netdev - Re: [PATCH v2 net-next 11/22] net: dsa: Allow drivers to modulate between presence and absence of tagging

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Date:   Tue, 9 Apr 2019 19:17:15 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>, vivien.didelot@...il.com,
        andrew@...n.ch, davem@...emloft.net
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        georg.waibel@...sor-technik.de
Subject: Re: [PATCH v2 net-next 11/22] net: dsa: Allow drivers to modulate
 between presence and absence of tagging



On 4/9/2019 5:56 PM, Vladimir Oltean wrote:
> Frames get processed by DSA and redirected to switch port net devices
> based on the ETH_P_XDSA multiplexed packet_type handler found by the
> network stack when calling eth_type_trans().
> 
> The running assumption is that once the DSA .rcv function is called, DSA
> is always able to decode the switch tag in order to change the skb->dev
> from its master.
> 
> However there are tagging protocols (such as the new
> DSA_TAG_PROTO_SJA1105) where this assumption is not completely true,
> since switch tagging piggybacks on the absence of a vlan_filtering
> bridge.
> 
> Having DSA receive untagged traffic would put it in an impossible
> situation: the eth_type_trans() function would invoke the DSA .rcv(),
> which could not change skb->dev, then eth_type_trans() would be invoked
> again, which again would call the DSA .rcv, and the packet would never
> be able to exit the DSA filter and would spiral in a loop until the
> whole system dies.
> 
> This happens because eth_type_trans() doesn't actually look at the skb
> (so as to identify a potential tag) when it deems it as being
> ETH_P_XDSA. It just checks whether skb->dev has a DSA private pointer
> installed (therefore it's a DSA master) and that there exists a .rcv
> callback (everybody except DSA_TAG_PROTO_NONE has that). This is
> understandable as there are many switch tags out there, and exhaustively
> checking for all of them is far from ideal.
> 
> The solution lies in the observation that a more nuanced check can be
> made when eth_type_trans() determines that switch tagging is used or
> not. In a way, this reverts patch "717ffbfb28ac net: dsa: remove
> dsa_uses_tagged_protocol", but instead of adding it back as a DSA
> function, it is now a boolean property. This is because the driver might
> actually know better when it can and can't support switch tagging.
> 
> With this patch, all tagging protocols can morph at runtime into the
> DSA_TAG_PROTO_NONE on receive, by setting cpu_dp->uses_tag_protocol = 0.
> This permits them to at least terminate traffic through the master net
> device. Their .rcv callback no longer even gets called in this mode.
> 
> Signed-off-by: Vladimir Oltean <olteanv@...il.com>
> ---

[snip]

> +	/* Property used to allow traffic at runtime to bypass the DSA
> +	 * filter in eth_type_trans and be processed as regular on the
> +	 * master net device.
> +	 */
> +	bool			uses_tag_protocol;

This gets used in the hot path can you make sure this is part of the
first cache line of a dsa_port? pahole is a good tool for determining
where a member is in a given structure. With that:

Reviewed-by: Florian Fainelli <f.fainelli@...il.com>
-- 
Florian