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.

 

After link is UP if we check the carrier_changes sysfs node - it will be
one only.

 

$ cat /sys/class/net/eth0/carrier_changes

1

 

After reverting the change - https://lkml.org/lkml/2016/1/9/173 (net:
phy: turn carrier off on phy attach) then I could see the carrier
changes incremented to 2 after Link UP.

$ cat /sys/class/net/eth0/carrier_changes

2

 

Thanks,

Bhadram.

------------------------------------------------------------------------
This email message is for the sole use of the intended recipient(s) and
may contain confidential information.  Any unauthorized review, use,
disclosure or distribution is prohibited.  If you are not the intended
recipient, please contact the sender by reply email and destroy all
copies of the original message.
------------------------------------------------------------------------

-- 
Florian