Re: VLAN and ARP failure on tg3 drivers

Re: VLAN and ARP failure on tg3 drivers

[Gertjan Hofman]

Dear Matt, Eric, Benny,

Sorry about the slow response to your fast replies. I think Benny is correct, the 'problem' lies in the fact that we were using a VLAN ID of 0, without knowing its special significance. User error.

I tested it with other VLAN id's (>0) and it appears to work fine. We are not entirely sure we understand  why it used to work with VLAN ID 0 on the Broadcom chips and still does with a number of different cards (with >2.6.27 kernels).  What is the 'correct' behaviour for this incorrect usage ? When a PC returns the ARP response to the machine with the BroadCom card, it will have the destination address of the VLAN device, but presumably the VLAN ID tag set to zero.  Hmmm. I can live with not knowing the answer I guess.


Thanks again,

Gertjan



 

--- On Fri, 10/23/09, Matt Carlson <mcarlson@broadcom.com> wrote:

> From: Matt Carlson <mcarlson@broadcom.com>
> Subject: Re: VLAN and ARP failure on tg3 drivers
> To: "Gertjan Hofman" <gertjan_hofman@yahoo.com>
> Cc: "netdev@vger.kernel.org" <netdev@vger.kernel.org>
> Date: Friday, October 23, 2009, 3:35 PM
> On Thu, Oct 22, 2009 at 09:52:42PM
> -0700, Gertjan Hofman wrote:
> > Dear Kernel developers,
> > 
> > A couple of weeks ago we tried to migrate from a
> 2.6.24? kernel to a 2.6.29 kernel and noticed our VLAN
> application no longer works.? The problem is easy to
> replicate:
> > 
> > 1. connect 2 PC's with a cross-over cable
> > 2. set up a fixed IP address to both PC's? (say
> 192.168.0.[1,2])
> > 3. create a vlan:? vconfig? add eth0 0.
> > 4. set IP addresses on the VLAN devices? (say
> 192.168.1.[1,2])
> > 5. try ping one machine from the other.
> > 
> > I tried to dig into the problem by using un-patched
> kernel.org kernels with Ubuntu .config files.? Kernels up to
> 2.6.26 work fine, kernels after and including 2.6.27 fail.
> The problem is that ARP messages are being dropped. If the
> ARP table is updated by hand on each machine, the
> communication across the VLAN works fine.
> > 
> > At first I thought the kernel VLAN code was the
> problem (we had an earlier issue with a regression in
> 2.6.24) but it looks like the problem is actually with the
> tg3 driver.? Our system uses Broadcom ethernet chips. I
> tried the same experiments with combination of boards that
> have Broadcom and none-Broadcom and the only time I see it
> fail is with the tg3? driver loaded.
> > 
> > Snooping with WireShark shows that a ARP request from
> the non-Broadcom machine is seen and even answered, but
> never appears back on the network. If the Broadcom machine
> orginates the ARP message, it never arrives at the
> destination. I tried lowering the size of the MTU to 1492 as
> well as giving each VLAN device a different MAC. No deal.
> > 
> > I tried to look at tg3 patch changes from 2.6.26 to
> 2.6.27 but I am not familiar enough with the Git system to
> extract the appropiate changes.? I am a bit surprised that I
> am not seeing any references to this on the web, the
> combination of >2.6.27 kernels, Broadcom and VLAN cant be
> that uncommon.
> > 
> > I would be happy to provide more information and to
> try tests if any one can suggest them.
> > 
> > Sincerely,
> > 
> > Gertjan
> 
> I don't see any reason why your setup should fail, but it
> doesn't hurt
> to gather more info about the problem.
> 
> What device are you experiencing this problem with? 
> Is management
> firmware enabled?  (`ethtool -i ethx`)
> 
>






      

Re: VLAN and ARP failure on tg3 drivers

[Benny Amorsen]

Gertjan Hofman <gertjan_hofman@yahoo.com> writes:

> Dear Matt, Eric, Benny,
>
> Sorry about the slow response to your fast replies. I think Benny is
> correct, the 'problem' lies in the fact that we were using a VLAN ID
> of 0, without knowing its special significance. User error.
>
> I tested it with other VLAN id's (>0) and it appears to work fine. We
> are not entirely sure we understand  why it used to work with VLAN ID
> 0 on the Broadcom chips and still does with a number of different
> cards (with >2.6.27 kernels).  What is the 'correct' behaviour for
> this incorrect usage ?

