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

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Date:   Tue, 31 Jul 2018 23:41:23 -0700
From:   okaya@...eaurora.org
To:     Christoph Hellwig <hch@....de>
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, okaya@...nel.org
Subject: Re: [PATCH] ia64: fix barrier placement for write* / dma mapping

+ my new email

On 2018-07-31 10:20, Christoph Hellwig wrote:
> memory-barriers.txt has been updated with the following requirement.
> 
> "When using writel(), a prior wmb() is not needed to guarantee that the
> cache coherent memory writes have completed before writing to the MMIO
> region."
> 
> The current writeX() and iowriteX() implementations on ia64 are not
> satisfying this requirement as the barrier is after the register write.
> 

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.

> This adds the missing memory barriers, and instead drops them from the
> dma sync routine where they are misplaced (and were missing in the
> more important map/unmap cases anyway).
> 
> All this doesn't affect the SN2 platform, which already has barrier
> in the I/O accessors, and none in dma mapping (but then again
> swiotlb doesn't have any either).
> 
> Signed-off-by: Christoph Hellwig <hch@....de>
> ---