netdev - Re: [PATCH] Make virtio_net support carrier detection

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Date:	Thu, 12 Mar 2009 14:47:09 +0200
From:	Pantelis Koukousoulas <pktoss@...il.com>
To:	Dan Williams <dcbw@...hat.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>, netdev@...r.kernel.org
Subject: Re: [PATCH] Make virtio_net support carrier detection

> The problem is that there's no "my carrier detection is accurate" flag
> for drivers.  Drivers that don't support carrier detection (say, my
> Belkin PCMCIA NE2k card) always report "carrier on", but of course don't
> support carrier detection.  So when NetworkManager looks at the device
> and sees "hey, there's a carrier!" it will activate it, because a
> carrier means the cable is plugged in or the PHY *thinks* a cable is
> plugged in.
>
> So NetworkManager checks whether the device supports ethtool get_link or
> MII register carrier status in lieu of a general kernel driver flag for
> "I support carrier detection".
>
> Carrier is not on/off, it needs to be tristate, like on/off/unknown.  If
> it was unknown, NM could make an intelligent decision about this without
> resorting to ethtool/MII checks.  But we don't have that.

This looks like an independent problem imho, one of kernel<->userspace
API. The current 'defacto' way of finding out if detection is supported or
not (ethtool) seems to give the needed information even if somewhat ugly.

The issue discussed here as I understand it is whether virtio should or
should not support carrier status reporting. IMHO it should, since
it is useful functionality and doesn't cost much.

Dan, What is your opinion on that?

Pantelis
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