netdev - RE: [PATCH] net: dsa: ksz: Increase the tag alignment

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Date:   Sat, 8 Dec 2018 00:52:08 +0000
From:   <Tristram.Ha@...rochip.com>
To:     <marex@...x.de>
CC:     <f.fainelli@...il.com>, <vivien.didelot@...oirfairelinux.com>,
        <andrew@...n.ch>, <Woojung.Huh@...rochip.com>,
        <UNGLinuxDriver@...rochip.com>, <Woojung.Huh@...rochip.com>,
        <davem@...emloft.net>, <netdev@...r.kernel.org>
Subject: RE: [PATCH] net: dsa: ksz: Increase the tag alignment

> -	padlen = (skb->len >= ETH_ZLEN) ? 0 : ETH_ZLEN - skb->len;
> +	padlen = (skb->len >= VLAN_ETH_ZLEN) ? 0 : VLAN_ETH_ZLEN - skb-
> >len;

The requirement is the tail tag should be at the end of frame before FCS.
When the length is less than 60 the MAC controller will pad the frame to
legal size.  That is why this function makes sure the padding is done
manually.  Increasing the size just increases the length of the frame and the
chance to allocate new socket buffer.

Example of using ping size of 18 will have the sizes of request and response
differ by 4 bytes.  Not that it matters much.

Are you concerned the MAC controller will remove the VLAN tag and so the frame
will not be sent? Or the switch removes the VLAN tag and is not able to send?