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Subject: Re: [RFC 09/11] coresight: etm-perf: Disable the path before
 capturing the trace data
To:     Suzuki K Poulose <suzuki.poulose@arm.com>,
        linux-arm-kernel@lists.infradead.org, coresight@lists.linaro.org
Cc:     linux-kernel@vger.kernel.org, mathieu.poirier@linaro.org,
        mike.leach@linaro.org
References: <1605012309-24812-1-git-send-email-anshuman.khandual@arm.com>
 <1605012309-24812-10-git-send-email-anshuman.khandual@arm.com>
 <1ad992f4-5981-6ff1-470b-875e30d3c229@arm.com>
From:   Anshuman Khandual <anshuman.khandual@arm.com>
Message-ID: <b947b100-0393-36da-68e8-36254fe823f9@arm.com>
Date:   Mon, 23 Nov 2020 11:38:53 +0530
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On 11/12/20 2:57 PM, Suzuki K Poulose wrote:
> On 11/10/20 12:45 PM, Anshuman Khandual wrote:
>> perf handle structure needs to be shared with the TRBE IRQ handler for
>> capturing trace data and restarting the handle. There is a probability
>> of an undefined reference based crash when etm event is being stopped
>> while a TRBE IRQ also getting processed. This happens due the release
>> of perf handle via perf_aux_output_end(). This stops the sinks via the
>> link before releasing the handle, which will ensure that a simultaneous
>> TRBE IRQ could not happen.
>>
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>> ---
>> This might cause problem with traditional sink devices which can be
>> operated in both sysfs and perf mode. This needs to be addressed
>> correctly. One option would be to move the update_buffer callback
>> into the respective sink devices. e.g, disable().
>>
>>   drivers/hwtracing/coresight/coresight-etm-perf.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/drivers/hwtracing/coresight/coresight-etm-perf.c b/drivers/hwtracing/coresight/coresight-etm-perf.c
>> index 534e205..1a37991 100644
>> --- a/drivers/hwtracing/coresight/coresight-etm-perf.c
>> +++ b/drivers/hwtracing/coresight/coresight-etm-perf.c
>> @@ -429,7 +429,9 @@ static void etm_event_stop(struct perf_event *event, int mode)
>>             size = sink_ops(sink)->update_buffer(sink, handle,
>>                             event_data->snk_config);
>> +        coresight_disable_path(path);
>>           perf_aux_output_end(handle, size);
>> +        return;
>>       }
> 
> As you mentioned, this is not ideal where another session could be triggered on
> the sink from a different ETM (not for per-CPU sink) in a different mode before
> you collect the buffer. I believe the best option is to leave the
> update_buffer() to disable_hw. This would need to pass on the "handle" to the
> disable_path.

Passing 'handle' into coresight_ops_sink->disable() would enable pushing
updated trace data into perf aux buffer. But do you propose to drop the
update_buffer() call back completely or just move it into disable() call
back (along with PERF_EF_UPDATE mode check) for all individual sinks for
now. May be, later it can be dropped off completely.

> 
> That way the races can be handled inside the sinks. Also, this aligns the
> perf mode of the sinks with that of the sysfs mode.

Did not get that, could you please elaborate ?