From mboxrd@z Thu Jan  1 00:00:00 1970
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Subject: Re: RFC on writel and writel_relaxed
Date: Wed, 28 Mar 2018 08:35:19 +1100
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To: Sinan Kaya <okaya@codeaurora.org>, Arnd Bergmann <arnd@arndb.de>, Jason Gunthorpe <jgg@ziepe.ca>
Cc: David Laight <David.Laight@aculab.com>, Oliver <oohall@gmail.com>, "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" <linuxppc-dev@lists.ozlabs.org>, "linux-rdma@vger.kernel.org" <linux-rdma@vger.kernel.org>, Alexander Duyck <alexander.h.duyck@redhat.com>, Will Deacon <will.deacon@arm.com>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, "netdev@vger.kernel.org" <netdev@vger.kernel.org>, Alexander Duyck <alexander.duyck@gmail.com>
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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."