DSA and skb->protocol

Hi Florian

I've been debugging a WARNING when using DSA with a D-Link
DIR665. I've had reports of the same warning with another device.

WARNING: CPU: 0 PID: 2014 at net/core/dev.c:2260 skb_warn_bad_offload+0xd0/0x104()
mv643xx_eth_port: caps=(0x0000000400014803, 0x00000000001b482b) len=1722 data_len=16 gso_size=1448 gso_type=1 gso_segs 2 ip_summed=3 encapsulation 0 features 400014801
Modules linked in:
CPU: 0 PID: 2014 Comm: sshd Tainted: G        W      3.17.0-rc1-00007-g2f06b2c08099-dirty #228
[<c000e2a0>] (unwind_backtrace) from [<c000bd88>] (show_stack+0x10/0x14)
[<c000bd88>] (show_stack) from [<c0016db8>] (warn_slowpath_common+0x6c/0x8c)
[<c0016db8>] (warn_slowpath_common) from [<c0016e08>] (warn_slowpath_fmt+0x30/0x40)
[<c0016e08>] (warn_slowpath_fmt) from [<c04ebad4>] (skb_warn_bad_offload+0xd0/0x104)
[<c04ebad4>] (skb_warn_bad_offload) from [<c03eea5c>] (skb_checksum_help+0x160/0x170)
[<c03eea5c>] (skb_checksum_help) from [<c03eef40>] (dev_hard_start_xmit+0x3f8/0x4c0)
[<c03eef40>] (dev_hard_start_xmit) from [<c04071ec>] (sch_direct_xmit+0x148/0x250)
[<c04071ec>] (sch_direct_xmit) from [<c03ef280>] (__dev_queue_xmit+0x278/0x5f4)
[<c03ef280>] (__dev_queue_xmit) from [<c04719b8>] (edsa_xmit+0xf8/0x2c8)
[<c04719b8>] (edsa_xmit) from [<c03eee24>] (dev_hard_start_xmit+0x2dc/0x4c0)
[<c03eee24>] (dev_hard_start_xmit) from [<c03ef3cc>] (__dev_queue_xmit+0x3c4/0x5f4)
[<c03ef3cc>] (__dev_queue_xmit) from [<c0415e88>] (ip_finish_output+0x64c/0x920)
[<c0415e88>] (ip_finish_output) from [<c0416e2c>] (ip_local_out_sk+0x34/0x38)
[<c0416e2c>] (ip_local_out_sk) from [<c0417144>] (ip_queue_xmit+0x128/0x388)
[<c0417144>] (ip_queue_xmit) from [<c042caa8>] (tcp_transmit_skb+0x534/0x93c)
[<c042caa8>] (tcp_transmit_skb) from [<c042cff8>] (tcp_write_xmit+0x148/0xbf8)
[<c042cff8>] (tcp_write_xmit) from [<c042dd7c>] (__tcp_push_pending_frames+0x30/0x9c)
[<c042dd7c>] (__tcp_push_pending_frames) from [<c041fbc8>] (tcp_sendmsg+0xc0/0xcec)
[<c041fbc8>] (tcp_sendmsg) from [<c0445710>] (inet_sendmsg+0x3c/0x70)
[<c0445710>] (inet_sendmsg) from [<c03d7d5c>] (sock_aio_write+0xcc/0xec)
[<c03d7d5c>] (sock_aio_write) from [<c00bb218>] (do_sync_write+0x7c/0xa4)
[<c00bb218>] (do_sync_write) from [<c00bbc40>] (vfs_write+0x108/0x1b0)
[<c00bbc40>] (vfs_write) from [<c00bc214>] (SyS_write+0x40/0x94)
[<c00bc214>] (SyS_write) from [<c0009480>] (ret_fast_syscall+0x0/0x2c)
---[ end trace b1b02d15aba4766a ]---

I think i understand what is going on, and one of your recent patches
may indicate you have seen something similar.

