Re: [PATCH net-next 08/13] net: dsa: hellcreek: deny tc-taprio changes to per-tc max SDU

[Vladimir Oltean]

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 payload,
> > > and without FCS, right?
> > 
> > Yes.
> > 
> > >
> > > 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.
> > 
> > Ok, i see. So, for 128 max-sdu we should end up with something like this
> > in the prio config register:
> > 
> >  schedule->max_sdu[tc] + VLAN_ETH_HLEN - 4
> 
> What does 4 represent? ETH_FCS_LEN?
> 
> So when schedule->max_sdu[tc] is 128, you write to HR_PTPRTCCFG_MAXSDU
> the value of 128 + 18 - 4 = 142, and this will pass packets (VLAN-tagged
> or not) with an skb->len (on xmit) <= 142, right?

Mistake, I meant skb->len <= 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.