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

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Date:   Fri, 16 Mar 2018 18:04:40 -0500
From:   "Steve Wise" <swise@...ngridcomputing.com>
To:     "'Jason Gunthorpe'" <jgg@...pe.ca>
Cc:     "'Sinan Kaya'" <okaya@...eaurora.org>, <netdev@...r.kernel.org>,
        <timur@...eaurora.org>, <sulrich@...eaurora.org>,
        <linux-arm-msm@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        "'Steve Wise'" <swise@...lsio.com>,
        "'Doug Ledford'" <dledford@...hat.com>,
        <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 Fri, Mar 16, 2018 at 04:05:10PM -0500, 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().
> 
> ?? Why is changing writex() to writeX() a NAK then?

Because I want it correct for PPC as well.

> 
> > 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.
> 
> That doesn't seem semanticaly sane.
> 
> __raw_writeX() should not appear in driver code, IMHO. Only the arch
> code can know what the exact semantics of that accessor are..
> 
> If ppc can't use writel_relaxed to optimize then we probably need yet
> another io accessor semantic defined :(


Anybody understand why the PPC implementation of writeX_relaxed() isn't
relaxed?


Steve.