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

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Message-ID: <67a9f7aa73f348558b2b3f097d2c040f@bgmail102.nvidia.com>
Date:   Mon, 4 Sep 2017 05:07:40 +0000
From:   Bhadram Varka <vbhadram@...dia.com>
To:     Florian Fainelli <f.fainelli@...il.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.

Hi Florian,

> -----Original Message-----
> From: Florian Fainelli [mailto:f.fainelli@...il.com]
> Sent: Saturday, September 02, 2017 7:25 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 08/31/2017 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
> 
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 we do we need to do netif_carrier_off() in phy_connect since we already doing the same after register_netdevice() ?

Thanks!
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