From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp.codeaurora.org (smtp.codeaurora.org [198.145.29.96]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 403dXg4v4dzDqnN for ; Sun, 18 Mar 2018 10:17:54 +1100 (AEDT) Subject: Re: [PATCH v3 18/18] infiniband: cxgb4: Eliminate duplicate barriers on weakly-ordered archs To: Jason Gunthorpe Cc: Steve Wise , netdev@vger.kernel.org, timur@codeaurora.org, sulrich@codeaurora.org, linux-arm-msm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, 'Steve Wise' , 'Doug Ledford' , linux-rdma@vger.kernel.org, linux-kernel@vger.kernel.org, 'Michael Werner' , 'Casey Leedom' , "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" References: <1521216991-28706-1-git-send-email-okaya@codeaurora.org> <1521216991-28706-19-git-send-email-okaya@codeaurora.org> <003601d3bd6a$783d6970$68b83c50$@opengridcomputing.com> <83387f6e-adcb-14e9-2c22-96abf9493cc6@codeaurora.org> <004501d3bd7b$505e70f0$f11b52d0$@opengridcomputing.com> <740c7d45-450e-c9b3-ceed-7bc7fcefbc5a@codeaurora.org> <71e37a55-537b-d75a-cfde-f188b7cfce8e@codeaurora.org> <1f5e3b14-05a1-08d0-c0cb-00805526448d@codeaurora.org> <20180317150520.GA23463@ziepe.ca> From: Sinan Kaya Message-ID: Date: Sat, 17 Mar 2018 14:30:10 -0400 MIME-Version: 1.0 In-Reply-To: <20180317150520.GA23463@ziepe.ca> Content-Type: text/plain; charset=utf-8 List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , +linuxppc-dev@lists.ozlabs.org On 3/17/2018 11:05 AM, Jason Gunthorpe wrote: > On Sat, Mar 17, 2018 at 12:25:14AM -0400, Sinan Kaya wrote: >> 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(); > > No! NAK! Adding *new* barriers to use the *wrong* accessor is crazy! > > Your first patch was right, replacing > wmb() > writel() > > With wmb(); writel_relaxed() is fine, and helps ARM. > > The problem with PPC has nothing to do with the writel, it is with the > spinlock, and we can't improve it without adding some new > infrastructure. Certainly don't hack around the problem by using > __raw_writel in multi-arch drivers! > > In userspace we settled on something like this as a pattern: > > mmio_wc_spin_lock() > writel_wc_mmio() > mmio_wc_spin_unlock() > > Where mmio_wc_spin_unlock incorporates the mmiowmb and is defined to > fully contain all MMIO access to WC and UC memory. > > Due to our userspace the pattern is specifically used with MMIO writes > to WC BAR memory, but it could be generalized.. > > This allows all known arches to use the best code in all cases. x86 > can even optimize a lot and combine the mmiowmb() into the > spin_unlock, apparently. Given both Dave [1] and Jason's feedback, we have to ask PowerPC developers to fix writel_relaxed(). Otherwise, PowerPC won't be able to take advantage of the optimizations introduced in this series. I don't think we need writel_really_relaxed() API. Somebody also has to take a task and work very hard to get rid of __raw_writeX() APIs in drivers/net directory. It looked like a very common practice though it clearly violates multiarch portability concerns Jason and Deve highlighted. This will be the next version: iff --git a/drivers/infiniband/hw/cxgb4/t4.h b/drivers/infiniband/hw/cxgb4/t4.h index 8369c7c..6e5658a 100644 --- a/drivers/infiniband/hw/cxgb4/t4.h +++ 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); + writeq_relaxed(*src, dst); src++; dst++; count--; @@ -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); } [1] https://lkml.org/lkml/2018/3/17/100 > > Jason > -- 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.