linux-kernel - RE: [PATCH v4 4/6] infiniband: cxgb4: Eliminate duplicate barriers on weakly-ordered archs

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Date:   Tue, 20 Mar 2018 10:10:40 -0500
From:   "Steve Wise" <swise@...ngridcomputing.com>
To:     "'Jason Gunthorpe'" <jgg@...pe.ca>,
        "'Sinan Kaya'" <okaya@...eaurora.org>
Cc:     <linux-rdma@...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-kernel@...r.kernel.org>
Subject: RE: [PATCH v4 4/6] infiniband: cxgb4: Eliminate duplicate barriers on weakly-ordered archs

> On Mon, Mar 19, 2018 at 10:47:46PM -0400, Sinan Kaya 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>
> >  drivers/infiniband/hw/cxgb4/t4.h | 14 +++++++-------
> >  1 file changed, 7 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/infiniband/hw/cxgb4/t4.h
> b/drivers/infiniband/hw/cxgb4/t4.h
> > index 8369c7c..6e5658a 100644
> > +++ b/drivers/infiniband/hw/cxgb4/t4.h
> > @@ -457,7 +457,7 @@ static inline void pio_copy(u64 __iomem *dst, u64
> *src)
> >  	int count = 8;
> >
> >  	while (count) {
> > -		writeq(*src, dst);
> > +		writeq_relaxed(*src, dst);
> >  		src++;
> >  		dst++;
> >  		count--;
> 
> This is another case where writes can be re-ordered.. IIRC dst is WC
> BAR memory, so the NIC should tolerate re-ordering, but Steve will
> have to ack this.
> 

Yes, this is WC BAR memory.  The goal is that pio_copy() will enable
write-combining this into a single 64B pci-e transaction.