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

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Date:   Tue, 9 Jun 2020 09:27:00 -0600
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.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

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.

>
> [1] https://lore.kernel.org/patchwork/patch/1253131/
>
> Thanks,
> Sai
>
> --
> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
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