netdev - RE: [PATCH 1/2] igb: Implement reading of reg RQDPC (Receive Queue Drop Packet Count)

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Date:	Thu, 14 May 2009 10:03:08 +0200
From:	Jesper Dangaard Brouer <hawk@...x.dk>
To:	"Williams, Mitch A" <mitch.a.williams@...el.com>
Cc:	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"David S. Miller" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"e1000-devel@...ts.sourceforge.net" 
	<e1000-devel@...ts.sourceforge.net>,
	"Ronciak, John" <john.ronciak@...el.com>,
	"Waskiewicz Jr, Peter P" <peter.p.waskiewicz.jr@...el.com>
Subject: RE: [PATCH 1/2] igb: Implement reading of reg RQDPC (Receive Queue
	Drop Packet Count)

On Wed, 2009-05-13 at 15:07 -0600, Williams, Mitch A wrote:
>  
> >-----Original Message-----
> >From: Jesper Dangaard Brouer [mailto:hawk@...x.dk] 
> >Sent: Monday, May 11, 2009 2:50 AM
> >
> >I have now tested it on a 82575 chip NIC. (I just got my 82575 NIC
> >working again (This 12 port monster from Hotlava Systems, just needed
> >more power on PCIe 100Watt)).
> >
> >I don't see the reason for doing special checks for the 82575. Reading
> >the RQDPC registers on 82575 always returns 0.  I don't see any harm in
> >that!?  (it also returns zero in overload situations)
> >
> >What do you want to redraw your NAK?
> >
> 
> Jesper, I still stand by my NAK.  It's never ever a good idea to read
> non-existent hardware registers.  You don't know what effect those
> reads will have on the hardware.  These addresses may be aliased to other
> registers without being documented.  Hardware designers do this all the
> time to save a few gates in the address decode logic, and these aliases
> may or may not be documented.  If this is the case, reading these
> registers will have unintended consequences.  You might be clearing some
> other statistic, or worse.
> 
> Since we just don't know, it's better to be safe than sorry.  Wrap
> those register reads so they only happen on 82576, and I'll happily
> ack your patch.

Fine, you argumented well for your case.

I'll repost some patches when time permits...

-- 
Med venlig hilsen / Best regards
  Jesper Brouer
  ComX Networks A/S
  Linux Network developer
  Cand. Scient Datalog / MSc.
  Author of http://adsl-optimizer.dk
  LinkedIn: http://www.linkedin.com/in/brouer

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