linux-kernel - Re: [PATCH] ia64: fix barrier placement for write* / dma mapping

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Date:   Wed, 1 Aug 2018 01:00:32 -0700
From:   Sinan Kaya <okaya@...nel.org>
To:     Christoph Hellwig <hch@....de>, okaya@...eaurora.org
Cc:     Tony Luck <tony.luck@...el.com>, Fenghua Yu <fenghua.yu@...el.com>,
        Arnd Bergmann <arnd@...db.de>, linux-ia64@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org
Subject: Re: [PATCH] ia64: fix barrier placement for write* / dma mapping

On 8/1/2018 12:29 AM, Christoph Hellwig wrote:
>> I asked this question to Tony Luck before. If I remember right,
>> his answer was:
>>
>> CPU guarantees outstanding writes to be flushed when a register write
>> instruction is executed and an additional barrier instruction is not
>> needed.
> That would be great.  It still doesn't explain the barriers in the
> dma sync routines.  Those have been there since the following commit
> in the history tree:

Yeah, I'll let Tony confirm my understanding.