VLAN 0 isn't incorrect, it's just surprising. When you send a packet
tagged with VLAN 0, it means that the packet should be interpreted as
being the same VLAN as a completely untagged packet.

So in theory, if both ends are using VLAN 0 and you aren't using eth0
for anything, traffic should flow, at least if both ends are on the same
kernel version. Feel free to debug why that isn't the case for you, of
course...


/Benny

Re: VLAN and ARP failure on tg3 drivers

[Eric Dumazet]

Benny Amorsen a écrit :
> Gertjan Hofman <gertjan_hofman@yahoo.com> writes:
> 
>> Dear Matt, Eric, Benny,
>>
>> Sorry about the slow response to your fast replies. I think Benny is
>> correct, the 'problem' lies in the fact that we were using a VLAN ID
>> of 0, without knowing its special significance. User error.
>>
>> I tested it with other VLAN id's (>0) and it appears to work fine. We
>> are not entirely sure we understand  why it used to work with VLAN ID
>> 0 on the Broadcom chips and still does with a number of different
>> cards (with >2.6.27 kernels).  What is the 'correct' behaviour for
>> this incorrect usage ?
> 
> VLAN 0 isn't incorrect, it's just surprising. When you send a packet
> tagged with VLAN 0, it means that the packet should be interpreted as
> being the same VLAN as a completely untagged packet.
> 
> So in theory, if both ends are using VLAN 0 and you aren't using eth0
> for anything, traffic should flow, at least if both ends are on the same
> kernel version. Feel free to debug why that isn't the case for you, of
> course...
> 

VLAN id 0 is not usable on current kernel because we use 16 bits in skb to
 store vlan_tci, and vlan_tci = 0 means there is no VLAN tagging.


We could use high order bit (0x8000) to tell if vlan tagging is set or not.

[PATCH] vlan: allow VLAN ID 0 to be used

[Eric Dumazet]

Eric Dumazet a écrit :
> VLAN id 0 is not usable on current kernel because we use 16 bits in skb to
>  store vlan_tci, and vlan_tci = 0 means there is no VLAN tagging.
> 
> 
> We could use high order bit (0x8000) to tell if vlan tagging is set or not.
> 

Here is the patch I cooked that permitted VLAN 0 to be used with tg3
(and other HW accelerated vlan nics I suppose)

[PATCH] vlan: allow VLAN ID 0 to be used

We currently use a 16 bit field (vlan_tci) to store VLAN ID on a skb.

0 value is used a special value, meaning VLAN ID not set.
This forbids use of VLAN ID 0

As VLAN ID is 12 bits, we can use high order bit as a flag, and
allow VLAN ID 0

Reported-by: Gertjan Hofman <gertjan_hofman@yahoo.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
---
diff --git a/include/linux/if_vlan.h b/include/linux/if_vlan.h
index 7ff9af1..7dfcdb5 100644
--- a/include/linux/if_vlan.h
+++ b/include/linux/if_vlan.h
@@ -105,8 +105,9 @@ static inline void vlan_group_set_device(struct vlan_group *vg,
 	array[vlan_id % VLAN_GROUP_ARRAY_PART_LEN] = dev;
 }
 
-#define vlan_tx_tag_present(__skb)	((__skb)->vlan_tci)
-#define vlan_tx_tag_get(__skb)		((__skb)->vlan_tci)
+#define VLAN_TAG_PRESENT		0x8000
+#define vlan_tx_tag_present(__skb)	((__skb)->vlan_tci & VLAN_TAG_PRESENT)
+#define vlan_tx_tag_get(__skb)		((__skb)->vlan_tci & 0x7fff)
 
 #if defined(CONFIG_VLAN_8021Q) || defined(CONFIG_VLAN_8021Q_MODULE)
 extern struct net_device *vlan_dev_real_dev(const struct net_device *dev);
@@ -231,7 +232,7 @@ static inline struct sk_buff *__vlan_put_tag(struct sk_buff *skb, u16 vlan_tci)
 static inline struct sk_buff *__vlan_hwaccel_put_tag(struct sk_buff *skb,
 						     u16 vlan_tci)
 {
-	skb->vlan_tci = vlan_tci;
+	skb->vlan_tci = VLAN_TAG_PRESENT | vlan_tci;
 	return skb;
 }
 
