netdev - RE: [net PATCH 1/2] e1000: Do not overestimate descriptor counts in Tx pre-check

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Date:	Wed, 9 Mar 2016 23:50:20 +0000
From:	"Brown, Aaron F" <aaron.f.brown@...el.com>
To:	Alexander Duyck <aduyck@...antis.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"jogreene@...hat.com" <jogreene@...hat.com>,
	"intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"sassmann@...hat.com" <sassmann@...hat.com>
Subject: RE: [net PATCH 1/2] e1000: Do not overestimate descriptor counts in
 Tx pre-check

> From: netdev-owner@...r.kernel.org [mailto:netdev-
> owner@...r.kernel.org] On Behalf Of Alexander Duyck
> Sent: Wednesday, March 2, 2016 1:16 PM
> To: netdev@...r.kernel.org; jogreene@...hat.com; intel-wired-
> lan@...ts.osuosl.org; Kirsher, Jeffrey T <jeffrey.t.kirsher@...el.com>;
> sassmann@...hat.com
> Subject: [net PATCH 1/2] e1000: Do not overestimate descriptor counts in Tx
> pre-check
> 
> The current code path is capable of grossly overestimating the number of
> descriptors needed to transmit a new frame.  This specifically occurs if
> the skb contains a number of 4K pages.  The issue is that the logic for
> determining the descriptors needed is ((S) >> (X)) + 1.  When X is 12 it
> means that we were indicating that we required 2 descriptors for each 4K
> page when we only needed one.
> 
> This change corrects this by instead adding (1 << (X)) - 1 to the S value
> instead of adding 1 after the fact.  This way we get an accurate descriptor
> needed count as we are essentially doing a DIV_ROUNDUP().
> 
> Reported-by: Ivan Suzdal <isuzdal@...antis.com>
> Signed-off-by: Alexander Duyck <aduyck@...antis.com>
> ---
>  drivers/net/ethernet/intel/e1000/e1000_main.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Tested-by: Aaron Brown <aaron.f.brown@...el.com>