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Subject: Re: RFC on writel and writel_relaxed
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
 Oliver <oohall@gmail.com>
Cc: linuxppc dev list <linuxppc-dev@lists.ozlabs.org>,
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From: Sinan Kaya <okaya@codeaurora.org>
Message-ID: <511b73e6-797c-9986-075c-c5b5d4eb1ed2@codeaurora.org>
Date: Sat, 24 Mar 2018 11:06:52 -0400
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On 3/23/2018 9:22 PM, Benjamin Herrenschmidt wrote:
>> Yes, I'm %100 sure. The answer is both writel() and barrier() generates
>> a write barrier instruction. I found this by searching the kernel disassembly
>> for back to back "dsb st" instruction.
> I'm not sure you are correct here. As I wrote below, the implementatoin
> of writel() contains an *explicit" memory barrier which is completely
> different to a barrier() instruction:

OK. I did some directed tests and I'm taking it back. 
barrier() is a compiler reordering statement only.

What got me confused was this sequence:

wmb()
barrier()
writel()

I thought that the second barrier instruction was coming from barrier() but
it was actually coming from writel().

-- 
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.