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

[Leizhen (ThunderTown)]

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.

> 
> Will
> 
> .
> 

-- 
Thanks!
BestRegards