RFC on writel and writel_relaxed

[Benjamin Herrenschmidt]

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.