Subject: Re: Debugging Ethernet issues

From: Sebastian Frias <[email-masked]>
Date: Wed, 16 Nov 2016 12:10:45 +0100

Hi Florian,

On 11/14/2016 10:00 PM, Florian Fainelli wrote:
> On 11/14/2016 12:27 PM, Mason wrote:
>> On 14/11/2016 19:20, Florian Fainelli wrote:
>>
>>> On 11/14/2016 09:59 AM, Sebastian Frias wrote:
>>>
>>>> Could you confirm that Mason's patch is correct and/or that it does not
>>>> has negative side-effects?
>>>
>>> The patch is not correct nor incorrect per-se, it changes the default
>>> policy of having pause frames advertised by default to not having them
>>> advertised by default. This influences both your Ethernet MAC and the
>>> link partner in that the result is either flow control is enabled
>>> (before) or it is not (with the patch). There must be something amiss if
>>> you see packet loss or some kind of problem like that with an early
>>> exchange such as DHCP. Flow control tend to kick in under higher packet
>>> rates (at least, that's what you expect).
>>
>> Did you note that, without the change under discussion (i.e. with
>> the eth driver as it is upstream), when the board is connected to
>> a 100 Mbps switch, then *nothing* works *systematically (no ping,
>> no DHCP; are there other relevant low-level network tools?).
> 
> No I missed that, way too many emails, really. So how about you compare
> the register settings that could be (that is, all that could be modified
> by the PHYLIB adjust_link function) and try to spot where things could
> go wrong? 

The registers that make this work or fail are modified in
'nb8800_pause_config()' which is called from 'adjust_link', see below.

>Any other register that can be influenced by the link speed?

It may not be the link-speed per se, but the way these "pause" parameters
are interacting.
Indeed, on the 1000 Mbps switch we get (1):

   "Link partner advertised pause frame use: Symmetric Receive-only"

And on the 100 Mbps switch we get (2):

   "Link partner advertised pause frame use: Symmetric"

Since the 100 Mbps is advertising "symmetric", the code in
'drivers/net/ethernet/aurora/nb8800.c:nb8800_pause_config()' enables the
flow control bit:

   nb8800_modl(priv, NB8800_RXC_CR, RCR_FL, priv->pause_tx);

If we force this to be disabled, the network works on 100 Mbps.

Keep in mind that the same driver (net and phy) work on a different board,
could it be just by chance?

> 
> It seems like a possible (yet after re-reading, very unlikely) scenario,
> considering that priv->speed, priv->duplex and priv->link are initially
> zero-initialized (because nb8800_priv is zero initialized) may not force
> a correct link transition and a full MAC reconfiguration in
> nb8800_link_reconfigure() where some of the cached values are used.
> 
> NB: you will see most drivers initialize the previous link, 

Could you suggest another driver to compare this with?
It is not easy to know which drivers follow the conventions so that
they can be used as examples.

>speed,
> duplex values to -1, because those are outside of the range of values
> that PHYLIB would assign to phydev->{link,duplex,speed}, and therefore,
> this is guaranteed to make the adjust_link callback that tries to
> minimize these settings to force a transition.
> 
>>
>> Also, maybe this comment was lost in my own noise:
>>
>> If I manually set the link up, then down, then run udhcpc
>> => then nothing works, as if something is wedged somewhere
>> (a kernel thread gets borked by a race condition?)
> 
> Well then start seriously debugging the problem: firs thing you need to
> check is is the RUNNING flag set on the interface (which indicates a
> carrier on?) without that, the networking stack won't even send packets.

'ifconfig' reports "eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>"

Packets get sent, as the statistics show for example the following values
increasing:

     tx_bytes: 756
     tx_frames: 3

However, the statistics show that RX values increase and then stop.
Basically, if we do:

# ethtool -S eth0 > /tmp/toto
# udhcpc (ctrl-C because it won't return because it keeps trying)
# ethtool -S eth0 > /tmp/titi
# diff /tmp/toto /tmp/titi

we can see that the RX values go from 0 to X, but get stuck at X, while
the TX values keep increasing:

     rx_bytes_ok: 887
     rx_frames_ok: 8
...
     tx_bytes: 1576
     tx_frames: 7

Like if the Flow Control is not working properly. However we don't know
how to double check it, since the older driver (for kernel 3.4) does not
seem to use the feature.

