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

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Date:   Thu, 31 Aug 2017 06:03:10 +0000
From:   Bhadram Varka <vbhadram@...dia.com>
To:     "andrew@...n.ch" <andrew@...n.ch>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>
CC:     linux-netdev <netdev@...r.kernel.org>
Subject: RE: netdev carrier changes is one even after ethernet link up.

+ netdev

From: Bhadram Varka 
Sent: Thursday, August 31, 2017 11:24 AM
To: 'andrew@...n.ch' <andrew@...n.ch>; 'f.fainelli@...il.com' <f.fainelli@...il.com>
Cc: linux-netdev <netdev@...r.kernel.org>
Subject: netdev carrier changes is one even after ethernet link up.

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.
 
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.
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