Re: [PATCH net] net: dsa: ksz: fix padding size of skb

[Jakub Kicinski]

On Fri, 16 Oct 2020 18:56:45 +0300 Vladimir Oltean wrote:
> > 3. "Manually" unsharing in dsa_slave_xmit(), reserving enough tailroom
> > for the tail tag (and ETH_ZLEN?). Would moving the "else" clause from
> > ksz_common_xmit()  to dsa_slave_xmit() do the job correctly?  
> 
> I was thinking about something like that, indeed. DSA knows everything
> about the tagger: its overhead, whether it's a tail tag or not. The xmit
> callback of the tagger should only be there to populate the tag where it
> needs to be. But reallocation, padding, etc etc, should all be dealt
> with by the common DSA xmit procedure. We want the taggers to be simple
> and reuse as much logic as possible, not to be bloated.

FWIW if you want to avoid the reallocs you may want to set
needed_tailroom on the netdev.