netdev - Re: RFC on writel and writel

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Date:   Wed, 28 Mar 2018 08:35:19 +1100
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Sinan Kaya <okaya@...eaurora.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

On Tue, 2018-03-27 at 10:46 -0400, Sinan Kaya wrote:
>  combined buffers.
> 
> Alex:
> "Don't bother. I can tell you right now that for x86 you have to have a
> wmb() before the writel().

No, this isn't the semantics of writel. You shouldn't need it unless
something changed and we need to revisit our complete understanding of
*all* MMIO accessor semantics.

At least for UC space, it has always been accepted (and enforced) that
writel would not require any other barrier to order vs. previous stores
to memory.

> Based on the comment in
> (https://www.spinics.net/lists/linux-rdma/msg62666.html):
>     Replacing wmb() + writel() with wmb() + writel_relaxed() will work on
>     PPC, it will just not give you a benefit today.
> 
> I say the patch set stays. This gives benefit on ARM, and has no
> effect on x86 and PowerPC. If you want to look at trying to optimize
> things further on PowerPC and such then go for it in terms of trying
> to implement the writel_relaxed(). Otherwise I say we call the ARM
> goodness a win and don't get ourselves too wrapped up in trying to fix
> this for all architectures."