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

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Date:   Tue, 27 Mar 2018 10:23:32 -0400
From:   Sinan Kaya <okaya@...eaurora.org>
To:     Lino Sanfilippo <LinoSanfilippo@....de>
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: 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.

> 
> Regards,
> Lino
> 


-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.