> If it is not set, why is not it set? Did nb8800_mac_config() get called
> in the first place to configure the MAC wrt. the link settings?
> 
> When you transmit, do transmit counters increase? That would indicate
> the TX DMA does its job. When transmission occurs, it is successful or
> is it reporting errors? If the PHY supports it, can you access PHY
> counters and look for success/error counters changing? Finally, try to
> put another golden (working) host and if your switch supports it,
> configure port mirroring to look at packets. If the switch does not
> support it, then try different link partners.
> 

So far, our limited testing indicates that only switches that advertise:

   "Link partner advertised pause frame use: Symmetric"

are problematic.

Would the information presented here give you guys other ideas of what
to look for? Are those switches known to give issues?
Could the "pause" feature be timing dependent? (since it works on other
boards)

Best regards,

Sebastian

----
(1): ethtool eth0 when connected to 1000 Mbps switch

        Supported ports: [ TP MII ]
        Supported link modes:   10baseT/Half 10baseT/Full 
                                100baseT/Half 100baseT/Full 
                                1000baseT/Full 
        Supported pause frame use: Symmetric
        Supports auto-negotiation: Yes
        Advertised link modes:  10baseT/Half 10baseT/Full 
                                100baseT/Half 100baseT/Full 
                                1000baseT/Full 
        Advertised pause frame use: Symmetric
        Advertised auto-negotiation: Yes
        Link partner advertised link modes:  10baseT/Half 10baseT/Full 
                                             100baseT/Half 100baseT/Full 
                                             1000baseT/Half 1000baseT/Full 
        Link partner advertised pause frame use: Symmetric Receive-only
        Link partner advertised auto-negotiation: Yes
        Speed: 1000Mb/s
        Duplex: Full
        Port: MII
        PHYAD: 4
        Transceiver: external
        Auto-negotiation: on
        Link detected: yes

(2): ethtool eth0 when connected to 100 Mbps switch

        Supported ports: [ TP MII ]
        Supported link modes:   10baseT/Half 10baseT/Full 
                                100baseT/Half 100baseT/Full 
                                1000baseT/Full 
        Supported pause frame use: Symmetric
        Supports auto-negotiation: Yes
        Advertised link modes:  10baseT/Half 10baseT/Full 
                                100baseT/Half 100baseT/Full 
                                1000baseT/Full 
        Advertised pause frame use: Symmetric
        Advertised auto-negotiation: Yes
        Link partner advertised link modes:  10baseT/Half 10baseT/Full 
                                             100baseT/Half 100baseT/Full 
        Link partner advertised pause frame use: Symmetric
        Link partner advertised auto-negotiation: Yes
        Speed: 100Mb/s
        Duplex: Full
        Port: MII
        PHYAD: 4
        Transceiver: external
        Auto-negotiation: on
        Link detected: yes

>>
>> Could not advertising pause frames result in making such a
>> race condition impossible? (I don't really believe in a race,
>> due to the 100% nature of the problem.)
>>
>>>> Right now we know that Mason's patch makes this work, but we do not understand
>>>> why nor its implications.
>>>
>>> You need to understand why, right now, the way this problem is
>>> presented, you came up with a workaround, not with the root cause or the
>>> solution. What does your link partner (switch?) reports, that is, what
>>> is the ethtool output when you have a link up from your nb8800 adapter?
>>
>> Isn't that what ethtool -a eth0 prints?
> 
> No, ethtool -a prints the local pause settings.
> 
>> How do I get the link partner information?
> 
> ethtool eth0:
> 
> # ethtool eth0
> Settings for eth0:
>         Supported ports: [ TP MII ]
>         Supported link modes:   10baseT/Half 10baseT/Full
>                                 100baseT/Half 100baseT/Full
>         Supported pause frame use: Symmetric Receive-only
>         Supports auto-negotiation: Yes
>         Advertised link modes:  10baseT/Half 10baseT/Full
>                                 100baseT/Half 100baseT/Full
>         Advertised pause frame use: No
>         Advertised auto-negotiation: Yes
>         Link partner advertised link modes:  10baseT/Half 10baseT/Full
>                                              100baseT/Half 100baseT/Full
> 
> 	^======================
> 
>         Link partner advertised pause frame use: Symmetric
>         Link partner advertised auto-negotiation: Yes
> 
> 	^========================
> 
>         Speed: 100Mb/s
>         Duplex: Full
>         Port: MII
>         PHYAD: 1
>         Transceiver: internal
>         Auto-negotiation: on
>         Supports Wake-on: gs
>         Wake-on: d
>         SecureOn password: 00:00:00:00:00:00
>         Current message level: 0x00000007 (7)
>                                drv probe link
>         Link detected: yes
> #
> 
> 
>> Just ethtool eth0?
> 
> Yes, just that.
>