Subject: RE: [PATCH v3 18/18] infiniband: cxgb4: Eliminate duplicate barriers on weakly-ordered archs

From: "Steve Wise" <[email-masked]>
Date: Fri, 16 Mar 2018 18:05:44 -0500

> 
> On 3/16/2018 5:05 PM, Steve Wise wrote:
> >> Code includes wmb() followed by writel(). writel() already has a barrier
> > on
> >> some architectures like arm64.
> >>
> >> This ends up CPU observing two barriers back to back before executing
> the
> >> register write.
> >>
> >> Since code already has an explicit barrier call, changing writel() to
> >> writel_relaxed().
> >>
> >> Signed-off-by: Sinan Kaya <[email-masked]>
> >
> > NAK - This isn't correct for PowerPC.  For PowerPC, writeX_relaxed() is just
> > writeX().
> >
> > I was just looking at this with Chelsio developers, and they said the
> > writeX() should be replaced with __raw_writeX(), not writeX_relaxed(), to
> > get rid of the extra barrier for all architectures.
> 
> OK. I can do that but isn't the problem at PowerPC adaptation?
> 
> /*
>  * We don't do relaxed operations yet, at least not with this semantic
>  */
> #define readb_relaxed(addr)	readb(addr)
> #define readw_relaxed(addr)	readw(addr)
> #define readl_relaxed(addr)	readl(addr)
> #define readq_relaxed(addr)	readq(addr)
> #define writeb_relaxed(v, addr)	writeb(v, addr)
> #define writew_relaxed(v, addr)	writew(v, addr)
> #define writel_relaxed(v, addr)	writel(v, addr)
> #define writeq_relaxed(v, addr)	writeq(v, addr)
> 
> Why don't we fix the PowerPC's relaxed operators? Is that a bigger task?

I don't know the answer, but perhaps the proper fix is to correctly implement these for PPC?

> 
> >From API perspective both __raw_writeX() and writeX_relaxed() are
> correct.
> It is just PowerPC doesn't seem the follow the definition yet.