[PATCH net v3 2/3] net: stmmac: Consider Tx VLAN offload tag length for maxSDU

[Russell King (Oracle)]

On Sat, Oct 18, 2025 at 07:36:26AM +0530, G Thomas, Rohan wrote:
> Hi Russell,
> 
> Thanks, I'll update the commit message.
> 
> On 10/17/2025 6:14 PM, Russell King (Oracle) wrote:
> > On Fri, Oct 17, 2025 at 02:11:20PM +0800, Rohan G Thomas via B4 Relay wrote:
> > > From: Rohan G Thomas <rohan.g.thomas at altera.com>
> > > 
> > > On hardware with Tx VLAN offload enabled, add the VLAN tag length to
> > > the skb length before checking the Qbv maxSDU if Tx VLAN offload is
> > > requested for the packet. Add 4 bytes for 802.1Q tag.
> > 
> > This needs to say _why_. Please describe the problem that the current
> > code suffers from. (e.g. the packet becomes too long for the queue to
> > handle, which causes it to be dropped - which is my guess.)
> > 
> > We shouldn't be guessing the reasons behind changes.
> > 
> 
> Queue maxSDU requirement of 802.1 Qbv standard requires mac to drop
> packets that exceeds maxSDU length and maxSDU doesn't include preamble,
> destination and source address, or FCS but includes ethernet type and VLAN
> header.
> 
> On hardware with Tx VLAN offload enabled, VLAN header length is not
> included in the skb->len, when Tx VLAN offload is requested. This leads
> to incorrect length checks and allows transmission of oversized packets.
> Add the VLAN_HLEN to the skb->len before checking the Qbv maxSDU if Tx
> VLAN offload is requested for the packet.
> 
> This patch ensures that the VLAN header length (`VLAN_HLEN`) is
> accounted for in the SDU length check when VLAN offload is requested.

Please put that in the commit message, thanks.

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