netdev - Aw: Re: [PATCH v7 0/7] netdev: intel: Eliminate duplicate barriers on weakly-ordered archs

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Date:   Tue, 27 Mar 2018 16:33:00 +0200
From:   "Lino Sanfilippo" <LinoSanfilippo@....de>
To:     "Sinan Kaya" <okaya@...eaurora.org>
Cc:     "Jeff Kirsher" <jeffrey.t.kirsher@...el.com>,
        "Alexander Duyck" <alexander.duyck@...il.com>,
        intel-wired-lan <intel-wired-lan@...ts.osuosl.org>,
        Netdev <netdev@...r.kernel.org>,
        "Timur Tabi" <timur@...eaurora.org>, sulrich@...eaurora.org,
        linux-arm-msm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Aw: Re: [PATCH v7 0/7] netdev: intel: Eliminate duplicate barriers
 on weakly-ordered archs


>
> On 3/27/2018 10:04 AM, Lino Sanfilippo wrote:
> > Hi,
> > 
> >> Double sorry now.
> >>
> >> I don't know if you have been following "RFC on writel and writel_relaxed" thread
> >> or not but there are some new developments about wmb() requirement. 
> > 
> > Just out of interest: Where can this thread be found?
> 
> https://www.spinics.net/lists/linux-rdma/msg62570.html
> 
> https://patchwork.kernel.org/patch/10309913/
> 
> 
> > 
> >>
> >> Basically, wmb() should never be used before writel() as writel() seem to
> >> provide coherency and observability guarantee.
> >>
> > 
> > AFAIU memory-barriers.txt writel() only guarantees correct order of accesses to
> > IO-memory not RAM vs. IO-memory (this may be the case for some architectures 
> > where the writel() implementation contains a wmb() but not for all).
> > For the RAM vs. IO-memory case at least a a wmb()/rmb() has to be used. 
> > Is this not correct? 
> 
> We are being told that if you use writel(), then you don't need a wmb() on
> all architectures.
> 
> Jason is seeking behavior clarification for write combined buffers.
> 

Interesting, thanks for the information!

Lino