Re: [PATCH 6/7] e1000: 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(). 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/e1000/e1000_main.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/net/ethernet/intel/e1000/e1000_main.c b/drivers/net/ethernet/intel/e1000/e1000_main.c
> index 3dd4aeb..e0e583a 100644
> --- a/drivers/net/ethernet/intel/e1000/e1000_main.c
> +++ b/drivers/net/ethernet/intel/e1000/e1000_main.c
> @@ -4573,7 +4573,7 @@ e1000_alloc_jumbo_rx_buffers(struct e1000_adapter *adapter,
>                  * such as IA-64).
>                  */
>                 wmb();
> -               writel(i, adapter->hw.hw_addr + rx_ring->rdt);
> +               writel_relaxed(i, adapter->hw.hw_addr + rx_ring->rdt);
>         }
>  }
>
> @@ -4688,7 +4688,7 @@ static void e1000_alloc_rx_buffers(struct e1000_adapter *adapter,
>                  * such as IA-64).
>                  */
>                 wmb();
> -               writel(i, hw->hw_addr + rx_ring->rdt);
> +               writel_relaxed(i, hw->hw_addr + rx_ring->rdt);
>         }
>  }
>

So you missed the writel in e1000_xmit_frame. You should probably get
that one too while you are doing these updates. The wmb() is in
e1000_tx_queue().