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

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Message-ID: <004501d3bd7b$505e70f0$f11b52d0$@opengridcomputing.com>
Date:   Fri, 16 Mar 2018 18:05:44 -0500
From:   "Steve Wise" <swise@...ngridcomputing.com>
To:     "'Sinan Kaya'" <okaya@...eaurora.org>, <netdev@...r.kernel.org>,
        <timur@...eaurora.org>, <sulrich@...eaurora.org>
Cc:     <linux-arm-msm@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        "'Steve Wise'" <swise@...lsio.com>,
        "'Doug Ledford'" <dledford@...hat.com>,
        "'Jason Gunthorpe'" <jgg@...pe.ca>, <linux-rdma@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        "'Michael Werner'" <werner@...lsio.com>,
        "'Casey Leedom'" <leedom@...lsio.com>
Subject: RE: [PATCH v3 18/18] infiniband: cxgb4: Eliminate duplicate barriers on weakly-ordered archs

> 
> 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 <okaya@...eaurora.org>
> >
> > 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.