int dev_hard_start_xmit() we have:

2607 
2608                 features = netif_skb_features(skb);

...
2637                         /* If packet is not checksummed and device does not
2638                          * support checksumming for this protocol, complete
2639                          * checksumming here.
2640                          */
2641                         if (skb->ip_summed == CHECKSUM_PARTIAL) {
2642                                 if (skb->encapsulation)
2643                                         skb_set_inner_transport_header(skb,
2644                                                 skb_checksum_start_offset(skb));
2645                                 else
2646                                         skb_set_transport_header(skb,
2647                                                 skb_checksum_start_offset(skb));
2648                                 if (!(features & NETIF_F_ALL_CSUM) &&
2649                                      skb_checksum_help(skb))
2650                                         goto out_kfree_skb;
2651                         }

This packet is CHECKSUM_PARTIAL, but it is also a GSO packet, with two
segments, so the skb_checksum_help() is throwing the warning. It is
expected that the hardware with do the checksum when using GSO. The
reason it is trying to do software checksums, not hardware, is because
features does not indicate the protocol is supported by hardware
checksumming. This i initially thought was odd, since this is a TCP/IP
packet, as you can see from the stack trace. However, 

netif_skb_features(skb) calls harmonize_features() which calls
can_checksum_protocol():

3206 static inline bool can_checksum_protocol(netdev_features_t features,
3207                                          __be16 protocol)
3208 {
3209         return ((features & NETIF_F_GEN_CSUM) ||
3210                 ((features & NETIF_F_V4_CSUM) &&
3211                  protocol == htons(ETH_P_IP)) ||
3212                 ((features & NETIF_F_V6_CSUM) &&
3213                  protocol == htons(ETH_P_IPV6)) ||
3214                 ((features & NETIF_F_FCOE_CRC) &&
3215                  protocol == htons(ETH_P_FCOE)));
3216 }

However looking in the skb, we see protocol is now ETH_P_EDSA, not
ETH_P_IP. Hence the hardware does not support this protocol.

This protocol value is because edsa_xmit() changes it in the slave
device TX path.

Your patch "net: dsa: change tag_protocol to an enum"
has a hunk:

making me think it is not actually needed to change
skb->protocol. When i remove this from edsa_xmit(), the warning goes
away and networking seems to work O.K.

Is this the right fix? Should we remove this setting of skb->protocol
in the other dsa xmit functions?

