4 bytes.
>
> If you are dong the capture on Linux, you can determine what tagging
> protocol is in use of an interface from a file under
> /sys/class/net/.../tagging.
>
> > In other words, can a port multiplex between the two types
> > ? or each port in a DSA device is known to utilize tagging or not ?
>
> Marvell SOHO switches using {E}DSA will have all frames using
> {E}DSA. It is possible at runtime in Linux to configure which of
> {E}DSA is used, depending on the hardware capabilities, but you won't
> see a stream of frames mixing them.
>
> When you are talking about 'Linux DSA', as opposed to 'Marvell DSA',
> the majority of switches will have all frames with tags. However it is
> not a hard rule. The sja1105 does some funky things for example.
>
> Andrew
>
--
*Regards*
*Shady Atef .*
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