[PATCH 2/2 Xenial SRU] e1000: Do not overestimate descriptor counts in Tx pre-check

[Tim Gardner]

From: Alexander Duyck <aduyck at mirantis.com>

BugLink: http://bugs.launchpad.net/bugs/1582328

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 at mirantis.com>
Signed-off-by: Alexander Duyck <aduyck at mirantis.com>
Tested-by: Aaron Brown <aaron.f.brown at intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher at intel.com>
(cherry picked from commit 847a1d6796c767f8b697ead60997b847a84b897b)
Signed-off-by: Tim Gardner <tim.gardner at canonical.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 0b81d93..ae90d4f 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)
 {
-- 
1.9.1