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

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Date:   Mon, 26 Jun 2017 21:41:47 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     Will Deacon <will.deacon@....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 2017/6/26 21:29, Leizhen (ThunderTown) wrote:
> 
> 
> On 2017/6/21 17:08, Will Deacon wrote:
>> 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.
> A good news. I got a new idea and I will post v2 later.
[PATCH 0/5] arm-smmu: performance optimization
[PATCH 1/5] iommu/arm-smmu-v3: put off the execution of TLBI* to reduce lock confliction

I just sent.

> 
>>
>> Will
>>
>> .
>>
> 

-- 
Thanks!
BestRegards