linux-kernel - [PATCH v8 1/7] i40e/i40evf: Eliminate duplicate barriers on weakly-ordered archs

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Date:   Mon,  2 Apr 2018 15:06:24 -0400
From:   Sinan Kaya <okaya@...eaurora.org>
To:     jeffrey.t.kirsher@...el.com
Cc:     netdev@...r.kernel.org, timur@...eaurora.org,
        sulrich@...eaurora.org, linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Sinan Kaya <okaya@...eaurora.org>,
        intel-wired-lan@...ts.osuosl.org, linux-kernel@...r.kernel.org
Subject: [PATCH v8 1/7] i40e/i40evf: Eliminate duplicate barriers on weakly-ordered archs

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@...eaurora.org>
---
 drivers/net/ethernet/intel/i40e/i40e_txrx.c   | 22 +++++++++-------------
 drivers/net/ethernet/intel/i40evf/i40e_txrx.c |  8 +-------
 2 files changed, 10 insertions(+), 20 deletions(-)

diff --git a/drivers/net/ethernet/intel/i40e/i40e_txrx.c b/drivers/net/ethernet/intel/i40e/i40e_txrx.c
index f174c72..1b9fa7a 100644
--- a/drivers/net/ethernet/intel/i40e/i40e_txrx.c
+++ b/drivers/net/ethernet/intel/i40e/i40e_txrx.c
@@ -186,7 +186,13 @@ static int i40e_program_fdir_filter(struct i40e_fdir_filter *fdir_data,
 	/* Mark the data descriptor to be watched */
 	first->next_to_watch = tx_desc;
 
-	writel(tx_ring->next_to_use, tx_ring->tail);
+	writel_relaxed(tx_ring->next_to_use, 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
+	 */
+	mmiowb();
+
 	return 0;
 
 dma_fail:
@@ -1523,12 +1529,6 @@ static inline void i40e_release_rx_desc(struct i40e_ring *rx_ring, u32 val)
 	/* update next to alloc since we have filled the ring */
 	rx_ring->next_to_alloc = val;
 
-	/* 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(val, rx_ring->tail);
 }
 
@@ -2274,11 +2274,7 @@ static void i40e_rx_buffer_flip(struct i40e_ring *rx_ring,
 
 static inline void i40e_xdp_ring_update_tail(struct i40e_ring *xdp_ring)
 {
-	/* Force memory writes to complete before letting h/w
-	 * know there are new descriptors to fetch.
-	 */
-	wmb();
-	writel_relaxed(xdp_ring->next_to_use, xdp_ring->tail);
+	writel(xdp_ring->next_to_use, xdp_ring->tail);
 }
 
 /**
@@ -3444,7 +3440,7 @@ static inline int i40e_tx_map(struct i40e_ring *tx_ring, struct sk_buff *skb,
 
 	/* notify HW of packet */
 	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
diff --git a/drivers/net/ethernet/intel/i40evf/i40e_txrx.c b/drivers/net/ethernet/intel/i40evf/i40e_txrx.c
index 12bd937..eb5556e 100644
--- a/drivers/net/ethernet/intel/i40evf/i40e_txrx.c
+++ b/drivers/net/ethernet/intel/i40evf/i40e_txrx.c
@@ -804,12 +804,6 @@ static inline void i40e_release_rx_desc(struct i40e_ring *rx_ring, u32 val)
 	/* update next to alloc since we have filled the ring */
 	rx_ring->next_to_alloc = val;
 
-	/* 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(val, rx_ring->tail);
 }
 
@@ -2379,7 +2373,7 @@ static inline void i40evf_tx_map(struct i40e_ring *tx_ring, struct sk_buff *skb,
 
 	/* notify HW of packet */
 	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
-- 
2.7.4