Thanks
	Andrew

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Hi Andrew,

On 09/14/2014 08:37 AM, Andrew Lunn wrote:
> Hi Florian
> 
> I've been debugging a WARNING when using DSA with a D-Link
> DIR665. I've had reports of the same warning with another device.
> 
> WARNING: CPU: 0 PID: 2014 at net/core/dev.c:2260 skb_warn_bad_offload+0xd0/0x104()
> mv643xx_eth_port: caps=(0x0000000400014803, 0x00000000001b482b) len=1722 data_len=16 gso_size=1448 gso_type=1 gso_segs 2 ip_summed=3 encapsulation 0 features 400014801
> Modules linked in:
> CPU: 0 PID: 2014 Comm: sshd Tainted: G        W      3.17.0-rc1-00007-g2f06b2c08099-dirty #228
> [<c000e2a0>] (unwind_backtrace) from [<c000bd88>] (show_stack+0x10/0x14)
> [<c000bd88>] (show_stack) from [<c0016db8>] (warn_slowpath_common+0x6c/0x8c)
> [<c0016db8>] (warn_slowpath_common) from [<c0016e08>] (warn_slowpath_fmt+0x30/0x40)
> [<c0016e08>] (warn_slowpath_fmt) from [<c04ebad4>] (skb_warn_bad_offload+0xd0/0x104)
> [<c04ebad4>] (skb_warn_bad_offload) from [<c03eea5c>] (skb_checksum_help+0x160/0x170)
> [<c03eea5c>] (skb_checksum_help) from [<c03eef40>] (dev_hard_start_xmit+0x3f8/0x4c0)
> [<c03eef40>] (dev_hard_start_xmit) from [<c04071ec>] (sch_direct_xmit+0x148/0x250)
> [<c04071ec>] (sch_direct_xmit) from [<c03ef280>] (__dev_queue_xmit+0x278/0x5f4)
> [<c03ef280>] (__dev_queue_xmit) from [<c04719b8>] (edsa_xmit+0xf8/0x2c8)
> [<c04719b8>] (edsa_xmit) from [<c03eee24>] (dev_hard_start_xmit+0x2dc/0x4c0)
> [<c03eee24>] (dev_hard_start_xmit) from [<c03ef3cc>] (__dev_queue_xmit+0x3c4/0x5f4)
> [<c03ef3cc>] (__dev_queue_xmit) from [<c0415e88>] (ip_finish_output+0x64c/0x920)
> [<c0415e88>] (ip_finish_output) from [<c0416e2c>] (ip_local_out_sk+0x34/0x38)
> [<c0416e2c>] (ip_local_out_sk) from [<c0417144>] (ip_queue_xmit+0x128/0x388)
> [<c0417144>] (ip_queue_xmit) from [<c042caa8>] (tcp_transmit_skb+0x534/0x93c)
> [<c042caa8>] (tcp_transmit_skb) from [<c042cff8>] (tcp_write_xmit+0x148/0xbf8)
> [<c042cff8>] (tcp_write_xmit) from [<c042dd7c>] (__tcp_push_pending_frames+0x30/0x9c)
> [<c042dd7c>] (__tcp_push_pending_frames) from [<c041fbc8>] (tcp_sendmsg+0xc0/0xcec)
> [<c041fbc8>] (tcp_sendmsg) from [<c0445710>] (inet_sendmsg+0x3c/0x70)
> [<c0445710>] (inet_sendmsg) from [<c03d7d5c>] (sock_aio_write+0xcc/0xec)
> [<c03d7d5c>] (sock_aio_write) from [<c00bb218>] (do_sync_write+0x7c/0xa4)
> [<c00bb218>] (do_sync_write) from [<c00bbc40>] (vfs_write+0x108/0x1b0)
> [<c00bbc40>] (vfs_write) from [<c00bc214>] (SyS_write+0x40/0x94)
> [<c00bc214>] (SyS_write) from [<c0009480>] (ret_fast_syscall+0x0/0x2c)
> ---[ end trace b1b02d15aba4766a ]---
> 
> I think i understand what is going on, and one of your recent patches
> may indicate you have seen something similar.
> 
> int dev_hard_start_xmit() we have:
> 
> 2607 
> 2608                 features = netif_skb_features(skb);
> 
> ...
> 2637                         /* If packet is not checksummed and device does not
> 2638                          * support checksumming for this protocol, complete
> 2639                          * checksumming here.
> 2640                          */
> 2641                         if (skb->ip_summed == CHECKSUM_PARTIAL) {
> 2642                                 if (skb->encapsulation)
> 2643                                         skb_set_inner_transport_header(skb,
> 2644                                                 skb_checksum_start_offset(skb));
> 2645                                 else
> 2646                                         skb_set_transport_header(skb,
> 2647                                                 skb_checksum_start_offset(skb));
> 2648                                 if (!(features & NETIF_F_ALL_CSUM) &&
> 2649                                      skb_checksum_help(skb))
> 2650                                         goto out_kfree_skb;
> 2651                         }
> 
> This packet is CHECKSUM_PARTIAL, but it is also a GSO packet, with two
> segments, so the skb_checksum_help() is throwing the warning. It is
> expected that the hardware with do the checksum when using GSO. The
> reason it is trying to do software checksums, not hardware, is because
> features does not indicate the protocol is supported by hardware
> checksumming. This i initially thought was odd, since this is a TCP/IP
> packet, as you can see from the stack trace. However, 
> 
> netif_skb_features(skb) calls harmonize_features() which calls
> can_checksum_protocol():
> 
> 3206 static inline bool can_checksum_protocol(netdev_features_t features,
> 3207                                          __be16 protocol)
> 3208 {
> 3209         return ((features & NETIF_F_GEN_CSUM) ||
> 3210                 ((features & NETIF_F_V4_CSUM) &&
> 3211                  protocol == htons(ETH_P_IP)) ||
> 3212                 ((features & NETIF_F_V6_CSUM) &&
> 3213                  protocol == htons(ETH_P_IPV6)) ||
> 3214                 ((features & NETIF_F_FCOE_CRC) &&
> 3215                  protocol == htons(ETH_P_FCOE)));
> 3216 }
> 
> However looking in the skb, we see protocol is now ETH_P_EDSA, not
> ETH_P_IP. Hence the hardware does not support this protocol.