@@ -284,7 +285,7 @@ static inline int __vlan_hwaccel_get_tag(const struct sk_buff *skb,
 					 u16 *vlan_tci)
 {
 	if (vlan_tx_tag_present(skb)) {
-		*vlan_tci = skb->vlan_tci;
+		*vlan_tci = vlan_tx_tag_get(skb);
 		return 0;
 	} else {
 		*vlan_tci = 0;

Re: [PATCH] vlan: allow VLAN ID 0 to be used

[David Miller]

From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Mon, 26 Oct 2009 17:13:58 +0100

> [PATCH] vlan: allow VLAN ID 0 to be used
> 
> We currently use a 16 bit field (vlan_tci) to store VLAN ID on a skb.
> 
> 0 value is used a special value, meaning VLAN ID not set.
> This forbids use of VLAN ID 0
> 
> As VLAN ID is 12 bits, we can use high order bit as a flag, and
> allow VLAN ID 0
> 
> Reported-by: Gertjan Hofman <gertjan_hofman@yahoo.com>
> Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>

This is going to need some more work.

IXGBE is already using the higher bits of ->vlan_tci internally,
your change breaks that.

QLGE explicitly initializes skb->vlan_tci to zero, you'll need to make
sure that's OK.

There is an explicit "if (skb->vlan_tci" (ie. zero vs. non-zero) test
in net/core/dev.c:netif_receive_skb()

net/core/skbuff.c:__copy_skb_header() does a straight copy, you'll
need to make sure that's still OK.

net/packet/af_packet.c:tpacket_rcv() and packet_recvmsg() report the
skb->vlan_tci value to userspace, that's broken now as userspace
doesn't expect that new bit to be there.

Re: [PATCH] vlan: allow VLAN ID 0 to be used

[Eric Dumazet]

David Miller a écrit :
> From: Eric Dumazet <eric.dumazet@gmail.com>
> Date: Mon, 26 Oct 2009 17:13:58 +0100
> 
>> [PATCH] vlan: allow VLAN ID 0 to be used
>>
>> We currently use a 16 bit field (vlan_tci) to store VLAN ID on a skb.
>>
>> 0 value is used a special value, meaning VLAN ID not set.
>> This forbids use of VLAN ID 0
>>
>> As VLAN ID is 12 bits, we can use high order bit as a flag, and
>> allow VLAN ID 0
>>
>> Reported-by: Gertjan Hofman <gertjan_hofman@yahoo.com>
>> Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
> 
> This is going to need some more work.
> 
> IXGBE is already using the higher bits of ->vlan_tci internally,
> your change breaks that.
> 
> QLGE explicitly initializes skb->vlan_tci to zero, you'll need to make
> sure that's OK.
> 
> There is an explicit "if (skb->vlan_tci" (ie. zero vs. non-zero) test
> in net/core/dev.c:netif_receive_skb()
> 
> net/core/skbuff.c:__copy_skb_header() does a straight copy, you'll
> need to make sure that's still OK.
> 
> net/packet/af_packet.c:tpacket_rcv() and packet_recvmsg() report the
> skb->vlan_tci value to userspace, that's broken now as userspace
> doesn't expect that new bit to be there.

Thanks a lot David for this extended review and guidelines

I hope I did not miss another important thing in this second version.

Again, tested on tg3 only, and on net-next-2.6 tree

[PATCH] vlan: allow null VLAN ID to be used

We currently use a 16 bit field (vlan_tci) to store VLAN ID/PRIO on a skb.

Null value is used as a special value, meaning vlan tagging not enabled.
This forbids use of null vlan ID.

As pointed by David, some drivers use the 3 high order bits (PRIO)

As VLAN ID is 12 bits, we can use the remaining bit (CFI) as a flag, and
allow null VLAN ID.

In case future code really wants to use VLAN_CFI_MASK, we'll have to use
a bit outside of vlan_tci.

