[PATCH] s2io: replace readq() with mmiowb() in s2io_xmit()

[Arthur Kepner <akepner@sgi.com>]

The neterion 10gige driver uses a readq() to flush some PIO writes
in s2io_xmit(). Using mmiowb() can, in some cases, reduce CPU 
utilization, and/or allow higher throughput. This is particularly 
true when TSO is off, and small MTUs are in use. 

For example, in one test measurement I just did with 2.6.12-rc2
on an Altix, MTUs were set to 1500 bytes and TSO turned off. 
With this patch, transmit throughput improved by ~20%. Throughput 
was ultimately bound by the CPU with or without the patch. With 
large MTUs (9600 bytes) or with TSO turned on, there was no 
significant change to throughput or CPU utilization.

Signed-off-by: Arthur Kepner <akepner@sgi.com>

--- linux.orig/drivers/net/s2io.c	2005-05-02 16:40:17.469733509 -0700
+++ linux/drivers/net/s2io.c	2005-05-02 16:40:25.001043632 -0700
@@ -2759,8 +2759,8 @@ static int s2io_xmit(struct sk_buff *skb
 #endif
 	writeq(val64, &tx_fifo->List_Control);
 
-	/* Perform a PCI read to flush previous writes */
-	val64 = readq(&bar0->general_int_status);
+	/* Perform a mmiowb() to order previous writes */
+	mmiowb();
 
 	put_off++;
 	put_off %= mac_control->tx_curr_put_info[queue].fifo_len + 1;

--
Arthur