[PATCH net] net: ethernet: mtk_eth_soc: fix DSA TX tag hwaccel for switch port 0

[Felix Fietkau]

On 07.02.23 11:30, Vladimir Oltean wrote:
> Arınç reports that on his MT7621AT Unielec U7621-06 board and MT7623NI
> Bananapi BPI-R2, packets received by the CPU over mt7530 switch port 0
> (of which this driver acts as the DSA master) are not processed
> correctly by software. More precisely, they arrive without a DSA tag
> (in packet or in the hwaccel area - skb_metadata_dst()), so DSA cannot
> demux them towards the switch's interface for port 0. Traffic from other
> ports receives a skb_metadata_dst() with the correct port and is demuxed
> properly.
> 
> Looking at mtk_poll_rx(), it becomes apparent that this driver uses the
> skb vlan hwaccel area:
> 
> 	union {
> 		u32		vlan_all;
> 		struct {
> 			__be16	vlan_proto;
> 			__u16	vlan_tci;
> 		};
> 	};
> 
> as a temporary storage for the VLAN hwaccel tag, or the DSA hwaccel tag.
> If this is a DSA master it's a DSA hwaccel tag, and finally clears up
> the skb VLAN hwaccel header.
> 
> I'm guessing that the problem is the (mis)use of API.
> skb_vlan_tag_present() looks like this:
> 
>   #define skb_vlan_tag_present(__skb)	(!!(__skb)->vlan_all)
> 
> So if both vlan_proto and vlan_tci are zeroes, skb_vlan_tag_present()
> returns precisely false. I don't know for sure what is the format of the
> DSA hwaccel tag, but I surely know that lowermost 3 bits of vlan_proto
> are 0 when receiving from port 0:
> 
> 	unsigned int port = vlan_proto & GENMASK(2, 0);
> 
> If the RX descriptor has no other bits set to non-zero values in
> RX_DMA_VTAG, then the call to __vlan_hwaccel_put_tag() will not, in
> fact, make the subsequent skb_vlan_tag_present() return true, because
> it's implemented like this:
> 
> static inline void __vlan_hwaccel_put_tag(struct sk_buff *skb,
> 					  __be16 vlan_proto, u16 vlan_tci)
> {
> 	skb->vlan_proto = vlan_proto;
> 	skb->vlan_tci = vlan_tci;
> }
> 
> What we need to do to fix this problem (assuming this is the problem) is
> to stop using skb->vlan_all as temporary storage for driver affairs, and
> just create some local variables that serve the same purpose, but
> hopefully better. Instead of calling skb_vlan_tag_present(), let's look
> at a boolean has_hwaccel_tag which we set to true when the RX DMA
> descriptors have something. Disambiguate based on netdev_uses_dsa()
> whether this is a VLAN or DSA hwaccel tag, and only call
> __vlan_hwaccel_put_tag() if we're certain it's a VLAN tag.
> 
> Arınç confirms that the treatment works, so this validates the
> assumption.
> 
> Link: https://lore.kernel.org/netdev/704f3a72-fc9e-714a-db54-272e17612637@arinc9.com/
> Fixes: 2d7605a72906 ("net: ethernet: mtk_eth_soc: enable hardware DSA untagging")
> Reported-by: Arınç ÜNAL <arinc.unal at arinc9.com>
> Tested-by: Arınç ÜNAL <arinc.unal at arinc9.com>
> Signed-off-by: Vladimir Oltean <vladimir.oltean at nxp.com>
Reviewed-by: Felix Fietkau <nbd at nbd.name>