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

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Date:   Tue, 09 Jun 2020 21:07:16 +0530
From:   Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
To:     Mathieu Poirier <mathieu.poirier@...aro.org>
Cc:     Mike Leach <mike.leach@...aro.org>,
        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 <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,

On 2020-06-09 20:57, Mathieu Poirier wrote:
> On Mon, 8 Jun 2020 at 08:07, Sai Prakash Ranjan
> <saiprakash.ranjan@...eaurora.org> wrote:
>> 
>> 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?
> 
> User space isn't a concern, especially after you've confirmed the
> problem occured during an ongoing sysfs session.
> 

Will post v3 with comments addressed after 5.8-rc1 is out.

Thanks,
Sai

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