[ 30/72] sfc: Only use TX push if a single descriptor is to be written

Greg Kroah-Hartman
2013-03-26T22:52:36Z

From: Ben Hutchings <bhutchings@solarflare.com>

[ 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@solarflare.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/net/ethernet/sfc/nic.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

--- a/drivers/net/ethernet/sfc/nic.c
+++ b/drivers/net/ethernet/sfc/nic.c
@@ -379,7 +379,8 @@ efx_may_push_tx_desc(struct efx_tx_queue
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