[PATCH v2] net: Google gve: Remove dma_wmb() before ringing doorbell

[PATCH v2] net: Google gve: Remove dma_wmb() before ringing doorbell

[Liran Alon]

Current code use dma_wmb() to ensure Rx/Tx descriptors are visible
to device before writing to doorbell.

However, these dma_wmb() are wrong and unnecessary. Therefore,
they should be removed.

iowrite32be() called from gve_rx_write_doorbell()/gve_tx_put_doorbell()
should guaratee that all previous writes to WB/UC memory is visible to
device before the write done by iowrite32be().

E.g. On ARM64, iowrite32be() calls __iowmb() which expands to dma_wmb()
and only then calls __raw_writel().

Reviewed-by: Si-Wei Liu <si-wei.liu@oracle.com>
Signed-off-by: Liran Alon <liran.alon@oracle.com>
---
 drivers/net/ethernet/google/gve/gve_rx.c | 2 --
 drivers/net/ethernet/google/gve/gve_tx.c | 6 ------
 2 files changed, 8 deletions(-)

diff --git a/drivers/net/ethernet/google/gve/gve_rx.c b/drivers/net/ethernet/google/gve/gve_rx.c
index edec61dfc868..9f52e72ff641 100644
--- a/drivers/net/ethernet/google/gve/gve_rx.c
+++ b/drivers/net/ethernet/google/gve/gve_rx.c
@@ -418,8 +418,6 @@ bool gve_clean_rx_done(struct gve_rx_ring *rx, int budget,
 	rx->cnt = cnt;
 	rx->fill_cnt += work_done;
 
-	/* restock desc ring slots */
-	dma_wmb();	/* Ensure descs are visible before ringing doorbell */
 	gve_rx_write_doorbell(priv, rx);
 	return gve_rx_work_pending(rx);
 }
diff --git a/drivers/net/ethernet/google/gve/gve_tx.c b/drivers/net/ethernet/google/gve/gve_tx.c
index f4889431f9b7..d0244feb0301 100644
--- a/drivers/net/ethernet/google/gve/gve_tx.c
+++ b/drivers/net/ethernet/google/gve/gve_tx.c
@@ -487,10 +487,6 @@ netdev_tx_t gve_tx(struct sk_buff *skb, struct net_device *dev)
 		 * may have added descriptors without ringing the doorbell.
 		 */
 
-		/* Ensure tx descs from a prior gve_tx are visible before
-		 * ringing doorbell.
-		 */
-		dma_wmb();
 		gve_tx_put_doorbell(priv, tx->q_resources, tx->req);
 		return NETDEV_TX_BUSY;
 	}
@@ -505,8 +501,6 @@ netdev_tx_t gve_tx(struct sk_buff *skb, struct net_device *dev)
 	if (!netif_xmit_stopped(tx->netdev_txq) && netdev_xmit_more())
 		return NETDEV_TX_OK;
 
-	/* Ensure tx descs are visible before ringing doorbell */
-	dma_wmb();
 	gve_tx_put_doorbell(priv, tx->q_resources, tx->req);
 	return NETDEV_TX_OK;
 }
-- 
2.20.1

Re: [PATCH v2] net: Google gve: Remove dma_wmb() before ringing doorbell

[David Miller]

From: Liran Alon <liran.alon@oracle.com>
Date: Fri,  3 Jan 2020 18:44:59 +0200

> Current code use dma_wmb() to ensure Rx/Tx descriptors are visible
> to device before writing to doorbell.
> 
> However, these dma_wmb() are wrong and unnecessary. Therefore,
> they should be removed.
> 
> iowrite32be() called from gve_rx_write_doorbell()/gve_tx_put_doorbell()
> should guaratee that all previous writes to WB/UC memory is visible to
> device before the write done by iowrite32be().
> 
> E.g. On ARM64, iowrite32be() calls __iowmb() which expands to dma_wmb()
> and only then calls __raw_writel().
> 
> Reviewed-by: Si-Wei Liu <si-wei.liu@oracle.com>
> Signed-off-by: Liran Alon <liran.alon@oracle.com>

Applied, thank you.