netdev - Re: [PATCH]: Resurrect napi

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Message-Id: <20070721.185905.18310463.davem@davemloft.net>
Date:	Sat, 21 Jul 2007 18:59:05 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	rusty@...tcorp.com.au
Cc:	netdev@...r.kernel.org, shemminger@...ux-foundation.org,
	jgarzik@...ox.com
Subject: Re: [PATCH]: Resurrect napi_poll patch.

From: Rusty Russell <rusty@...tcorp.com.au>
Date: Sun, 22 Jul 2007 09:10:23 +1000

> In five years' time, the "napi_struct for simple drivers" in net_device
> will look confusing.  Since your change touches all NAPI drivers anyway,
> it'd be nice to go straight to "everyone allocates their own NAPI
> struct" in one jump.

Good point.

But note that you'd be adding a pointer deref, the current sequence:

napi_struct --> netdev --> driver_private

involves all pointer arithmetic and no derefs, whereas:

napi_struct --> driver_private --> netdev

will involve at least on deref to get to the netdev from the
driver_private.

(Ignore the fact that netdev_priv() is not currently optimized
 as it used to be, that's an abberation of the current multi-queue
 implementation and will be fixed).

So this suggestion does have real downsides.
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