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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 14:21:16 +0000
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On Wed, Sep 21, 2022 at 05:12:06PM +0300, Vladimir Oltean wrote:
> On Wed, Sep 21, 2022 at 01:46:22PM +0200, Kurt Kanzenbach wrote:
> > >> 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 paylo=
ad,
> > > and without FCS, right?
> >=20
> > Yes.
> >=20
> > >
> > > 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.
> >=20
> > Ok, i see. So, for 128 max-sdu we should end up with something like thi=
s
> > in the prio config register:
> >=20
> >  schedule->max_sdu[tc] + VLAN_ETH_HLEN - 4
>=20
> What does 4 represent? ETH_FCS_LEN?
>=20
> So when schedule->max_sdu[tc] is 128, you write to HR_PTPRTCCFG_MAXSDU
> the value of 128 + 18 - 4 =3D 142, and this will pass packets (VLAN-tagge=
d
> or not) with an skb->len (on xmit) <=3D 142, right?

Mistake, I meant skb->len <=3D 146 (max_sdu[tc] + VLAN_ETH_HLEN). So the
hardware calculates the max frame len to include FCS, and skb->len is
without FCS, hence the 4 discrepancy.=