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

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Message-ID: <20201016110311.6a43e10d@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Fri, 16 Oct 2020 11:03:11 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Christian Eggers <ceggers@...i.de>,
        Kurt Kanzenbach <kurt@...utronix.de>,
        Woojung Huh <woojung.huh@...rochip.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Microchip Linux Driver Support <UNGLinuxDriver@...rochip.com>,
        "David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] net: dsa: ksz: fix padding size of skb

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.