linux-kernel - Re: [PATCH 4/5] coresight: trbe: Move irq

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Date:   Sun, 18 Jul 2021 11:43:49 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Suzuki K Poulose <suzuki.poulose@....com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     coresight@...ts.linaro.org, linux-kernel@...r.kernel.org,
        al.grant@....com, leo.yan@...aro.org, mathieu.poirier@...aro.org,
        mike.leach@...aro.org, peterz@...radead.org, Tamas.Zsoldos@....com,
        will@...nel.org
Subject: Re: [PATCH 4/5] coresight: trbe: Move irq_work queue processing



On 7/15/21 2:03 PM, Suzuki K Poulose wrote:
> On 15/07/2021 04:23, Anshuman Khandual wrote:
>>
>>
>> On 7/12/21 5:08 PM, Suzuki K Poulose wrote:
>>> The TRBE driver issues the irq_work_run() to ensure that
>>> any pending disable event callback has been processed. But this
>>> is done before we mark the AUX buffer as TRUNCATED, making
>>> the call pointless. Move the call after the current handle
>>> has been closed.
>>
>> So there is a possibility that a disable event gets missed before
>> the buffer is marked TRUNCATED ? But even then would not another
> 
> No, it is the other way around. A disable event is triggered
> in response to the TRUNCATED flag.
> 
>> disable event be triggered again subsequently ? Just trying to
>> understand what is the actual problem because of the current
>> placement of irq_work_run() ?
> 
> That begs the question, What is the purpose of irq_work_run() ?
> See below.
> 
>>
>>>
>>> Fixes: 3fbf7f011f24 ("coresight: sink: Add TRBE driver")
>>> Reported-by: Tamas Zsoldos <Tamas.Zsoldos@....com>
>>> Cc: Anshuman Khandual <anshuman.khandual@....com>
>>> Cc: Mathieu Poirier <mathieu.poirier@...aro.org>
>>> Cc: Mike Leach <mike.leach@...aro.org>
>>> Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
>>> Cc: Leo Yan <leo.yan@...aro.org>
>>> Cc: Will Deacon <will@...nel.org>
>>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
>>> ---
>>>   drivers/hwtracing/coresight/coresight-trbe.c | 14 ++++++++------
>>>   1 file changed, 8 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
>>> index ec38cf17b693..c0c264264427 100644
>>> --- a/drivers/hwtracing/coresight/coresight-trbe.c
>>> +++ b/drivers/hwtracing/coresight/coresight-trbe.c
>>> @@ -723,6 +723,14 @@ static void trbe_handle_overflow(struct perf_output_handle *handle)
>>>       perf_aux_output_flag(handle, PERF_AUX_FLAG_CORESIGHT_FORMAT_RAW |
>>>                        PERF_AUX_FLAG_TRUNCATED);
>>>       perf_aux_output_end(handle, size);
>>> +
>>> +    /*
>>> +     * Ensure perf callbacks have completed. Since we
>>> +     * always TRUNCATE the buffer on IRQ, the event
>>> +     * is scheduled to be disabled. Make sure that happens
>>> +     * as soon as possible.
>>> +     */
>>> +    irq_work_run();
>>>   }
>>>     static bool is_perf_trbe(struct perf_output_handle *handle)
>>> @@ -777,12 +785,6 @@ static irqreturn_t arm_trbe_irq_handler(int irq, void *dev)
>>>       if (!is_perf_trbe(handle))
>>>           return IRQ_NONE;
>>>   -    /*
>>> -     * Ensure perf callbacks have completed, which may disable
>>> -     * the trace buffer in response to a TRUNCATION flag.
>>> -     */
> 
> This comment here explains what the irq_work_run() is supposed to do.
> This clearly indicates that the event will be disabled in response
> to TRUNCATION. Now, we haven't updated the buffer flags yet here.
> It happens *after* this call, in handle_overflow(). So, this is placed
> incorrectly. The purpose of this patch is to make sure that we actually
> complete the callbacks in response to the TRUNCATED flag we set with
> this IRQ.

Got it.

> 
>>> -    irq_work_run();
>>> -
>>>       act = trbe_get_fault_act(status);
>>>       switch (act) {
>>>       case TRBE_FAULT_ACT_WRAP:
>>>
> 
> Suzuki
>