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

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Message-ID: <20180801072947.GD20224@lst.de>
Date:   Wed, 1 Aug 2018 09:29:47 +0200
From:   Christoph Hellwig <hch@....de>
To:     okaya@...eaurora.org
Cc:     Christoph Hellwig <hch@....de>, 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, okaya@...nel.org
Subject: Re: [PATCH] ia64: fix barrier placement for write* / dma mapping

On Tue, Jul 31, 2018 at 11:41:23PM -0700, okaya@...eaurora.org 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:

commit 66b99421d118a5ddd98a72913670b0fcf0a38d45
Author: Andrew Morton <akpm@...l.org>
Date:   Sat Mar 13 17:05:37 2004 -0800

    [PATCH] DMA: Fill gaping hole in DMA API interfaces.

    From: "David S. Miller" <davem@...hat.com>

which in fact only added them for the HP zx1 platform, and doesn't
contain any good explanation of why we need a barrier.

So I guess the right answer might be to just remove these barriers
without replacement.