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

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Date:   Fri, 16 Oct 2020 18:56:45 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Christian Eggers <ceggers@...i.de>
Cc:     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>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] net: dsa: ksz: fix padding size of skb

On Fri, Oct 16, 2020 at 02:44:46PM +0200, Christian Eggers wrote:
> Machine:
> - ARMv7 (i.MX6ULL), SMP_CACHE_BYTES is 64
> - DSA device: Microchip KSZ9563 (I am currently working on time stamping support)

I have a board very similar to this on which I am going to test.

> Last, CONFIG_SLOB must be selected.

Interesting, do you know why?

> 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.