netdev - Re: [PATCH v3] ipconfig wait for carrier

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Date:	Thu, 19 May 2011 14:32:47 -0500
From:	Dan Williams <dcbw@...hat.com>
To:	Micha Nelissen <micha@...i.hopto.org>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH v3] ipconfig wait for carrier

On Thu, 2011-05-19 at 20:52 +0200, Micha Nelissen wrote:
> Dan Williams wrote:
> > On Wed, 2011-05-18 at 18:14 -0400, David Miller wrote:
> >> Please fix ic_is_init_dev() to return a proper boolean "false" instead
> >> of "0" when IFF_LOOPBACK is set.
> 
> Ok. Had an int before, but boolean is better.
> 
> > Shouldn't the code still wait at *least* one second?  Not all drivers
> > support carrier detect, and those that don't set the carrier always-on.
> > Thus older devices that used to have 1s to get carrier in line (even if
> > they don't report it) now have only 10ms.
> > 
> > I think it should wait at least one second like the code currently does,
> > and then if the carrier still isn't up, wait longer.
> 
> What is the 1 second based on?
> 
> If a driver does not support carrier detect, then this code will wait
> for the timeout period. Or do those older drivers set carrier detect
> immediately when device is probed?

Older devices that do not support carrier detect have the carrier always
on, ie IFF_RUNNING is always set, and netif_carrier_ok() always returns
1.  There is currently no way to determine whether a device supports
carrier detection or not, since drivers do not set a flag anywhere to
that effect (though I'd like it if they did).

I think the bit we're talking about is the change to CONF_POST_OPEN and
the corresponding change from ssleep(1) -> msleep(10).  For drivers that
don't support carrier detect, the patch would effectively decrease the
time that these drivers have to establish a carrier (even if they don't
report it to the kernel!) from 1 second to 10ms.  Then the code in
ic_open_devs() will immediately fall down to have_carrier because these
drivers have the carrier always one.

Net result: previously there was a 1 second window for this older
hardware to establish a link, now there is a 10ms window.  That's a
behavior change that could break stuff that used to work.

Dan

> Micha
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