linux-kernel - Re: [PATCH v2 14/17] coresight: trbe: Make sure we have enough space

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Date:   Wed, 22 Sep 2021 11:16:48 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     linux-kernel@...r.kernel.org, maz@...nel.org,
        catalin.marinas@....com, mark.rutland@....com, james.morse@....com,
        leo.yan@...aro.org, mike.leach@...aro.org,
        mathieu.poirier@...aro.org, will@...nel.org, lcherian@...vell.com,
        coresight@...ts.linaro.org
Subject: Re: [PATCH v2 14/17] coresight: trbe: Make sure we have enough space

On 22/09/2021 10:58, Anshuman Khandual wrote:
> 
> 
> On 9/21/21 7:11 PM, Suzuki K Poulose wrote:
>> The TRBE driver makes sure that there is enough space for a meaningful
>> run, otherwise pads the given space and restarts the offset calculation
>> once. But there is no guarantee that we may find space or hit "no space".
> 
> So what happens currently when it neither finds the required minimum buffer
> space for a meaningful run nor does it hit the "no space" scenario ?

It tries once today and assumes that it will either hit :

  - No space
    OR
  - Enough space

which is reasonable, given the minimum space needed is a few bytes.
But this may no longer be true with other erratum workaround.

> 
>> Make sure that we repeat the step until, either :
>>    - We have the minimum space
>>     OR
>>    - There is NO space at all.
>>
>> Cc: Anshuman Khandual <anshuman.khandual@....com>
>> Cc: Mathieu Poirier <mathieu.poirier@...aro.org>
>> Cc: Mike Leach <mike.leach@...aro.org>
>> Cc: Leo Yan <leo.yan@...aro.org>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
>> ---
>>   drivers/hwtracing/coresight/coresight-trbe.c | 6 +++++-
>>   1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
>> index 3373f4e2183b..02f9e00e2091 100644
>> --- a/drivers/hwtracing/coresight/coresight-trbe.c
>> +++ b/drivers/hwtracing/coresight/coresight-trbe.c
>> @@ -451,10 +451,14 @@ static unsigned long trbe_normal_offset(struct perf_output_handle *handle)
>>   	 * If the head is too close to the limit and we don't
>>   	 * have space for a meaningful run, we rather pad it
>>   	 * and start fresh.
>> +	 *
>> +	 * We might have to do this more than once to make sure
>> +	 * we have enough required space.
> 
> OR no space at all, as explained in the commit message.
> Hence this comment needs an update.
> 
>>   	 */
>> -	if (limit && ((limit - head) < trbe_min_trace_buf_size(handle))) {
>> +	while (limit && ((limit - head) < trbe_min_trace_buf_size(handle))) {
>>   		trbe_pad_buf(handle, limit - head);
>>   		limit = __trbe_normal_offset(handle);
>> +		head = PERF_IDX2OFF(handle->head, buf);
> 
> Should the loop be bound with a retry limit as well ?

No. We will eventually hit No-space as we keep on padding
the buffer.

Suzuki