netdev - Re: [PATCH 2/2] ixgbe: fix select

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Date:	Fri, 20 Mar 2009 09:24:34 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	"Waskiewicz Jr, Peter P" <peter.p.waskiewicz.jr@...el.com>
Cc:	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	David Miller <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"e1000-devel@...ts.sourceforge.net" 
	<e1000-devel@...ts.sourceforge.net>
Subject: Re: [PATCH 2/2] ixgbe: fix select_queue management

On Fri, 20 Mar 2009 00:23:39 -0700 (Pacific Daylight Time)
"Waskiewicz Jr, Peter P" <peter.p.waskiewicz.jr@...el.com> wrote:

> On Thu, 19 Mar 2009, Stephen Hemminger wrote:
> 
> > Convert ixgbe to use net_device_ops properly.
> > Rather than changing the select_queue function pointer
> > just check the flag.
> > 
> > Signed-off-by: Stephen Hemminger <shemminger@...tta.com>
> 
> Thanks Stephen.  I was looking at reassigning a DCB netdev_ops struct when 
> DCB is enabled, and then having a default netdev_ops struct when it's not 
> enabled.  I agree the check is cleaner the way you have it below, but it's 
> another conditional check in the Tx hotpath, which we have too many of in 
> the first place.

Changing number of tx queues is actually the fastest, since then
indirection is not needed

> On a related side note, why is the netdev_ops member of net_device 
> declared const?

The purpose of having an ops structure is two fold. First, the ops
can be in read-only section (if driver wants) to avoid cache issues.
More importantly only one instance is necessary when there are multiple
boards, or 1000's of vlans.

> Cheers,
> -PJ Waskiewicz
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