[ 026/171 ] sfc: Only use TX push if a single descriptor is to be written

[Steven Rostedt]

3.6.11.2 stable review patch.
If anyone has any objections, please let me know.

------------------

From: Ben Hutchings <bhutchings@xxxxxxxxxxxxxx>

[ Upstream commit fae8563b25f73dc584a07bcda7a82750ff4f7672 ]

Using TX push when notifying the NIC of multiple new descriptors in
the ring will very occasionally cause the TX DMA engine to re-use an
old descriptor.  This can result in a duplicated or partly duplicated
packet (new headers with old data), or an IOMMU page fault.  This does
not happen when the pushed descriptor is the only one written.

TX push also provides little latency benefit when a packet requires
more than one descriptor.

Signed-off-by: Ben Hutchings <bhutchings@xxxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 drivers/net/ethernet/sfc/nic.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/sfc/nic.c b/drivers/net/ethernet/sfc/nic.c
index 326d799..a1c3d80 100644
--- a/drivers/net/ethernet/sfc/nic.c
+++ b/drivers/net/ethernet/sfc/nic.c
@@ -377,7 +377,8 @@ efx_may_push_tx_desc(struct efx_tx_queue *tx_queue, unsigned int write_count)
 		return false;
 
 	tx_queue->empty_read_count = 0;
-	return ((empty_read_count ^ write_count) & ~EFX_EMPTY_COUNT_VALID) == 0;
+	return ((empty_read_count ^ write_count) & ~EFX_EMPTY_COUNT_VALID) == 0
+		&& tx_queue->write_count - write_count == 1;
 }
 
 /* For each entry inserted into the software descriptor ring, create a
-- 
1.7.10.4


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