From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stephen Hemminger <stephen@networkplumber.org>
Subject: Re: [PATCH v2 10/10] kni: add API to set link status on
 kernel interface
Date: Tue, 11 Sep 2018 14:52:48 -0700
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Cc: Igor Ryzhov <iryzhov@nfware.com>, Ferruh Yigit <ferruh.yigit@intel.com>,
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To: Dan Gora <dg@adax.com>
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On Tue, 11 Sep 2018 18:45:49 -0300
Dan Gora <dg@adax.com> wrote:

> Hi All,
> 
> So I implemented the "write to /sys/devices/virtual/net/*/carrier"
> method to change the link status, but there is one more minor thing
> that I wanted to point out about this approach.  The problem is that
> you cannot read or write the '/sys/devices/virtual/net/*/carrier' file
> while the interface is marked 'down'.  This means that link status
> changes can only be performed by the DPDK application while the
> interface is in the "up" state.  With the ioctl method, you can change
> the carrier state at pretty much any time.
> 
> Is this a problem?  It's not a huge one, I guess, but it is something
> else to consider.
> 
> Please let me know what you all think.
> 
> thanks
> dan

The carrier state has no meaning when device is down, at least for physical
devices. Because often the PHY is powered off when the device is marked down.