linux-kernel - Re: [PATCH 1/1] iommu/arm-smmu-v3: replace writel with writel_relaxed in queue_inc

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Date:   Wed, 21 Jun 2017 10:08:03 +0100
From:   Will Deacon <will.deacon@....com>
To:     "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
Cc:     Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        iommu <iommu@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Zefan Li <lizefan@...wei.com>, Xinwei Hu <huxinwei@...wei.com>,
        Tianhong Ding <dingtianhong@...wei.com>,
        Hanjun Guo <guohanjun@...wei.com>
Subject: Re: [PATCH 1/1] iommu/arm-smmu-v3: replace writel with
 writel_relaxed in queue_inc_prod

On Wed, Jun 21, 2017 at 09:28:23AM +0800, Leizhen (ThunderTown) wrote:
> On 2017/6/20 19:35, Robin Murphy wrote:
> > On 20/06/17 12:04, Zhen Lei wrote:
> >> This function is protected by spinlock, and the latter will do memory
> >> barrier implicitly. So that we can safely use writel_relaxed. In fact, the
> >> dmb operation will lengthen the time protected by lock, which indirectly
> >> increase the locking confliction in the stress scene.
> > 
> > If you remove the DSB between writing the commands (to Normal memory)
> > and writing the pointer (to Device memory), how can you guarantee that
> > the complete command is visible to the SMMU and it isn't going to try to
> > consume stale memory contents? The spinlock is irrelevant since it's
> > taken *before* the command is written.
> OK, I see, thanks. Let's me see if there are any other methods. And I think
> that this may should be done well by hardware.

FWIW, I did use the _relaxed variants wherever I could when I wrote the
driver. There might, of course, be bugs, but it's not like the normal case
for drivers where the author didn't consider the _relaxed accessors
initially.

Will