netdev - Re: RFC on writel and writel

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Date:   Tue, 27 Mar 2018 16:01:08 +0100
From:   Jose Abreu <Jose.Abreu@...opsys.com>
To:     Sinan Kaya <okaya@...eaurora.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Arnd Bergmann <arnd@...db.de>, Jason Gunthorpe <jgg@...pe.ca>
CC:     David Laight <David.Laight@...lab.com>, Oliver <oohall@...il.com>,
        "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" 
        <linuxppc-dev@...ts.ozlabs.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        Alexander Duyck <alexander.h.duyck@...hat.com>,
        Will Deacon <will.deacon@....com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: RFC on writel and writel_relaxed

Hi,

On 27-03-2018 15:46, Sinan Kaya wrote:
>
> Sinan
> "We are being told that if you use writel(), then you don't need a wmb() on
> all architectures."
>
> Alex:
> "I'm not sure who told you that but that is incorrect, at least for
> x86. If you attempt to use writel() without the wmb() we will have to
> NAK the patches. We will accept the wmb() with writel_releaxed() since
> that solves things for ARM."
>

So this means we should always use writel() + wmb() in *all*
accesses? I don't know about x86 but arc architecture doesn't
have a wmb() in the writel() function (in some configs).

I see the point in net drivers while you have dma + io accesses
but for most drivers this shouldn't be needed, right?

What about ordering of writes? Is it guaranteed that one write
will happen before the next one ?

Best Regards,
Jose Miguel Abreu