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

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Date:	Wed, 02 Mar 2016 16:16:01 -0500
From:	Alexander Duyck <aduyck@...antis.com>
To:	netdev@...r.kernel.org, jogreene@...hat.com,
	intel-wired-lan@...ts.osuosl.org, 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(-)

diff --git a/drivers/net/ethernet/intel/e1000/e1000_main.c b/drivers/net/ethernet/intel/e1000/e1000_main.c
index 3fc7bde..d213fb4 100644
--- a/drivers/net/ethernet/intel/e1000/e1000_main.c
+++ b/drivers/net/ethernet/intel/e1000/e1000_main.c
@@ -3106,7 +3106,7 @@ static int e1000_maybe_stop_tx(struct net_device *netdev,
 	return __e1000_maybe_stop_tx(netdev, size);
 }

-#define TXD_USE_COUNT(S, X) (((S) >> (X)) + 1)
+#define TXD_USE_COUNT(S, X) (((S) + ((1 << (X)) - 1)) >> (X))
 static netdev_tx_t e1000_xmit_frame(struct sk_buff *skb,
 				    struct net_device *netdev)
 {