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From: John Baldwin <jhb@freebsd.org>
To: Adrian Chadd <adrian@freebsd.org>
Date: Mon, 17 Sep 2012 15:03:12 -0400
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	(bigwig.baldwin.cx); Mon, 17 Sep 2012 15:04:10 -0400 (EDT)
Cc: freebsd-net@freebsd.org, Ryan Stone <rysto32@gmail.com>
Subject: Re: What's the latest on fixing IFF_DRV_OACTIVE/if_start/etc?
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On Monday, September 17, 2012 1:45:12 pm Adrian Chadd wrote:
> On 17 September 2012 10:16, John Baldwin <jhb@freebsd.org> wrote:
> 
> > I think for if_bridge the fix is that it no longer uses if_start. :)
> 
> :)
> 
> > For real hardware you will get some sort of TX completion interrupt that will
> > restart the transmit queue.  Virtual software-only interfaces such as vlan(4)
> > and if_bridge(4) don't have that luxury though, and the best bet for them is
> > to probably have them use if_transmit instead.  vlan(4) and if_bridge(4) are
> > already fixed for that (if_bridge was only fixed a week or so ago in HEAD).
> 
> I'm still not convinced that going the if_start route (with
> process-to-completion) is going to work well when forwarding gobs of
> packets on anything bar ${BIG_IRON}, but that aside..

Eh?  For virtual interfaces, if_transmit introduces less overhead than
using if_start (no locking, queueing, dequeueing, etc.).  I expect that
to be a net win for smaller boards.

> It may be nice to introduce a virtual TX completion callback? Ie, a
> child driver could signal that it's successfully drained its TX queue,
> notifying any parent drivers that they can send more?

That could work, but I generally think if_transmit is a better route for
these sorts of things.  That turns these interfaces into simple filters
rather than building up their own queue, etc.

-- 
John Baldwin