[PATCH v2] 8139too: move wmb before TX DMA start

[Andreas Oberritter <obi@linuxtv.org>]

From: Andreas Oberritter <obi@linuxtv.org>

The write barrier should be used before starting a DMA transfer. This fixes
a problem, where almost all packets received on another machine had garbled
content. Tested with an RTL8100C on a MIPS machine.

Signed-off-by: Andreas Oberritter <obi@linuxtv.org>
---
Added a comment on Ben's request explaining the reason for the barrier.

diff --git a/drivers/net/8139too.c b/drivers/net/8139too.c
index 53bd903..3c5fac6 100644
--- a/drivers/net/8139too.c
+++ b/drivers/net/8139too.c
@@ -1716,13 +1716,18 @@ static int rtl8139_start_xmit (struct sk_buff *skb, struct net_device *dev)
 	}
 
 	spin_lock_irqsave(&tp->lock, flags);
+	/*
+	 * Writing to TxStatus triggers a DMA transfer of the data
+	 * copied to tp->tx_buf[entry] above. Use a memory barrier
+	 * to make sure that the device sees the updated data.
+	 */
+	wmb();
 	RTL_W32_F (TxStatus0 + (entry * sizeof (u32)),
 		   tp->tx_flag | max(len, (unsigned int)ETH_ZLEN));
 
 	dev->trans_start = jiffies;
 
 	tp->cur_tx++;
-	wmb();
 
 	if ((tp->cur_tx - NUM_TX_DESC) == tp->dirty_tx)
 		netif_stop_queue (dev);