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

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Message-ID: <ab945588-7c50-1ff0-21d0-246d28c92431@denx.de>
Date:   Sat, 8 Dec 2018 05:25:24 +0100
From:   Marek Vasut <marex@...x.de>
To:     Tristram.Ha@...rochip.com
Cc:     f.fainelli@...il.com, vivien.didelot@...oirfairelinux.com,
        andrew@...n.ch, Woojung.Huh@...rochip.com,
        UNGLinuxDriver@...rochip.com, davem@...emloft.net,
        netdev@...r.kernel.org
Subject: Re: [PATCH] net: dsa: ksz: Increase the tag alignment

On 12/08/2018 01:52 AM, Tristram.Ha@...rochip.com wrote:
>> -	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?

With TI CPSW in dual-ethernet configuration, which adds internal VLAN
tag at the end of the frame, the KSZ switch fails. The CPU will send out
packets and the switch will reject them as corrupted. It needs this
extra VLAN tag padding.

-- 
Best regards,
Marek Vasut