netdev - Re: netdev: operstate UNKNOWN for loopback and other devices?

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Message-ID: <Z6IA60rfe1oNxlYz@shredder>
Date: Tue, 4 Feb 2025 13:58:35 +0200
From: Ido Schimmel <idosch@...dia.com>
To: Yong Wang <yongwang@...dia.com>
Cc: "stephen@...workplumber.org" <stephen@...workplumber.org>,
	"kuba@...nel.org" <kuba@...nel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Andy Roulin <aroulin@...dia.com>, Nikhil Dhar <ndhar@...dia.com>
Subject: Re: netdev: operstate UNKNOWN for loopback and other devices?

On Mon, Feb 03, 2025 at 09:17:08PM +0200, Yong Wang wrote:
> 
> On Wed, 20 Nov 2024 09:08:32 -0800
> Stephen Hemminger <stephen@...workplumber.org> wrote:
> 
> 
> >On Tue, 19 Nov 2024 19:23:53 -0800
> >Jakub Kicinski <kuba@...nel.org> wrote:
> >
> >> On Tue, 19 Nov 2024 15:37:03 -0800 Stephen Hemminger wrote:
> >> > It looks like loopback and other software devices never get the operstate
> >> > set correctly. Not a serious problem, but it is incorrect.
> >> > 
> >> > For example:
> >> > $ ip -br link
> >> > lo               UNKNOWN        00:00:00:00:00:00 <LOOPBACK,UP,LOWER_UP> 
> >> > 
> >> > tap0             UNKNOWN        ca:ff:ed:bf:96:a0 <BROADCAST,PROMISC,UP,LOWER_UP> 
> >> > tap1             UNKNOWN        36:f5:16:d1:4c:15 <BROADCAST,PROMISC,UP,LOWER_UP> 
> >> > 
> >> > For wireless and ethernet devices kernel reports UP and DOWN correctly.
> >> > 
> >> > Looks like some missing bits in dev_open but not sure exactly where.  
> >> 
> >> I thought it means the driver doesn't have any notion of the carrier,
> >> IOW the carrier will never go down. Basically those drivers don't
> >> call netif_carrier_{on,off}() at all, and rely on carrier being on
> >> by default at netdev registration.
> >
> >Tap device does have concept of pseudo carrier. If application has file descriptor
> >open it reports carrier, if the device is present but application has not opened
> >it then carrier is reported down.
> >
> 
> The UNKNOWN operstate sometimes is misleading, for loopback device, the fix seems 
> simple, we can just set 'dev->operstate = IF_OPER_UP' in its initialization function

This comes from the VRF driver which is the only driver I found that
does that:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b87ab6b8e517563be372b69bdabd96c672458547

> or add ndo_open handler to call netif_carrier_no, as discussed in thread at
> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=754987#89.

I don't see a lot of value in changing this ancient behavior, but I
don't think the loopback device should be singled out either. Otherwise,
some time from now someone will complain about a different device.