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

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Date:   Mon, 10 Dec 2018 14:29:16 +0100
From:   Marek Vasut <marex@...x.de>
To:     Florian Fainelli <f.fainelli@...il.com>, Tristram.Ha@...rochip.com
Cc:     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 06:35 PM, Florian Fainelli wrote:
> Le 12/7/18 à 8:25 PM, Marek Vasut a écrit :
>> 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.
> 
> Oh so they add the internal VLAN at the end of the frame, not the
> beginning? That is quite surprising but that would not be the one single
> oddity found with CPSW I am afraid.. The way I would approach this is
> with layering where the padding needs to occur:
> 
> - within the tag driver you need to make sure there is enough room to
> insert the KSZ tag
> 
> - within the ethernet MAC driver (which comes last in the transmit
> path), you need to make sure there is enough room to insert that trailer
> VLAN tag to permit internal transmission

So you think this is a bug in the CPSW instead ?

-- 
Best regards,
Marek Vasut