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From:   Vladimir Oltean <vladimir.oltean@nxp.com>
To:     Kurt Kanzenbach <kurt@linutronix.de>
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Subject: Re: [PATCH net-next 08/13] net: dsa: hellcreek: deny tc-taprio
 changes to per-tc max SDU
Thread-Topic: [PATCH net-next 08/13] net: dsa: hellcreek: deny tc-taprio
 changes to per-tc max SDU
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Date:   Wed, 21 Sep 2022 11:29:17 +0000
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On Wed, Sep 21, 2022 at 01:23:24PM +0200, Kurt Kanzenbach wrote:
> On Thu Sep 15 2022, Vladimir Oltean wrote:
> > On Thu, Sep 15, 2022 at 08:15:54AM +0200, Kurt Kanzenbach wrote:
> >> > So the maxSDU hardware register tracks exactly the L2 payload size, =
like
> >> > the software variable does, or does it include the Ethernet header s=
ize
> >> > and/or FCS?
> >>=20
> >> This is something I'm not sure about. I'll ask the HW engineer when he=
's
> >> back from vacation.
> >
> > You can also probably figure this out by limiting the max-sdu to a valu=
e
> > like 200 and seeing what frame sizes pass through.
>=20
> So, configured to 128 and 132 bytes (including VLAN Ethernet header) is
> the maximum frame size which passes through.

Frame size means MAC DA + MAC SA + VLAN header + Ethertype + L2 payload,
and without FCS, right?

Because max_sdu 128 only counts the L2 payload, so the maximum frame
size that passes should be 142 octets, or 146 octets with VLAN.

The same is true with the port MTU. When it's 1500, the maximum frame
size is 1514, or 1518 with VLAN.=