#define VLAN_PRIO_MASK         0xe000 /* Priority Code Point */
#define VLAN_PRIO_SHIFT        13
#define VLAN_CFI_MASK          0x1000 /* Canonical Format Indicator */
#define VLAN_TAG_PRESENT       VLAN_CFI_MASK
#define VLAN_VID_MASK          0x0fff /* VLAN Identifier */

Reported-by: Gertjan Hofman <gertjan_hofman@yahoo.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
---
 include/linux/if_vlan.h |   14 +++++++++-----
 net/8021q/vlan.h        |    2 +-
 net/8021q/vlan_dev.c    |    2 +-
 net/core/dev.c          |    2 +-
 net/packet/af_packet.c  |    5 +++--
 5 files changed, 15 insertions(+), 10 deletions(-)

diff --git a/include/linux/if_vlan.h b/include/linux/if_vlan.h
index 7ff9af1..8898cbe 100644
--- a/include/linux/if_vlan.h
+++ b/include/linux/if_vlan.h
@@ -63,7 +63,11 @@ static inline struct vlan_ethhdr *vlan_eth_hdr(const struct sk_buff *skb)
 	return (struct vlan_ethhdr *)skb_mac_header(skb);
 }
 
-#define VLAN_VID_MASK	0xfff
+#define VLAN_PRIO_MASK		0xe000 /* Priority Code Point */
+#define VLAN_PRIO_SHIFT		13
+#define VLAN_CFI_MASK		0x1000 /* Canonical Format Indicator */
+#define VLAN_TAG_PRESENT	VLAN_CFI_MASK
+#define VLAN_VID_MASK		0x0fff /* VLAN Identifier */
 
 /* found in socket.c */
 extern void vlan_ioctl_set(int (*hook)(struct net *, void __user *));
@@ -105,8 +109,8 @@ static inline void vlan_group_set_device(struct vlan_group *vg,
 	array[vlan_id % VLAN_GROUP_ARRAY_PART_LEN] = dev;
 }
 
-#define vlan_tx_tag_present(__skb)	((__skb)->vlan_tci)
-#define vlan_tx_tag_get(__skb)		((__skb)->vlan_tci)
+#define vlan_tx_tag_present(__skb)	((__skb)->vlan_tci & VLAN_TAG_PRESENT)
+#define vlan_tx_tag_get(__skb)		((__skb)->vlan_tci & ~VLAN_TAG_PRESENT)
 
 #if defined(CONFIG_VLAN_8021Q) || defined(CONFIG_VLAN_8021Q_MODULE)
 extern struct net_device *vlan_dev_real_dev(const struct net_device *dev);
@@ -231,7 +235,7 @@ static inline struct sk_buff *__vlan_put_tag(struct sk_buff *skb, u16 vlan_tci)
 static inline struct sk_buff *__vlan_hwaccel_put_tag(struct sk_buff *skb,
 						     u16 vlan_tci)
 {
-	skb->vlan_tci = vlan_tci;
+	skb->vlan_tci = VLAN_TAG_PRESENT | vlan_tci;
 	return skb;
 }
 
@@ -284,7 +288,7 @@ static inline int __vlan_hwaccel_get_tag(const struct sk_buff *skb,
 					 u16 *vlan_tci)
 {
 	if (vlan_tx_tag_present(skb)) {
-		*vlan_tci = skb->vlan_tci;
+		*vlan_tci = vlan_tx_tag_get(skb);
 		return 0;
 	} else {
 		*vlan_tci = 0;
diff --git a/net/8021q/vlan.h b/net/8021q/vlan.h
index 82570bc..4ade5ed 100644
--- a/net/8021q/vlan.h
+++ b/net/8021q/vlan.h
@@ -89,7 +89,7 @@ static inline u32 vlan_get_ingress_priority(struct net_device *dev,
 {
 	struct vlan_dev_info *vip = vlan_dev_info(dev);
 
-	return vip->ingress_priority_map[(vlan_tci >> 13) & 0x7];
+	return vip->ingress_priority_map[(vlan_tci >> VLAN_PRIO_SHIFT) & 0x7];
 }
 
 #ifdef CONFIG_VLAN_8021Q_GVRP
diff --git a/net/8021q/vlan_dev.c b/net/8021q/vlan_dev.c
index 4198ec5..e370197 100644
--- a/net/8021q/vlan_dev.c
+++ b/net/8021q/vlan_dev.c
@@ -393,7 +393,7 @@ int vlan_dev_set_egress_priority(const struct net_device *dev,
 	struct vlan_dev_info *vlan = vlan_dev_info(dev);
 	struct vlan_priority_tci_mapping *mp = NULL;
 	struct vlan_priority_tci_mapping *np;
-	u32 vlan_qos = (vlan_prio << 13) & 0xE000;
+	u32 vlan_qos = (vlan_prio << VLAN_PRIO_SHIFT) & VLAN_PRIO_MASK;
 
