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

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Date:   Tue, 3 Apr 2018 13:47:05 -0400
From:   Sinan Kaya <okaya@...eaurora.org>
To:     "Kalderon, Michal" <Michal.Kalderon@...ium.com>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "timur@...eaurora.org" <timur@...eaurora.org>,
        "sulrich@...eaurora.org" <sulrich@...eaurora.org>
Cc:     "linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "Elior, Ariel" <Ariel.Elior@...ium.com>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 3/3] RDMA/qedr: eliminate duplicate barriers on
 weakly-ordered archs #2

On 4/3/2018 3:42 AM, Kalderon, Michal wrote:
> The wmb before writel are used to make sure the
> HW observes the changes in memory before we trigger the doorbell. 

According to Linus, writel() guarantees observability. No extra
barrier is necessary.

https://www.mail-archive.com/netdev@vger.kernel.org/msg225806.html

There shouldn't be any wmb() in drivers unless it is used for a
very well-known reason. 

APIs like readX() and writeX() guarantee observability.

-- 
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.