linux-kernel - Re: [PATCH v5 3/3] RDMA/qedr: eliminate duplicate barriers on weakly-ordered archs #2

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Date:   Mon, 2 Apr 2018 22:29:53 -0400
From:   Sinan Kaya <okaya@...eaurora.org>
To:     linux-rdma@...r.kernel.org, timur@...eaurora.org,
        sulrich@...eaurora.org
Cc:     linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Michal Kalderon <Michal.Kalderon@...ium.com>,
        Ariel Elior <Ariel.Elior@...ium.com>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 3/3] RDMA/qedr: eliminate duplicate barriers on
 weakly-ordered archs #2

On 3/22/2018 12:26 PM, Sinan Kaya wrote:
> @@ -860,7 +860,7 @@ static void doorbell_cq(struct qedr_cq *cq, u32 cons, u8 flags)
>  	wmb();
>  	cq->db.data.agg_flags = flags;
>  	cq->db.data.value = cpu_to_le32(cons);
> -	writeq(cq->db.raw, cq->db_addr);
> +	writeq_relaxed(cq->db.raw, cq->db_addr);

Given the direction to get rid of wmb() in front of writeX() functions, I have been
reviewing this code. Under normal circumstances, I can get rid of all wmb() as follows.

However, I started having my doubts now. Are these wmb() used as a SMP barrier too?
I can't find any smp_Xmb() in drivers/infiniband/hw/qedr directory.

static void doorbell_cq(struct qedr_cq *cq, u32 cons, u8 flags)
 {
-       /* Flush data before signalling doorbell */
-       wmb();
        cq->db.data.agg_flags = flags;
        cq->db.data.value = cpu_to_le32(cons);
        writeq(cq->db.raw, cq->db_addr);
@@ -1870,8 +1868,7 @@ static int qedr_update_qp_state(struct qedr_dev *dev,
                         */

                        if (rdma_protocol_roce(&dev->ibdev, 1)) {
-                               wmb();
-                               writel_relaxed(qp->rq.db_data.raw, qp->rq.db);
+                               writel(qp->rq.db_data.raw, qp->rq.db);
                                /* Make sure write takes effect */
                                mmiowb();
                        }
@@ -3275,8 +3272,7 @@ int qedr_post_send(struct ib_qp *ibqp, struct ib_send_wr *wr,
         * unchanged). For performance reasons we avoid checking for this
         * redundant doorbell.
         */
-       wmb();
-       writel_relaxed(qp->sq.db_data.raw, qp->sq.db);
+       writel(qp->sq.db_data.raw, qp->sq.db);

        /* Make sure write sticks */
        mmiowb();
@@ -3362,9 +3358,6 @@ int qedr_post_recv(struct ib_qp *ibqp, struct ib_recv_wr *wr,

                qedr_inc_sw_prod(&qp->rq);

-               /* Flush all the writes before signalling doorbell */
-               wmb();





-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.