 	/* See if a priority mapping exists.. */
 	mp = vlan->egress_priority_map[skb_prio & 0xF];
diff --git a/net/core/dev.c b/net/core/dev.c
index fa88dcd..5ab8668 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2303,7 +2303,7 @@ int netif_receive_skb(struct sk_buff *skb)
 	if (!skb->tstamp.tv64)
 		net_timestamp(skb);
 
-	if (skb->vlan_tci && vlan_hwaccel_do_receive(skb))
+	if (vlan_tx_tag_present(skb) && vlan_hwaccel_do_receive(skb))
 		return NET_RX_SUCCESS;
 
 	/* if we've gotten here through NAPI, check netpoll */
diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index ff752c6..33e68f2 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -79,6 +79,7 @@
 #include <linux/module.h>
 #include <linux/init.h>
 #include <linux/mutex.h>
+#include <linux/if_vlan.h>
 
 #ifdef CONFIG_INET
 #include <net/inet_common.h>
@@ -766,7 +767,7 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
 			getnstimeofday(&ts);
 		h.h2->tp_sec = ts.tv_sec;
 		h.h2->tp_nsec = ts.tv_nsec;
-		h.h2->tp_vlan_tci = skb->vlan_tci;
+		h.h2->tp_vlan_tci = vlan_tx_tag_get(skb);
 		hdrlen = sizeof(*h.h2);
 		break;
 	default:
@@ -1493,7 +1494,7 @@ static int packet_recvmsg(struct kiocb *iocb, struct socket *sock,
 		aux.tp_snaplen = skb->len;
 		aux.tp_mac = 0;
 		aux.tp_net = skb_network_offset(skb);
-		aux.tp_vlan_tci = skb->vlan_tci;
+		aux.tp_vlan_tci = vlan_tx_tag_get(skb);
 
 		put_cmsg(msg, SOL_PACKET, PACKET_AUXDATA, sizeof(aux), &aux);
 	}

Re: [PATCH] vlan: allow VLAN ID 0 to be used

[David Miller]

From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Tue, 27 Oct 2009 02:34:57 +0100

> I hope I did not miss another important thing in this second version.
> 
> Again, tested on tg3 only, and on net-next-2.6 tree
> 
> [PATCH] vlan: allow null VLAN ID to be used

Looks good, applied to net-next-2.6

Someone now needs to convert IXGBE to use VLAN_PRIO_MASK and
VLAN_PRIO_SHIFT instead of it's internal macros.

Re: [PATCH] vlan: allow VLAN ID 0 to be used

[Benny Amorsen]

Eric Dumazet <eric.dumazet@gmail.com> writes:

> Here is the patch I cooked that permitted VLAN 0 to be used with tg3
> (and other HW accelerated vlan nics I suppose)
>
> [PATCH] vlan: allow VLAN ID 0 to be used
>
> We currently use a 16 bit field (vlan_tci) to store VLAN ID on a skb.
>
> 0 value is used a special value, meaning VLAN ID not set.
> This forbids use of VLAN ID 0

Are you sure you actually want to do this?

VLAN 0 IS special. Frames received on VLAN 0 should be treated just as
if they had no VLAN tag at all, except that they have an 802.1p value.

Sending frames with VLAN 0 should have something to do with whether
the sender wants to use 802.1p, which doesn't really have much to do
with VLAN's at all...

It would be nice if the unsuspecting user was at least warned that their
use of VLAN 0 is non-standard and may cause surprising results like
leakage into the "native" VLAN. That could be done in /sbin/ip or
/sbin/vconfig, of course.


