Messages in this thread | | | Date | Wed, 03 Jun 2020 17:30:55 +0530 | From | Sai Prakash Ranjan <> | Subject | Re: [PATCH 2/2] coresight: tmc: Add shutdown callback for TMC ETR/ETF |
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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].
[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 -- QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation
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