netdev - [PATCH 1/2] net: AWS ENA: Remove unncessary wmb() to flush bounce buffer

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Message-Id: <20200102180830.66676-2-liran.alon@oracle.com>
Date:   Thu,  2 Jan 2020 20:08:29 +0200
From:   Liran Alon <liran.alon@...cle.com>
To:     netanel@...zon.com, davem@...emloft.net, netdev@...r.kernel.org
Cc:     saeedb@...zon.com, zorik@...zon.com, sameehj@...zon.com,
        igorch@...zon.com, akiyano@...zon.com, evgenys@...zon.com,
        gtzalik@...zon.com, ndagan@...zon.com, matua@...zon.com,
        galpress@...zon.com, Liran Alon <liran.alon@...cle.com>,
        Håkon Bugge <haakon.bugge@...cle.com>
Subject: [PATCH 1/2] net: AWS ENA: Remove unncessary wmb() to flush bounce buffer

Current code executes wmb() in order to flush writes to bounce buffer
before copying it to device-memory (PCI BAR mapped as WC) to ensure
consistent data is written to device-memory.

However, this wmb() is unnecessary. This is because all reads from the
buffer are guaranteed to be consistent with previous writes to the buffer
done from the same task (Which is the only one that writes to the buffer).

i.e. If a single CPU runs both the writes to the buffer and the reads
from the buffer, the reads are guaranteed to read most up-to-date data
in program order (E.g. Due to store-to-load-forwarding mechanism).
Otherwise, there is a context-switch, and that should make writes before
context-switch globally visible as-well.

Reviewed-by: Håkon Bugge <haakon.bugge@...cle.com>
Signed-off-by: Liran Alon <liran.alon@...cle.com>
---
 drivers/net/ethernet/amazon/ena/ena_eth_com.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/drivers/net/ethernet/amazon/ena/ena_eth_com.c b/drivers/net/ethernet/amazon/ena/ena_eth_com.c
index 2845ac277724..742578ac1240 100644
--- a/drivers/net/ethernet/amazon/ena/ena_eth_com.c
+++ b/drivers/net/ethernet/amazon/ena/ena_eth_com.c
@@ -93,11 +93,6 @@ static int ena_com_write_bounce_buffer_to_dev(struct ena_com_io_sq *io_sq,
 			 io_sq->qid, io_sq->entries_in_tx_burst_left);
 	}

-	/* Make sure everything was written into the bounce buffer before
-	 * writing the bounce buffer to the device
-	 */
-	wmb();
-
 	/* The line is completed. Copy it to dev */
 	__iowrite64_copy(io_sq->desc_addr.pbuf_dev_addr + dst_offset,
 			 bounce_buffer, (llq_info->desc_list_entry_size) / 8);
-- 
2.20.1