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

[Alexander Duyck]

On Tue, Mar 13, 2018 at 8:20 PM, Sinan Kaya <okaya@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
> ---
>  drivers/net/ethernet/intel/ixgbevf/ixgbevf.h      | 5 ++++-
>  drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c | 7 +++++++
>  2 files changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/ethernet/intel/ixgbevf/ixgbevf.h b/drivers/net/ethernet/intel/ixgbevf/ixgbevf.h
> index f695242..64d0e0b 100644
> --- a/drivers/net/ethernet/intel/ixgbevf/ixgbevf.h
> +++ b/drivers/net/ethernet/intel/ixgbevf/ixgbevf.h
> @@ -244,9 +244,12 @@ static inline u16 ixgbevf_desc_unused(struct ixgbevf_ring *ring)
>         return ((ntc > ntu) ? 0 : ring->count) + ntc - ntu - 1;
>  }
>
> +/* Assumes caller has executed a write barrier to order memory and device
> + * requests.
> + */
>  static inline void ixgbevf_write_tail(struct ixgbevf_ring *ring, u32 value)
>  {
> -       writel(value, ring->tail);
> +       writel_relaxed(value, ring->tail);
>  }
>
>  #define IXGBEVF_RX_DESC(R, i)  \
> diff --git a/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c b/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
> index 9b3d43d..0ba7f59 100644
> --- a/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
> +++ b/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
> @@ -3643,6 +3643,13 @@ static void ixgbevf_tx_map(struct ixgbevf_ring *tx_ring,
>
>         tx_ring->next_to_use = i;
>
> +       /* Force memory writes to complete before letting h/w
> +        * know there are new descriptors to fetch.  (Only
> +        * applicable for weak-ordered memory model archs,
> +        * such as IA-64).
> +        */
> +       wmb();

This memory barrier is redundant. There is a wmb() that is called
about 10 lines before this.

> +
>         /* notify HW of packet */
>         ixgbevf_write_tail(tx_ring, i);
>
> --
> 2.7.4
>