Commit 847a1d67 authored by Alexander Duyck's avatar Alexander Duyck Committed by Jeff Kirsher
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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: default avatarIvan Suzdal <>
Signed-off-by: default avatarAlexander Duyck <>
Tested-by: default avatarAaron Brown <>
Signed-off-by: default avatarJeff Kirsher <>
parent 8e2cc0e6
......@@ -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)
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