linux-kernel - Re: [PATCH v4 1/6] RDMA/bnxt_re: Eliminate duplicate barriers on weakly-ordered archs

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Date:   Tue, 20 Mar 2018 08:48:01 -0600
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Sinan Kaya <okaya@...eaurora.org>
Cc:     linux-rdma@...r.kernel.org, timur@...eaurora.org,
        sulrich@...eaurora.org, linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Selvin Xavier <selvin.xavier@...adcom.com>,
        Devesh Sharma <devesh.sharma@...adcom.com>,
        Somnath Kotur <somnath.kotur@...adcom.com>,
        Sriharsha Basavapatna <sriharsha.basavapatna@...adcom.com>,
        Doug Ledford <dledford@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/6] RDMA/bnxt_re: Eliminate duplicate barriers on
 weakly-ordered archs

On Mon, Mar 19, 2018 at 10:47:43PM -0400, Sinan Kaya 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@...eaurora.org>
>  drivers/infiniband/hw/bnxt_re/qplib_rcfw.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/infiniband/hw/bnxt_re/qplib_rcfw.c b/drivers/infiniband/hw/bnxt_re/qplib_rcfw.c
> index 8329ec6..4a6b981 100644
> +++ b/drivers/infiniband/hw/bnxt_re/qplib_rcfw.c
> @@ -181,10 +181,10 @@ static int __send_message(struct bnxt_qplib_rcfw *rcfw, struct cmdq_base *req,
>  
>  	/* ring CMDQ DB */
>  	wmb();
> -	writel(cmdq_prod, rcfw->cmdq_bar_reg_iomem +
> -	       rcfw->cmdq_bar_reg_prod_off);
> -	writel(RCFW_CMDQ_TRIG_VAL, rcfw->cmdq_bar_reg_iomem +
> -	       rcfw->cmdq_bar_reg_trig_off);
> +	writel_relaxed(cmdq_prod, rcfw->cmdq_bar_reg_iomem +
> +		       rcfw->cmdq_bar_reg_prod_off);
> +	writel_relaxed(RCFW_CMDQ_TRIG_VAL, rcfw->cmdq_bar_reg_iomem +
> +		       rcfw->cmdq_bar_reg_trig_off);

Woah, this may not be safe..

The definition of writel_relaxed() is that it is fully unordered, so
the above two writes may change order now. Broadcom guys would have to
ack if that it is OK or not for their hardware.

In general this is not an OK approach for a mechanical
conversion.. Only the first writel can be convereted.

You need to check all your patches to make sure there are no
subsequent writel's in the places touched.

Jason