Usually, the hardware needs to be told there is a DSA/EDSA tag before
the actual Ethernet frame, I don't have the mv643xx_eth documentation
handy, but I suppose there should be something like this available.

> 
> This protocol value is because edsa_xmit() changes it in the slave
> device TX path.
> 
> Your patch "net: dsa: change tag_protocol to an enum"
> has a hunk:
> 
> diff --git a/net/dsa/tag_brcm.c b/net/dsa/tag_brcm.c
> index e0b759ec209e..8fbc21c0de78 100644
> --- a/net/dsa/tag_brcm.c
> +++ b/net/dsa/tag_brcm.c
> @@ -91,7 +91,6 @@ static netdev_tx_t brcm_tag_xmit(struct sk_buff *skb, struct net_device *dev)
>         /* Queue the SKB for transmission on the parent interface, but
>          * do not modify its EtherType
>          */
> -       skb->protocol = htons(ETH_P_BRCMTAG);
>         skb->dev = p->parent->dst->master_netdev;
>         dev_queue_xmit(skb);
> 
> making me think it is not actually needed to change
> skb->protocol. When i remove this from edsa_xmit(), the warning goes
> away and networking seems to work O.K.
> 
> Is this the right fix? Should we remove this setting of skb->protocol
> in the other dsa xmit functions?

Adding Lennert here, I suspect that having the skb->protocol assignment
was initially done to help drivers such as mv643xx_eth and others doing
DSA to be able to identify these packets properly in the transmit path.

Unless there is something else, I would be inclined to remove the
skb->protocol assignment from tag_dsa.c and tag_edsa.c. In case the
driver needs to consult what is configured, it should either:

- look at dev->dsa_ptr->tag_protocol if we do not need to know on a
per-packet basis what's the tagging protocol used (using
netdev_uses_dsa() + a helper function we'd introduce)

