netdev - Re: netdev carrier changes is one even after ethernet link up.

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Date:   Fri, 1 Sep 2017 18:55:14 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Bhadram Varka <vbhadram@...dia.com>,
        "andrew@...n.ch" <andrew@...n.ch>
Cc:     linux-netdev <netdev@...r.kernel.org>
Subject: Re: netdev carrier changes is one even after ethernet link up.

On 08/31/2017 10:49 PM, Bhadram Varka wrote:
> Thanks for responding. Now responding inline
> 
>> -----Original Message-----
>> From: Florian Fainelli [mailto:f.fainelli@...il.com]
>> Sent: Friday, September 01, 2017 5:53 AM
>> To: Bhadram Varka <vbhadram@...dia.com>; andrew@...n.ch
>> Cc: linux-netdev <netdev@...r.kernel.org>
>> Subject: Re: netdev carrier changes is one even after ethernet link up.
>>
>> On 08/30/2017 10:53 PM, Bhadram Varka wrote:
>>> Hi,
>>>
>>>
>>>
>>> I have observed that carrier_changes is one even in case of the
>>> ethernet link is up.
>>>
>>>
>>>
>>> After investigating the code below is my observation –
>>>
>>>
>>>
>>> ethernet_driver_probe()
>>>
>>> +--->phy_connect()
>>>
>>> |     +--->phy_attach_direct()
>>>
>>> |           +---> netif_carrier_off()    : which increments
>>> carrier_changes to one.
>>>
>>> +--->register_netdevice() : will the carrier_changes becomes zero here ?
>>>
>>> +--->netif_carrier_off(): not increment the carrier_changes since
>>> __LINK_STATE_NOCARRIER already set.
>>>
>>>
>>>
>>> From ethernet driver open will start the PHY and trigger the
>>> phy_state_machine.
>>>
>>> Phy_state_machine workqueue calling netif_carrier_on() once the link is
>> UP.
>>>
>>> netif_carrier_on() increments the carrier_changes by one.
>>
>> If the call trace is correct, then there is at least two problems here:
>>
>> - phy_connect() does start the PHY machine which means that as soon as it
>> detects a link state of any kind (up or down) it can call
>> netif_carrier_off() respectively netif_carrier_on()
>>
>> - as soon as you call register_netdevice() notifiers run and other parts of the
>> kernel or user-space programs can see an inconsistent link state
>>
>> I would suggest doing the following sequence instead:
>>
>> netif_carrier_off()
>> register_netdevice()
>> phy_connect()
>>
>> Which should result in a consistent link state and carrier value.
>>
> Yes, It will address the issue. 
> 
> If we did the phy_conect in ndo_open it will make the carrier changes as two. But if we did in probe function then it's not working.
> 
> In ethernet driver probe - (below sequence is not working)
> phy_connect()
> register_netdevice()
> netif_carrier_off()
> 
> working sequence:
> In probe():
> register_netdevice()
> ndo_open:
>            phy_connect()
> 
> After reverting - https://lkml.org/lkml/2016/1/9/173 this works if we do phy_connect in probe as well.

But as mentioned before you should not be doing the PHY probe in your
driver's probe function for different reasons:

- the probe function's responsibility is to initialize the driver and
the HW to a state where they both have everything needed but it should
be in quiesced state. There is no guarantee that your network device may
ever be used after probe unless something calls ndo_open(), you should
therefore keep all resources to a minimum: memory allocated, HW powered
down etc.

- there is a race condition between the PHY state machine started in
phy_connect(), and when register_netdevice() is called and notifiers
running which can lead to an inconsistent state for the carrier

So considering that your driver does not do that, I am not sure what you
are expecting...
-- 
Florian