Subject: Re: [PATCH v4 17/17] net: ena: Eliminate duplicate barriers on weakly-ordered archs

From: [email-masked]
Date: Sun, 25 Mar 2018 09:33:35 -0400

On 2018-03-25 08:06, Belgazal, Netanel wrote:
> I think you should either add a parameter to
> ena_com_write_sq_doorbell() or add ena_com_write_sq_doorbell_rel().
> Right now, you have unused function.

That is true. I got rid of ena_com_write_sq_doorbell_rel.

> 
> On 3/20/18, 4:43 AM, "Sinan Kaya" <[email-masked]> wrote:
> 
>     Code includes barrier() followed by writel(). writel() already has 
> a
>     barrier
>     on some architectures like arm64.
> 
>     This ends up CPU observing two barriers back to back before 
> executing the
>     register write.
> 
>     Create a new wrapper function with relaxed write operator. Use the 
> new
>     wrapper when a write is following a barrier().
> 
>     Since code already has an explicit barrier call, changing writel() 
> to
>     writel_relaxed().
> 
>     Signed-off-by: Sinan Kaya <[email-masked]>
>     ---
>      drivers/net/ethernet/amazon/ena/ena_com.c     |  6 ++++--
>      drivers/net/ethernet/amazon/ena/ena_eth_com.h | 22 
> ++++++++++++++++++++--
>      drivers/net/ethernet/amazon/ena/ena_netdev.c  |  4 ++--
>      3 files changed, 26 insertions(+), 6 deletions(-)
> 
>     diff --git a/drivers/net/ethernet/amazon/ena/ena_com.c
> b/drivers/net/ethernet/amazon/ena/ena_com.c
>     index bf2de52..b6e628f 100644
>     --- a/drivers/net/ethernet/amazon/ena/ena_com.c
>     +++ b/drivers/net/ethernet/amazon/ena/ena_com.c
>     @@ -631,7 +631,8 @@ static u32 ena_com_reg_bar_read32(struct
> ena_com_dev *ena_dev, u16 offset)
>      	 */
>      	wmb();
> 
>     -	writel(mmio_read_reg, ena_dev->reg_bar + 
> ENA_REGS_MMIO_REG_READ_OFF);
>     +	writel_relaxed(mmio_read_reg,
>     +		       ena_dev->reg_bar + ENA_REGS_MMIO_REG_READ_OFF);
> 
>      	for (i = 0; i < timeout; i++) {
>      		if (read_resp->req_id == mmio_read->seq_num)
>     @@ -1826,7 +1827,8 @@ void ena_com_aenq_intr_handler(struct
> ena_com_dev *dev, void *data)
> 
>      	/* write the aenq doorbell after all AENQ descriptors were read 
> */
>      	mb();
>     -	writel((u32)aenq->head, dev->reg_bar + 
> ENA_REGS_AENQ_HEAD_DB_OFF);
>     +	writel_relaxed((u32)aenq->head,
>     +		       dev->reg_bar + ENA_REGS_AENQ_HEAD_DB_OFF);
>      }
> 
>      int ena_com_dev_reset(struct ena_com_dev *ena_dev,
>     diff --git a/drivers/net/ethernet/amazon/ena/ena_eth_com.h
> b/drivers/net/ethernet/amazon/ena/ena_eth_com.h
>     index 2f76572..09ef7cd 100644
>     --- a/drivers/net/ethernet/amazon/ena/ena_eth_com.h
>     +++ b/drivers/net/ethernet/amazon/ena/ena_eth_com.h
>     @@ -107,7 +107,8 @@ static inline int
> ena_com_sq_empty_space(struct ena_com_io_sq *io_sq)
>      	return io_sq->q_depth - 1 - cnt;
>      }
> 
>     -static inline int ena_com_write_sq_doorbell(struct ena_com_io_sq 
> *io_sq)
>     +static inline int ena_com_write_sq_doorbell(struct ena_com_io_sq 
> *io_sq,
>     +					    bool relaxed)
>      {
>      	u16 tail;
> 
>     @@ -116,7 +117,24 @@ static inline int
> ena_com_write_sq_doorbell(struct ena_com_io_sq *io_sq)
>      	pr_debug("write submission queue doorbell for queue: %d tail: 
> %d\n",
>      		 io_sq->qid, tail);
> 
>     -	writel(tail, io_sq->db_addr);
>     +	if (relaxed)
>     +		writel_relaxed(tail, io_sq->db_addr);
>     +	else
>     +		writel(tail, io_sq->db_addr);
>     +
>     +	return 0;
>     +}
>     +
>     +static inline int ena_com_write_sq_doorbell_rel(struct
> ena_com_io_sq *io_sq)
>     +{
>     +	u16 tail;
>     +
>     +	tail = io_sq->tail;
>     +
>     +	pr_debug("write submission queue doorbell for queue: %d tail: 
> %d\n",
>     +		 io_sq->qid, tail);
>     +
>     +	writel_relaxed(tail, io_sq->db_addr);
> 
>      	return 0;
>      }
>     diff --git a/drivers/net/ethernet/amazon/ena/ena_netdev.c
> b/drivers/net/ethernet/amazon/ena/ena_netdev.c
>     index 6975150..0530201 100644
>     --- a/drivers/net/ethernet/amazon/ena/ena_netdev.c
>     +++ b/drivers/net/ethernet/amazon/ena/ena_netdev.c
>     @@ -556,7 +556,7 @@ static int ena_refill_rx_bufs(struct ena_ring
> *rx_ring, u32 num)
>      		 * issue a doorbell
>      		 */
>      		wmb();
>     -		ena_com_write_sq_doorbell(rx_ring->ena_com_io_sq);
>     +		ena_com_write_sq_doorbell(rx_ring->ena_com_io_sq, true);
>      	}
> 
>      	rx_ring->next_to_use = next_to_use;
>     @@ -2151,7 +2151,7 @@ static netdev_tx_t ena_start_xmit(struct
> sk_buff *skb, struct net_device *dev)
> 
>      	if (netif_xmit_stopped(txq) || !skb->xmit_more) {
>      		/* trigger the dma engine */
>     -		ena_com_write_sq_doorbell(tx_ring->ena_com_io_sq);
>     +		ena_com_write_sq_doorbell(tx_ring->ena_com_io_sq, false);
>      		u64_stats_update_begin(&tx_ring->syncp);
>      		tx_ring->tx_stats.doorbells++;
>      		u64_stats_update_end(&tx_ring->syncp);
>     --
>     2.7.4