- or, if we we need that information to be per-packet, have a
DSA-specific control block and a set of helpers to retrieve that information
--
Florian
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On 09/14/2014 08:37 AM, Andrew Lunn wrote:
> Hi Florian
> 
> I've been debugging a WARNING when using DSA with a D-Link
> DIR665. I've had reports of the same warning with another device.
> 
> WARNING: CPU: 0 PID: 2014 at net/core/dev.c:2260 skb_warn_bad_offload+0xd0/0x104()
> mv643xx_eth_port: caps=(0x0000000400014803, 0x00000000001b482b) len=1722 data_len=16 gso_size=1448 gso_type=1 gso_segs 2 ip_summed=3 encapsulation 0 features 400014801
> Modules linked in:
> CPU: 0 PID: 2014 Comm: sshd Tainted: G        W      3.17.0-rc1-00007-g2f06b2c08099-dirty #228
> [<c000e2a0>] (unwind_backtrace) from [<c000bd88>] (show_stack+0x10/0x14)
> [<c000bd88>] (show_stack) from [<c0016db8>] (warn_slowpath_common+0x6c/0x8c)
> [<c0016db8>] (warn_slowpath_common) from [<c0016e08>] (warn_slowpath_fmt+0x30/0x40)
> [<c0016e08>] (warn_slowpath_fmt) from [<c04ebad4>] (skb_warn_bad_offload+0xd0/0x104)
> [<c04ebad4>] (skb_warn_bad_offload) from [<c03eea5c>] (skb_checksum_help+0x160/0x170)
> [<c03eea5c>] (skb_checksum_help) from [<c03eef40>] (dev_hard_start_xmit+0x3f8/0x4c0)
> [<c03eef40>] (dev_hard_start_xmit) from [<c04071ec>] (sch_direct_xmit+0x148/0x250)
> [<c04071ec>] (sch_direct_xmit) from [<c03ef280>] (__dev_queue_xmit+0x278/0x5f4)
> [<c03ef280>] (__dev_queue_xmit) from [<c04719b8>] (edsa_xmit+0xf8/0x2c8)
> [<c04719b8>] (edsa_xmit) from [<c03eee24>] (dev_hard_start_xmit+0x2dc/0x4c0)
> [<c03eee24>] (dev_hard_start_xmit) from [<c03ef3cc>] (__dev_queue_xmit+0x3c4/0x5f4)
> [<c03ef3cc>] (__dev_queue_xmit) from [<c0415e88>] (ip_finish_output+0x64c/0x920)
> [<c0415e88>] (ip_finish_output) from [<c0416e2c>] (ip_local_out_sk+0x34/0x38)
> [<c0416e2c>] (ip_local_out_sk) from [<c0417144>] (ip_queue_xmit+0x128/0x388)
> [<c0417144>] (ip_queue_xmit) from [<c042caa8>] (tcp_transmit_skb+0x534/0x93c)
> [<c042caa8>] (tcp_transmit_skb) from [<c042cff8>] (tcp_write_xmit+0x148/0xbf8)
> [<c042cff8>] (tcp_write_xmit) from [<c042dd7c>] (__tcp_push_pending_frames+0x30/0x9c)
> [<c042dd7c>] (__tcp_push_pending_frames) from [<c041fbc8>] (tcp_sendmsg+0xc0/0xcec)
> [<c041fbc8>] (tcp_sendmsg) from [<c0445710>] (inet_sendmsg+0x3c/0x70)
> [<c0445710>] (inet_sendmsg) from [<c03d7d5c>] (sock_aio_write+0xcc/0xec)
> [<c03d7d5c>] (sock_aio_write) from [<c00bb218>] (do_sync_write+0x7c/0xa4)
> [<c00bb218>] (do_sync_write) from [<c00bbc40>] (vfs_write+0x108/0x1b0)
> [<c00bbc40>] (vfs_write) from [<c00bc214>] (SyS_write+0x40/0x94)
> [<c00bc214>] (SyS_write) from [<c0009480>] (ret_fast_syscall+0x0/0x2c)
> ---[ end trace b1b02d15aba4766a ]---
> 
> I think i understand what is going on, and one of your recent patches
> may indicate you have seen something similar.
> 
> int dev_hard_start_xmit() we have:
> 
> 2607 
> 2608                 features = netif_skb_features(skb);
> 
> ...
> 2637                         /* If packet is not checksummed and device does not
> 2638                          * support checksumming for this protocol, complete
> 2639                          * checksumming here.
> 2640                          */
> 2641                         if (skb->ip_summed == CHECKSUM_PARTIAL) {
> 2642                                 if (skb->encapsulation)
> 2643                                         skb_set_inner_transport_header(skb,
> 2644                                                 skb_checksum_start_offset(skb));
> 2645                                 else
> 2646                                         skb_set_transport_header(skb,
> 2647                                                 skb_checksum_start_offset(skb));
> 2648                                 if (!(features & NETIF_F_ALL_CSUM) &&
> 2649                                      skb_checksum_help(skb))
> 2650                                         goto out_kfree_skb;
> 2651                         }
> 
> This packet is CHECKSUM_PARTIAL, but it is also a GSO packet, with two
> segments, so the skb_checksum_help() is throwing the warning. It is
> expected that the hardware with do the checksum when using GSO. The
> reason it is trying to do software checksums, not hardware, is because
> features does not indicate the protocol is supported by hardware
> checksumming. This i initially thought was odd, since this is a TCP/IP
> packet, as you can see from the stack trace. However, 
> 
> netif_skb_features(skb) calls harmonize_features() which calls
> can_checksum_protocol():
> 
> 3206 static inline bool can_checksum_protocol(netdev_features_t features,
> 3207                                          __be16 protocol)
> 3208 {
> 3209         return ((features & NETIF_F_GEN_CSUM) ||
> 3210                 ((features & NETIF_F_V4_CSUM) &&
> 3211                  protocol == htons(ETH_P_IP)) ||
> 3212                 ((features & NETIF_F_V6_CSUM) &&
> 3213                  protocol == htons(ETH_P_IPV6)) ||
> 3214                 ((features & NETIF_F_FCOE_CRC) &&
> 3215                  protocol == htons(ETH_P_FCOE)));
> 3216 }
> 
> However looking in the skb, we see protocol is now ETH_P_EDSA, not
> ETH_P_IP. Hence the hardware does not support this protocol.
> 
> This protocol value is because edsa_xmit() changes it in the slave
> device TX path.
> 
> Your patch "net: dsa: change tag_protocol to an enum"
> has a hunk:
> 
> diff --git a/net/dsa/tag_brcm.c b/net/dsa/tag_brcm.c
> index e0b759ec209e..8fbc21c0de78 100644
> --- a/net/dsa/tag_brcm.c
> +++ b/net/dsa/tag_brcm.c
> @@ -91,7 +91,6 @@ static netdev_tx_t brcm_tag_xmit(struct sk_buff *skb, struct net_device *dev)
>         /* Queue the SKB for transmission on the parent interface, but
>          * do not modify its EtherType
>          */
> -       skb->protocol = htons(ETH_P_BRCMTAG);
>         skb->dev = p->parent->dst->master_netdev;
>         dev_queue_xmit(skb);
> 
> making me think it is not actually needed to change
> skb->protocol. When i remove this from edsa_xmit(), the warning goes
> away and networking seems to work O.K.
> 
> Is this the right fix? Should we remove this setting of skb->protocol
> in the other dsa xmit functions?
> 
> Thanks
> 	Andrew
> 

