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

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Date:   Mon, 08 Jun 2020 19:37:43 +0530
From:   Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
To:     Mathieu Poirier <mathieu.poirier@...aro.org>,
        Mike Leach <mike.leach@...aro.org>
Cc:     Suzuki K Poulose <suzuki.poulose@....com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-msm@...r.kernel.org,
        Coresight ML <coresight@...ts.linaro.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Robin Murphy <robin.murphy@....com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH 2/2] coresight: tmc: Add shutdown callback for TMC ETR/ETF

Hi Mathieu, Mike

On 2020-06-04 12:57, Sai Prakash Ranjan wrote:
> 

[...]

>> 
>> Robin has a point - user space is long gone at this time.  As such the 
>> first
>> question to ask is what kind of CS session was running at the time the 
>> system
>> was shutting down.  Was it a perf session of a sysfs session?
>> 
>> I'm guessing it was a sysfs session because user space has been blown 
>> away a
>> while back and part of that process should have killed all perf 
>> sessions.
> 
> I was enabling trace via sysfs.
> 
>> 
>> If I am correct then simply switching off the ETR HW in the shutdown() 
>> amba bus
>> callback should be fine - otherwise Mike's approach is mandatory.  
>> There is
>> also the exchange between Robin and Sai about removing the SMMU 
>> shutdown
>> callback, but that thread is still incomplete.
>> 
> 
> If Robin is hinting at removing SMMU shutdown callback, then I think 
> adding
> all these shutdown callbacks to all clients of SMMU can be avoided. Git 
> blaming
> the thing shows it was added to avoid some kexec memory corruption.
> 

I think I misread the cryptic hint from Robin and it is not right to 
remove
SMMU shutdown callback. For more details on why that was a bad idea and 
would
break kexec, please refer to [1].

As for the coresight, can I disable the ETR only in the tmc shutdown 
callback
or are we still concerned about the userspace coming into picture?

[1] https://lore.kernel.org/patchwork/patch/1253131/

Thanks,
Sai

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