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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 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.

Robin.

> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> ---
>  drivers/iommu/arm-smmu-v3.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
> index 380969a..d2fbee3 100644
> --- a/drivers/iommu/arm-smmu-v3.c
> +++ b/drivers/iommu/arm-smmu-v3.c
> @@ -728,7 +728,7 @@ static void queue_inc_prod(struct arm_smmu_queue *q)
>  	u32 prod = (Q_WRP(q, q->prod) | Q_IDX(q, q->prod)) + 1;
> 
>  	q->prod = Q_OVF(q, q->prod) | Q_WRP(q, prod) | Q_IDX(q, prod);
> -	writel(q->prod, q->prod_reg);
> +	writel_relaxed(q->prod, q->prod_reg);
>  }
> 
>  /*
> --
> 2.5.0
> 
> 

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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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