Date:   Thu, 5 Jan 2017 10:23:45 +0100
From:   Zefir Kurtisi <zefir.kurtisi@...atec.com>
To:     Florian Fainelli <f.fainelli@...il.com>, netdev@...r.kernel.org
Cc:     andrew@...n.ch
Subject: Re: [PATCH] phy state machine: failsafe leave invalid RUNNING state

On 01/04/2017 10:44 PM, Florian Fainelli wrote:
> On 01/04/2017 08:10 AM, Zefir Kurtisi wrote:
>> On 01/04/2017 04:30 PM, Florian Fainelli wrote:
>>>
>>>
>>> On 01/04/2017 07:27 AM, Zefir Kurtisi wrote:
>>>> On 01/04/2017 04:13 PM, Florian Fainelli wrote:
>>>>>
>>>>>
>>>>> On 01/04/2017 07:04 AM, Zefir Kurtisi wrote:
>>>>>> While in RUNNING state, phy_state_machine() checks for link changes by
>>>>>> comparing phydev->link before and after calling phy_read_status().
>>>>>> This works as long as it is guaranteed that phydev->link is never
>>>>>> changed outside the phy_state_machine().
>>>>>>
>>>>>> If in some setups this happens, it causes the state machine to miss
>>>>>> a link loss and remain RUNNING despite phydev->link being 0.
>>>>>>
>>>>>> This has been observed running a dsa setup with a process continuously
>>>>>> polling the link states over ethtool each second (SNMPD RFC-1213
>>>>>> agent). Disconnecting the link on a phy followed by a ETHTOOL_GSET
>>>>>> causes dsa_slave_get_settings() / dsa_slave_get_link_ksettings() to
>>>>>> call phy_read_status() and with that modify the link status - and
>>>>>> with that bricking the phy state machine.
>>>>>
>>>>> That's the interesting part of the analysis, how does this brick the PHY
>>>>> state machine? Is the PHY driver changing the link status in the
>>>>> read_status callback that it implements?
>>>>>
>>>> phydev->read_status points to genphy_read_status(), where the first call goes to
>>>> genphy_update_link() which updates the link status.
>>>>
>>>> Thereafter phy_state_machine():RUNNING won't be able to detect the link loss
>>>> anymore unless the link state changes again.
>>>>
>>>>
>>>> I was trying to figure out if there is a rule that forbids changing phydev->link
>>>> from outside the state machine, but found several places where it happens (either
>>>> directly, or over genphy_read_status() or over genphy_update_link()).
>>>>
>>>> Curious how this did not show up before, since within the dsa setup it is very
>>>> easy to trigger:
>>>> a) physically disconnect link
>>>> b) within one second run ethtool ethX
>>>
>>> You need to be more specific here about what "the dsa setup" is, drivers
>>> involved, which ports of the switch you are seeing this with (user
>>> facing, CPU port, DSA port?) etc.
>>>
>> I am working on top of LEDE and with that at kernel 4.4.21 - alas I checked the
>> related source files and believe the effect should be reproducible with HEAD.
>>
>> The setup is as follows:
>> mv88e6321:
>> * ports 0+1 connected to fibre-optics transceivers at fixed 100 Mbps
>> * port 4 is CPU port
>> * custom phy driver (replacement for marvell.ko) only populated with
>>   * .config_init to
>>     * set fixed speed for ports 0+1 (when in FO mode)
>>     * run genphy_config_init() for all other modes (here: CPU port)
>>   * .config_aneg=genphy_config_aneg, .read_status=genphy_read_status
>>
>>
>> To my understanding, the exact setup is irrelevant - to reproduce the issue it is
>> enough to have a means of running genphy_update_link() (as done in e.g.
>> mediatek/mtk_eth_soc.c, dsa/slave.c), or genphy_read_status() (as done in e.g.
>> hisilicon/hns/hns_enet.c) or phy_read_status() (as done in e.g.
>> ethernet/ti/netcp_ethss.c, ethernet/aeroflex/greth.c, etc.). In the observed
>> drivers it is mostly implemented in the ETHTOOL_GSET execution path.
>>
>> Once you get the link state updated outside the phy state machine, it remains in
>> invalid RUNNING. To prevent that invalid state, to my understanding upper layer
>> drivers (Ethernet, dsa) must not modify link-states in any case (including calling
>> the functions noted above), or we need the proposed fail-safe mechanism to prevent
>> getting stuck.
> 
> OK, I see the code path involved now, sorry -ENOCOFFEE when I initially
> responded. Yes, clearly, we should not be mangling the PHY device's link
> by calling genphy_read_status(). At first glance, none of the users
> below should be doing what they are doing, but let's kick a separate
> patch series to collect feedback from the driver writes.
> 
> Thanks!
> 
Ok, thanks for taking time.

The kbuild test robot error is due to 'struct device dev' been removed from
phy_device struct since 4.4.21. Does it make sense to provide a v2 fixing that, or
do you expect that this fail-safe mechanism is not needed once all Ethernet/dsa
drivers are fixed?

I think it won't hurt to add the check simply to ensure that it got fixed and the
issue is not popping up thereafter.


Cheers,
Zefir

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