Re: [Intel-wired-lan] [PATCH v6 7/7] ixgbevf: eliminate duplicate barriers on weakly-ordered archs

[Alexander Duyck <alexander.duyck@gmail.com>]

On Fri, Mar 23, 2018 at 11:21 AM, Sinan Kaya <okaya@codeaurora.org> wrote:
> Code includes wmb() followed by writel() in multiple places. 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.
>
> Since code already has an explicit barrier call, changing writel() to
> writel_relaxed().
>
> Signed-off-by: Sinan Kaya <okaya@codeaurora.org>
> ---
>  drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c | 21 ++++++++++++++++++---
>  1 file changed, 18 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c b/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
> index 815cb1a..9e684b1 100644
> --- a/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
> +++ b/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
> @@ -725,7 +725,12 @@ static void ixgbevf_alloc_rx_buffers(struct ixgbevf_ring *rx_ring,
>                  * such as IA-64).
>                  */
>                 wmb();
> -               writel(i, rx_ring->tail);
> +               writel_relaxed(i, rx_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();
>         }

The mmiowb shouldn't be needed for Rx. Only one CPU will be running
NAPI for the queue and we will synchronize this with a full writel
anyway when we re-enable the interrupts.

>  }
>
> @@ -1232,7 +1237,12 @@ static int ixgbevf_clean_rx_irq(struct ixgbevf_q_vector *q_vector,
>                  * know there are new descriptors to fetch.
>                  */
>                 wmb();
> -               writel(xdp_ring->next_to_use, xdp_ring->tail);
> +               writel_relaxed(xdp_ring->next_to_use, xdp_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();
>         }
>
>         u64_stats_update_begin(&rx_ring->syncp);
> @@ -4004,7 +4014,12 @@ static void ixgbevf_tx_map(struct ixgbevf_ring *tx_ring,
>         tx_ring->next_to_use = i;
>
>         /* notify HW of packet */
> -       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
> +        */
> +       mmiowb();
>
>         return;
>  dma_error:
> --
> 2.7.4
>
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