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Subject[PATCH v8 4/7] igb: eliminate duplicate barriers on weakly-ordered archs
Date
memory-barriers.txt has been updated as follows:

"When using writel(), a prior wmb() is not needed to guarantee that the
cache coherent memory writes have completed before writing to the MMIO
region."

Remove old IA-64 comments in the code along with unneeded wmb() in front
of writel().

There are places in the code where wmb() has been used as a double barrier
for CPU and IO in place of smp_wmb() and wmb() as an optimization. For
such places, keep the wmb() but replace the following writel() with
writel_relaxed() to have a sequence as

wmb()
writel_relaxed()
mmio_wb()

Signed-off-by: Sinan Kaya <okaya@codeaurora.org>
---
drivers/net/ethernet/intel/igb/igb_main.c | 14 ++------------
1 file changed, 2 insertions(+), 12 deletions(-)

diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
index c1c0bc3..c3f7130 100644
--- a/drivers/net/ethernet/intel/igb/igb_main.c
+++ b/drivers/net/ethernet/intel/igb/igb_main.c
@@ -5652,11 +5652,7 @@ static int igb_tx_map(struct igb_ring *tx_ring,
/* set the timestamp */
first->time_stamp = jiffies;

- /* Force memory writes to complete before letting h/w know there
- * are new descriptors to fetch. (Only applicable for weak-ordered
- * memory model archs, such as IA-64).
- *
- * We also need this memory barrier to make certain all of the
+ /* We need this memory barrier to make certain all of the
* status bits have been updated before next_to_watch is written.
*/
wmb();
@@ -5674,7 +5670,7 @@ static int igb_tx_map(struct igb_ring *tx_ring,
igb_maybe_stop_tx(tx_ring, DESC_NEEDED);

if (netif_xmit_stopped(txring_txq(tx_ring)) || !skb->xmit_more) {
- writel(i, tx_ring->tail);
+ writel_relaxed(i, tx_ring->tail);

/* we need this if more than one processor can write to our tail
* at a time, it synchronizes IO on IA64/Altix systems
@@ -8073,12 +8069,6 @@ void igb_alloc_rx_buffers(struct igb_ring *rx_ring, u16 cleaned_count)
/* update next to alloc since we have filled the ring */
rx_ring->next_to_alloc = i;

- /* Force memory writes to complete before letting h/w
- * know there are new descriptors to fetch. (Only
- * applicable for weak-ordered memory model archs,
- * such as IA-64).
- */
- wmb();
writel(i, rx_ring->tail);
}
}
--
2.7.4
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