diff mbox series

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

Message ID 20200102180830.66676-2-liran.alon@oracle.com
State Changes Requested
Delegated to: David Miller
Headers show
Series net: AWS ENA: Fix memory barrier usage when using LLQ | expand

Commit Message

Liran Alon Jan. 2, 2020, 6:08 p.m. UTC 
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@oracle.com>
Signed-off-by: Liran Alon <liran.alon@oracle.com>
---
 drivers/net/ethernet/amazon/ena/ena_eth_com.c | 5 -----
 1 file changed, 5 deletions(-)

Comments

Liran Alon Jan. 17, 2020, 3:54 p.m. UTC | #1 
Ping on merging this patch (Patch 2/2 of series should be dropped).

-Liran

> On 2 Jan 2020, at 20:08, Liran Alon <liran.alon@oracle.com> wrote:
> 
> 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@oracle.com>
> Signed-off-by: Liran Alon <liran.alon@oracle.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
>
diff mbox series

Patch

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);