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 16:13:47 -0600
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Steve Wise <swise@...ngridcomputing.com>
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?

> 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 :(

Jason