linux-kernel - Re: [PATCH V2] scsi: mpt3sas: remove redundant wmb

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Message-ID: <yq1y3uvq0c1.fsf@oracle.com>
Date:   Wed, 19 Apr 2017 22:28:30 -0400
From:   "Martin K. Petersen" <martin.petersen@...cle.com>
To:     Sinan Kaya <okaya@...eaurora.org>
Cc:     linux-scsi@...r.kernel.org, timur@...eaurora.org,
        linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Sathya Prakash <sathya.prakash@...adcom.com>,
        Chaitra P B <chaitra.basappa@...adcom.com>,
        Suganath Prabu Subramani 
        <suganath-prabu.subramani@...adcom.com>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        MPT-FusionLinux.pdl@...adcom.com (open list:LSILOGIC MPT FUSION DRIVERS
        (FC/SAS/SPI)), linux-kernel@...r.kernel.org (open list)
Subject: Re: [PATCH V2] scsi: mpt3sas: remove redundant wmb

Sinan Kaya <okaya@...eaurora.org> writes:

> Due to relaxed ordering requirements on multiple architectures,
> drivers are required to use wmb/rmb/mb combinations when they need to
> guarantee observability between the memory and the HW.
>
> The mpt3sas driver is already using wmb() for this purpose.  However,
> it issues a writel following wmb(). writel() function on arm/arm64
> arhictectures have an embedded wmb() call inside.
>
> This results in unnecessary performance loss and code duplication.
>
> writel already guarantees ordering for both cpu and bus. we don't need
> additional wmb()

Broadcom folks, please review!

-- 
Martin K. Petersen	Oracle Linux Engineering