netdev - Re: [PATCH v3 18/18] infiniband: cxgb4: Eliminate duplicate barriers on weakly-ordered archs

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Date:   Sat, 17 Mar 2018 00:25:14 -0400
From:   Sinan Kaya <okaya@...eaurora.org>
To:     Steve Wise <swise@...ngridcomputing.com>, netdev@...r.kernel.org,
        timur@...eaurora.org, sulrich@...eaurora.org
Cc:     linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        'Steve Wise' <swise@...lsio.com>,
        'Doug Ledford' <dledford@...hat.com>,
        'Jason Gunthorpe' <jgg@...pe.ca>, linux-rdma@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        'Michael Werner' <werner@...lsio.com>,
        'Casey Leedom' <leedom@...lsio.com>
Subject: Re: [PATCH v3 18/18] infiniband: cxgb4: Eliminate duplicate barriers
 on weakly-ordered archs

On 3/17/2018 12:03 AM, Sinan Kaya wrote:
> On 3/16/2018 11:40 PM, Sinan Kaya wrote:
>> I'll change writel_relaxed() with __raw_writel() in the series like you suggested
>> and also look at your other comments.
> 
> I spoke too soon.
> 
> Now that I realized, code needs to follow one of the following patterns for correctness
> 
> 1)
> wmb()
> writel()/writel_relaxed()
> 
> or 
> 
> 2)
> wmb()
> __raw_wrltel()
> mmiowb()
> 
> but definitely not
> 
> wmb()
> __raw_wrltel()
> 
> Since #1 == #2, I'll stick to my current implementation of writel_relaxed()
> 
> Changing writel() to writel_relaxed() or __raw_writel() isn't enough. PowerPC needs mmiowb()
> for correctness. ARM's mmiowb() implementation is empty.
> 
> So, there is no one size fits all solution with the current state of affairs.
> 
> 

I think I finally got what you mean.

Code seems to have

wmb()
writel()/writeq()
wmb()

this can be safely replaced with

wmb()
__raw_writel()/__raw_writeq()
wmb()

This will work on all arches. Below is the new version. Let me know if this is OK.

+++ b/drivers/infiniband/hw/cxgb4/t4.h
@@ -457,7 +457,7 @@ static inline void pio_copy(u64 __iomem *dst, u64 *src)
        int count = 8;

        while (count) {
-               writeq(*src, dst);
+               __raw_writeq(*src, dst);
                src++;
                dst++;
                count--;
@@ -477,15 +477,16 @@ static inline void t4_ring_sq_db(struct t4_wq *wq, u16 inc, union t4_wr *wqe)
                                 (u64 *)wqe);
                } else {
                        pr_debug("DB wq->sq.pidx = %d\n", wq->sq.pidx);
-                       writel(PIDX_T5_V(inc) | QID_V(wq->sq.bar2_qid),
-                              wq->sq.bar2_va + SGE_UDB_KDOORBELL);
+                       __raw_writel(PIDX_T5_V(inc) | QID_V(wq->sq.bar2_qid),
+                                    wq->sq.bar2_va + SGE_UDB_KDOORBELL);
                }

                /* Flush user doorbell area writes. */
                wmb();
                return;
        }
-       writel(QID_V(wq->sq.qid) | PIDX_V(inc), wq->db);
+       __raw_writel(QID_V(wq->sq.qid) | PIDX_V(inc), wq->db);
+       mmiowmb()
 }

 static inline void t4_ring_rq_db(struct t4_wq *wq, u16 inc,
@@ -502,15 +503,16 @@ static inline void t4_ring_rq_db(struct t4_wq *wq, u16 inc,
                                 (void *)wqe);
                } else {
                        pr_debug("DB wq->rq.pidx = %d\n", wq->rq.pidx);
-                       writel(PIDX_T5_V(inc) | QID_V(wq->rq.bar2_qid),
-                              wq->rq.bar2_va + SGE_UDB_KDOORBELL);
+                       __raw_writel(PIDX_T5_V(inc) | QID_V(wq->rq.bar2_qid),
+                                    wq->rq.bar2_va + SGE_UDB_KDOORBELL);
                }

                /* Flush user doorbell area writes. */
                wmb();
                return;
        }
-       writel(QID_V(wq->rq.qid) | PIDX_V(inc), wq->db);
+       __raw_writel(QID_V(wq->rq.qid) | PIDX_V(inc), wq->db);
+       mmiowmb();
 }


-- 
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.