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

-----Original Message-----
From: Florian Fainelli [mailto:f.fainelli@gmail.com]
Sent: Saturday, September 02, 2017 7:25 AM
To: Bhadram Varka <redacted>; andrew@lunn.ch
Cc: linux-netdev <redacted>
Subject: Re: netdev carrier changes is one even after ethernet link up.

On 08/31/2017 10:49 PM, Bhadram Varka wrote:

quoted

Thanks for responding. Now responding inline

quoted

-----Original Message-----
From: Florian Fainelli [mailto:f.fainelli@gmail.com]
Sent: Friday, September 01, 2017 5:53 AM
To: Bhadram Varka <redacted>; andrew@lunn.ch
Cc: linux-netdev <redacted>
Subject: Re: netdev carrier changes is one even after ethernet link up.

On 08/30/2017 10:53 PM, Bhadram Varka wrote:

quoted

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.

quoted

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.

quoted

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

Agreed. But I could see lot of drivers performing the phy_connect() in driver_probe() function itself.
Also observed that these drivers are performing netif_carrier_off() after register_netdevice().

My point is that why do we need to do netif_carrier_off() in phy_connect() since we already doing the same after register_netdevice() in MAC driver ?

Thanks!
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