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

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Date:   Wed, 3 Jun 2020 14:40:52 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
Cc:     Mathieu Poirier <mathieu.poirier@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Mike Leach <mike.leach@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, coresight@...ts.linaro.org,
        Stephen Boyd <swboyd@...omium.org>
Subject: Re: [PATCH 2/2] coresight: tmc: Add shutdown callback for TMC ETR/ETF

On 2020-06-03 13:26, Sai Prakash Ranjan wrote:
> Hi Robin,
> 
> On 2020-06-03 17:51, Robin Murphy wrote:
>> 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 ;)
>>
> 
> That is what happens in SMMU shutdown callback right? It is the 
> reboot/shutdown flow.

Yes, that's where it happens, but my point is *why* it happens at all.

hint: `git log --grep=shutdown drivers/iommu/`

If we could assume the system is always about to be powered off or 
reset, we wouldn't need to do anything to the SMMU either ;)

Robin.

> 
>     arm_smmu_device_shutdown()
>      platform_drv_shutdown()
>       device_shutdown()
>        kernel_restart_prepare()
>         kernel_restart()
> 
> Thanks,
> Sai
>