netdev - Re: [PATCH] phy state machine: failsafe leave invalid RUNNING state

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Date:   Wed, 4 Jan 2017 13:44:16 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Zefir Kurtisi <zefir.kurtisi@...atec.com>, netdev@...r.kernel.org
Cc:     andrew@...n.ch
Subject: Re: [PATCH] phy state machine: failsafe leave invalid RUNNING state

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!
-- 
Florian