linux-kernel - RE: [PATCH v3 18/18] infiniband: cxgb4: Eliminate duplicate barriers on weakly-ordered archs

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Date:   Fri, 16 Mar 2018 16:05:10 -0500
From:   "Steve Wise" <swise@...ngridcomputing.com>
To:     "'Sinan Kaya'" <okaya@...eaurora.org>, <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

> 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>

NAK - This isn't correct for PowerPC.  For PowerPC, writeX_relaxed() is just
writeX().  

I was just looking at this with Chelsio developers, and they said the
writeX() should be replaced with __raw_writeX(), not writeX_relaxed(), to
get rid of the extra barrier for all architectures.

Also, t4.h:pio_copy() needs to  use __raw_writeq() to enable the write
combining fastpath for ARM and PowerPC.  The code as it stands doesn't
achieve any write combining on PowerPC at least.  

And the writel()s at the end of the ring functions (the non bar2 udb path)
needs a mmiowb() afterwards if you're going to use __raw_writeX() there.
However that path is only used for very old hardware (T4), so I wouldn't
worry about them. 

Steve.


> ---
>  drivers/infiniband/hw/cxgb4/t4.h | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/infiniband/hw/cxgb4/t4.h
> b/drivers/infiniband/hw/cxgb4/t4.h
> index 8369c7c..7a48c9e 100644
> --- a/drivers/infiniband/hw/cxgb4/t4.h
> +++ b/drivers/infiniband/hw/cxgb4/t4.h
> @@ -477,15 +477,15 @@ 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);
> +			writel_relaxed(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);
> +	writel_relaxed(QID_V(wq->sq.qid) | PIDX_V(inc), wq->db);
>  }
> 
>  static inline void t4_ring_rq_db(struct t4_wq *wq, u16 inc,
> @@ -502,15 +502,15 @@ 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);
> +			writel_relaxed(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);
> +	writel_relaxed(QID_V(wq->rq.qid) | PIDX_V(inc), wq->db);
>  }
> 
>  static inline int t4_wq_in_error(struct t4_wq *wq)
> --
> 2.7.4