/Benny

Re: [PATCH] vlan: allow VLAN ID 0 to be used

[Eric Dumazet]

Benny Amorsen a écrit :
> Eric Dumazet <eric.dumazet@gmail.com> writes:
> 
>> Here is the patch I cooked that permitted VLAN 0 to be used with tg3
>> (and other HW accelerated vlan nics I suppose)
>>
>> [PATCH] vlan: allow VLAN ID 0 to be used
>>
>> We currently use a 16 bit field (vlan_tci) to store VLAN ID on a skb.
>>
>> 0 value is used a special value, meaning VLAN ID not set.
>> This forbids use of VLAN ID 0
> 
> Are you sure you actually want to do this?
> 
> VLAN 0 IS special. Frames received on VLAN 0 should be treated just as
> if they had no VLAN tag at all, except that they have an 802.1p value.
> 
> Sending frames with VLAN 0 should have something to do with whether
> the sender wants to use 802.1p, which doesn't really have much to do
> with VLAN's at all...
> 
> It would be nice if the unsuspecting user was at least warned that their
> use of VLAN 0 is non-standard and may cause surprising results like
> leakage into the "native" VLAN. That could be done in /sbin/ip or
> /sbin/vconfig, of course.
> 

Quotting http://en.wikipedia.org/wiki/IEEE_802.1Q

VLAN Identifier (VID): a 12-bit field specifying the VLAN to which the frame belongs.
 A value of 0 means that the frame doesn't belong to any VLAN; in this case the 802.1Q
 tag specifies only a priority and is referred to as a priority tag.
A value of hex FFF is reserved for implementation use.
All other values may be used as VLAN identifiers, allowing up to 4094 VLANs


So we expect to generate a 802.1Q frame, even with a VID=0 field.
Before patch, device sends a non 802.1Q frame, which is not what was wanted by user.
(Maybe he wants to check its device/network is able to transport 1522 bytes frames, who knows...)

To use non tagged frames, user selects eth0 device, and to send tagged frames, he selects eth0.0


Now, maybe eth0 and eth0.0 should share same IP addresses, because incoming frame
with ID=0 tag should be received by eth0 device, but I am not sure standard requires this.

Re: [PATCH] vlan: allow VLAN ID 0 to be used

[Patrick McHardy]

Eric Dumazet wrote:
> Benny Amorsen a écrit :
>> Eric Dumazet <eric.dumazet@gmail.com> writes:
>>
>>> Here is the patch I cooked that permitted VLAN 0 to be used with tg3
>>> (and other HW accelerated vlan nics I suppose)
>>>
>>> [PATCH] vlan: allow VLAN ID 0 to be used
>>>
>>> We currently use a 16 bit field (vlan_tci) to store VLAN ID on a skb.
>>>
>>> 0 value is used a special value, meaning VLAN ID not set.
>>> This forbids use of VLAN ID 0
>> Are you sure you actually want to do this?
>>
>> VLAN 0 IS special. Frames received on VLAN 0 should be treated just as
>> if they had no VLAN tag at all, except that they have an 802.1p value.
>>
>> Sending frames with VLAN 0 should have something to do with whether
>> the sender wants to use 802.1p, which doesn't really have much to do
>> with VLAN's at all...
>>
>> It would be nice if the unsuspecting user was at least warned that their
>> use of VLAN 0 is non-standard and may cause surprising results like
>> leakage into the "native" VLAN. That could be done in /sbin/ip or
>> /sbin/vconfig, of course.
>>
> 
> Quotting http://en.wikipedia.org/wiki/IEEE_802.1Q
> 
> VLAN Identifier (VID): a 12-bit field specifying the VLAN to which the frame belongs.
>  A value of 0 means that the frame doesn't belong to any VLAN; in this case the 802.1Q
>  tag specifies only a priority and is referred to as a priority tag.
> A value of hex FFF is reserved for implementation use.
> All other values may be used as VLAN identifiers, allowing up to 4094 VLANs
> 
> 
> So we expect to generate a 802.1Q frame, even with a VID=0 field.
> Before patch, device sends a non 802.1Q frame, which is not what was wanted by user.
> (Maybe he wants to check its device/network is able to transport 1522 bytes frames, who knows...)
> 
> To use non tagged frames, user selects eth0 device, and to send tagged frames, he selects eth0.0

I agree. Quoting IEEE802.1Q:

0 The null VLAN ID. Indicates that the tag header contains only priority
  information; no VLAN identifier is present in the frame. ...

which implies that its valid to use 0 as tag in the header.

> Now, maybe eth0 and eth0.0 should share same IP addresses, because incoming frame
> with ID=0 tag should be received by eth0 device, but I am not sure standard requires this.

I don't think the standard makes any demands regarding this.

The current behaviour seems better to me since its symetrical
to the output path, where the VLAN device is necessary to be
able to specify a priority mapping.