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

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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

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[PATCH 1/1] iommu/arm-smmu-v3: replace writel with writel_relaxed in queue_inc_prod Zhen Lei <thunder.leizhen@huawei.com> - 2017-06-20 13:10 +0200
  Re: [PATCH 1/1] iommu/arm-smmu-v3: replace writel with writel_relaxed  in queue_inc_prod Robin Murphy <robin.murphy@arm.com> - 2017-06-20 13:40 +0200
    Re: [PATCH 1/1] iommu/arm-smmu-v3: replace writel with writel_relaxed  in queue_inc_prod "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com> - 2017-06-21 03:30 +0200
      Re: [PATCH 1/1] iommu/arm-smmu-v3: replace writel with  writel_relaxed in queue_inc_prod Will Deacon <will.deacon@arm.com> - 2017-06-21 11:10 +0200
        Re: [PATCH 1/1] iommu/arm-smmu-v3: replace writel with writel_relaxed  in queue_inc_prod "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com> - 2017-06-26 15:40 +0200
          Re: [PATCH 1/1] iommu/arm-smmu-v3: replace writel with writel_relaxed  in queue_inc_prod "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com> - 2017-06-26 15:50 +0200

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