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 17:46:16 -0400
From:   Sinan Kaya <okaya@...eaurora.org>
To:     Steve Wise <swise@...ngridcomputing.com>, 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?

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