netdev - Re: RFC on writel and writel

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Date:   Wed, 28 Mar 2018 08:33:16 +1100
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Alexander Duyck <alexander.duyck@...il.com>,
        Will Deacon <will.deacon@....com>
Cc:     Sinan Kaya <okaya@...eaurora.org>, Arnd Bergmann <arnd@...db.de>,
        Jason Gunthorpe <jgg@...pe.ca>,
        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>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: RFC on writel and writel_relaxed

On Tue, 2018-03-27 at 11:54 -0700, Alexander Duyck wrote:
> As far as I know the code has been this way for a while, something
> like 2002, when the barrier was already present in e1000. However
> there it was calling out weakly ordered models "such as IA-64". Since
> then pretty much all the hardware based network drivers at this point
> have similar code floating around with wmb() in place to prevent
> issues on weak ordered memory systems.
> 
> So in any case we still need to be careful as there are architectures
> that are depending on this even if they might not be x86. :-/

Well, we need to clarify that once and for all, because as I wrote
earlier, it was decreed by Linus more than a decade ago that writel
would be fully ordered by itself vs. previous memory stores (at least
on UC memory).

This is why we added sync's to writel on powerpc and later ARM added
similar barriers to theirs.

This is also why writel_relaxed was added (though much later), since
what writel_relaxed does is to life that specific requirement.

IE. If what you say is true and wmb() is needed on x86, then
writel_relaxed is now completely useless...

Ben.