tg3: Optimize rx double copy test

On a PCIX bus, the 5701 has a bug which requires the driver to double
copy all rx packets.  The rx code uses the rx_offset device member as a
flag to determine if this workaround should take effect.  The following
patch will modify the rx_offset member such that this test will become
less clear.

The patch starts by integrating the workaround check into the packet
length check.  It rounds out the implementation by relaxing the
workaround restrictions if the platform has efficient unaligned
accesses.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 25 insertions, 7 deletions

diff --git a/drivers/net/tg3.c b/drivers/net/tg3.c
index 3e89323..5791405 100644
--- a/drivers/net/tg3.c
+++ b/drivers/net/tg3.c
@@ -144,6 +144,24 @@
 
 #define TG3_RSS_MIN_NUM_MSIX_VECS	2
 
+/* Due to a hardware bug, the 5701 can only DMA to memory addresses
+ * that are at least dword aligned when used in PCIX mode.  The driver
+ * works around this bug by double copying the packet.  This workaround
+ * is built into the normal double copy length check for efficiency.
+ *
+ * However, the double copy is only necessary on those architectures
+ * where unaligned memory accesses are inefficient.  For those architectures
+ * where unaligned memory accesses incur little penalty, we can reintegrate
+ * the 5701 in the normal rx path.  Doing so saves a device structure
+ * dereference by hardcoding the double copy threshold in place.
+ */
+#define TG3_RX_COPY_THRESHOLD		256
+#if NET_IP_ALIGN == 0 || defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
+	#define TG3_RX_COPY_THRESH(tp)	TG3_RX_COPY_THRESHOLD
+#else
+	#define TG3_RX_COPY_THRESH(tp)	((tp)->rx_copy_thresh)
+#endif
+
 /* minimum number of free TX descriptors required to wake up TX process */
 #define TG3_TX_WAKEUP_THRESH(tnapi)		((tnapi)->tx_pending / 4)
 
@@ -4639,12 +4657,7 @@ static int tg3_rx(struct tg3_napi *tnapi, int budget)
 		len = ((desc->idx_len & RXD_LEN_MASK) >> RXD_LEN_SHIFT) -
 		      ETH_FCS_LEN;
 
-		if (len > RX_COPY_THRESHOLD &&
-		    tp->rx_offset == NET_IP_ALIGN) {
-		    /* rx_offset will likely not equal NET_IP_ALIGN
-		     * if this is a 5701 card running in PCI-X mode
-		     * [see tg3_get_invariants()]
-		     */
+		if (len > TG3_RX_COPY_THRESH(tp)) {
 			int skb_size;
 
 			skb_size = tg3_alloc_rx_skb(tp, tpr, opaque_key,
@@ -13469,9 +13482,14 @@ static int __devinit tg3_get_invariants(struct tg3 *tp)
 		tp->tg3_flags &= ~TG3_FLAG_POLL_SERDES;
 
 	tp->rx_offset = NET_IP_ALIGN;
+	tp->rx_copy_thresh = TG3_RX_COPY_THRESHOLD;
 	if (GET_ASIC_REV(tp->pci_chip_rev_id) == ASIC_REV_5701 &&
-	    (tp->tg3_flags & TG3_FLAG_PCIX_MODE) != 0)
+	    (tp->tg3_flags & TG3_FLAG_PCIX_MODE) != 0) {
 		tp->rx_offset = 0;
+#ifndef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
+		tp->rx_copy_thresh = ~0;
+#endif
+	}
 
 	tp->rx_std_max_post = TG3_RX_RING_SIZE;