No, if anything they should all be using ETH_P_XDSA to indicate the
protocol between the switch and the host network interface.  When you
triggered this issue did you by any chance change some of the settings
on the host netdev for the switch?

From what I can tell it looks like the issue might be related to the
fact that the slave features are based off of the vlan_features for the
host device.  If for example the vlan_features on your host device were
recently changed that could be one cause for this issue.

The fix would probably be to update skb_network_protocol so that it can
sort out ETH_P_XDSA tags and get the Ethertype from the frame.

Thanks,

Alex


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> > making me think it is not actually needed to change
> > skb->protocol. When i remove this from edsa_xmit(), the warning goes
> > away and networking seems to work O.K.
> > 
> > Is this the right fix? Should we remove this setting of skb->protocol
> > in the other dsa xmit functions?
> > 
> > Thanks
> > 	Andrew
> > 
> 
> No, if anything they should all be using ETH_P_XDSA to indicate the
> protocol between the switch and the host network interface.  When you
> triggered this issue did you by any chance change some of the settings
> on the host netdev for the switch?

This is the first time i've used DSA. I'm adding support for a new
board which has a switch. So it is not too easy for me to go backwards
and see when the WARNING started appearing.
 
> The fix would probably be to update skb_network_protocol so that it can
> sort out ETH_P_XDSA tags and get the Ethertype from the frame.

