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Subject: Re: RFC on writel and writel_relaxed
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Jason Gunthorpe <jgg@ziepe.ca>, okaya@codeaurora.org
Cc: Arnd Bergmann <arnd@arndb.de>, David Laight <David.Laight@aculab.com>,
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Date: Wed, 28 Mar 2018 08:27:12 +1100
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On Tue, 2018-03-27 at 08:12 -0600, Jason Gunthorpe wrote:
> > I have been converting wmb+writel to wmb+writel_relaxed. (About 30 patches)
> > 
> > I will have to just remove the wmb and keep writel, then repost.
> 
> Okay, but before you do that, can we get a statement how this works
> for WC?
> 
> Some of these writels are to WC memory, do they need the wmb()?!?

This is an issue as we don't have well defined semantics for WC.

At this point, I would suggest staying away from that (ie, not changing
them). We need to look into it.

I know for example that on powerpc I cannot give you any weaker
semantic on WC for writel (I have to put a full sync in there), but I
am trying to see if I can make writel_relaxed both work with the
existing semantics and provide combining. But it's not yet a given (our
weaker IO barrier, eieio, isn't architecturally defined to do anything
on G=0 space, looking with the HW guys at what the HW actually does).

Cheers,
Ben.