The hardware must not see that is given ownership of a buffer until it is
completely written, and when the driver receives ownership of a buffer,
it must ensure that any other reads to the buffer reflect its final
state. Thus, I/O barriers are added where required.
Without this patch, I have observed GCC reordering the setting of
bdp->length and bdp->status in gfar_new_skb. Hardware reordering
was also theoretically possible.
Signed-off-by: Scott Wood <[EMAIL PROTECTED]>
---
I've added the requested comments to the source code about
the use of eieio(). Jeff, please consider for 2.6.22, as it
fixes a bug that has been observed.
drivers/net/gianfar.c | 11 +++++++++++
1 files changed, 11 insertions(+), 0 deletions(-)
diff --git a/drivers/net/gianfar.c b/drivers/net/gianfar.c
index b666a0c..f5b3cba 100644
--- a/drivers/net/gianfar.c
+++ b/drivers/net/gianfar.c
@@ -1025,6 +1025,15 @@ static int gfar_start_xmit(struct sk_buff *skb, struct
net_device *dev)
dev->trans_start = jiffies;
+ /* The powerpc-specific eieio() is used, as wmb() has too strong
+ * semantics (it requires synchronization between cacheable and
+ * uncacheable mappings, which eieio doesn't provide and which we
+ * don't need), thus requiring a more expensive sync instruction. At
+ * some point, the set of architecture-independent barrier functions
+ * should be expanded to include weaker barriers.
+ */
+
+ eieio();
txbdp->status = status;
/* If this was the last BD in the ring, the next one */
@@ -1301,6 +1310,7 @@ struct sk_buff * gfar_new_skb(struct net_device *dev,
struct rxbd8 *bdp)
bdp->length = 0;
/* Mark the buffer empty */
+ eieio();
bdp->status |= (RXBD_EMPTY | RXBD_INTERRUPT);
return skb;
@@ -1484,6 +1494,7 @@ int gfar_clean_rx_ring(struct net_device *dev, int
rx_work_limit)
bdp = priv->cur_rx;
while (!((bdp->status & RXBD_EMPTY) || (--rx_work_limit < 0))) {
+ rmb();
skb = priv->rx_skbuff[priv->skb_currx];
if (!(bdp->status &
--
1.5.0.3
-
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