I'm not sure how easy getting the correct Ethertype from the frame
is. The DSA tag can go in different places, depending on what tagging
protocol is used.

For EDSA, which is what the board/switch i'm using uses, the tag is
placed between the source address and the ethertype. If the frame is
not an 802.1q, the tag is 8 bytes. If it is 802.1q the tag is 6
bytes. The first 2 bytes are an Ethertype, indicating EDSA is being
used. You need to look at byte 4 of the tag to know how long it is, in
order to find the second Ethertype in the frame, for the SDU.

Similarly DSA tagging, is either 2 bytes or 4 bytes, and you need to
look at byte 0 of the tag to determine its length. There is no
Ethertype to know that DSA is being used.

Trailer tagging is different. It places 4 bytes at the end of the
frame, applying padding for frames < 60 bytes. The Ethertype is not
touched.

Florian recently added brcm tagging. This again goes between the
Source address and the Ethertype, and does not define a valid
Ethertype.

So unless you know what tagging protocol is in use, it is very hard to
peer inside the frame to work out what the real SDU Ethertype is. So
it seems easier to just pass it in skb->protocol.

   Andrew
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On 09/15/2014 11:32 AM, Andrew Lunn wrote:
>>> making me think it is not actually needed to change
>>> skb->protocol. When i remove this from edsa_xmit(), the warning goes
>>> away and networking seems to work O.K.
>>>
>>> Is this the right fix? Should we remove this setting of skb->protocol
>>> in the other dsa xmit functions?
>>>
>>> Thanks
>>> 	Andrew
>>>
>>
>> No, if anything they should all be using ETH_P_XDSA to indicate the
>> protocol between the switch and the host network interface.  When you
>> triggered this issue did you by any chance change some of the settings
>> on the host netdev for the switch?
> 
> This is the first time i've used DSA. I'm adding support for a new
> board which has a switch. So it is not too easy for me to go backwards
> and see when the WARNING started appearing.

We probably need something like a fix_feature() callback and/or a
NETDEV_FEAT_CHANGE() notifier callback to make sure we properly
recompute the slave network devices features based on the master network
device.

>  
>> The fix would probably be to update skb_network_protocol so that it can
>> sort out ETH_P_XDSA tags and get the Ethertype from the frame.
> 
> I'm not sure how easy getting the correct Ethertype from the frame
> is. The DSA tag can go in different places, depending on what tagging
> protocol is used.
> 
> For EDSA, which is what the board/switch i'm using uses, the tag is
> placed between the source address and the ethertype. If the frame is
> not an 802.1q, the tag is 8 bytes. If it is 802.1q the tag is 6
> bytes. The first 2 bytes are an Ethertype, indicating EDSA is being
> used. You need to look at byte 4 of the tag to know how long it is, in
> order to find the second Ethertype in the frame, for the SDU.
> 
> Similarly DSA tagging, is either 2 bytes or 4 bytes, and you need to
> look at byte 0 of the tag to determine its length. There is no
> Ethertype to know that DSA is being used.
> 
> Trailer tagging is different. It places 4 bytes at the end of the
> frame, applying padding for frames < 60 bytes. The Ethertype is not
> touched.
> 
> Florian recently added brcm tagging. This again goes between the
> Source address and the Ethertype, and does not define a valid
> Ethertype.
> 
> So unless you know what tagging protocol is in use, it is very hard to
> peer inside the frame to work out what the real SDU Ethertype is. So
> it seems easier to just pass it in skb->protocol.

Right, which is probably why the tag xmit() functions do override
skb->protocol to directly provide a value to the master device driver,
that way we do not have to go deep down into dissecting the frame.

I am not sure if there are possible scenarios where we may have to
handle mixed EDSA+DSA traffic, I suppose that might exist with a
combination of Marvell switches. If that's the case, we still need a
per-SKB information as opposed to looking directly into dev->dsa_ptr.
--
Florian
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