Re: [PATCH 2/2] coresight: tmc: Add shutdown callback for TMC ETR/ETF

[Robin Murphy <robin.murphy@arm.com>]

On 2020-06-03 13:00, Sai Prakash Ranjan wrote:
> Hi Robin, Mathieu
> 
> On 2020-06-03 17:07, Robin Murphy wrote:
>> On 2020-06-01 22:28, Mathieu Poirier wrote:
>>> That being said I'm sure that dependencies on an IOMMU isn't a 
>>> problem confined
>>> to coresight. I am adding Robin Murphy, who added this commit [1], to 
>>> the thread
>>> in the hope that he can provide guidance on the right way to do this.
>>
>> Right, it's not specific to CoreSight, and it's not even specific to
>> IOMMUs really. In short, blame kexec ;)
>>
> 
> Yes it is not specific to coresight, we are targeting this for all
> consumers/clients of SMMU(atleast on SC7180 SoC). We have display throwing
> NoC/interconnect errors[1] during reboot after SMMU is disabled.
> This is also not specific to kexec either as you explained here [2] about
> a case with display which is exacly what is happening in our system [1].

Sure, but those instances are begging the question of why the SMMU is 
disabled at reboot in the first place ;)

Robin.

> 
> [1] 
> https://lore.kernel.org/lkml/1591009402-681-1-git-send-email-mkrishn@codeaurora.org/ 
> 
> [2] 
> https://lore.kernel.org/lkml/5858bdac-b7f9-ac26-0c0d-c9653cef841d@arm.com/
> 
>> The fundamental thing is that devices should stop any DMA activity at
>> shutdown. For a normal poweroff you can typically get away without
>> doing so, but over kexec, ongoing DMA traffic may corrupt memory in
>> the new kernel (at worst, I think even DMA reads could potentially
>> cause unexpected cache behaviour that might lead to mishaps, given the
>> right combination of memory attributes).
>>
>> IOMMUs merely help to make the situation more serious. For similar
>> kexec reasons, they need to disable any existing translations at
>> shutdown (imagine if the second kernel didn't have an IOMMU driver).
>> And at that point, even the normal poweroff case becomes problematic,
>> because any device DMA that hasn't been shut down beforehand is now
>> not necessarily going benignly to memory as it would in the no-IOMMU
>> case above, but potentially to random physical addresses, with all the
>> hilarity ensuing that you would expect from that.
>>
> 
> Thanks,
> Sai