linux-kernel - Re: [PATCH v4 13/13] coresight: docs: Document etm4x timestamp interval option

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Message-ID: <67142b63-b413-4ed6-8526-b18b38667d42@linaro.org>
Date: Fri, 14 Nov 2025 09:59:46 +0000
From: James Clark <james.clark@...aro.org>
To: Randy Dunlap <rdunlap@...radead.org>
Cc: coresight@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
 Suzuki K Poulose <suzuki.poulose@....com>, Mike Leach
 <mike.leach@...aro.org>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jonathan Corbet <corbet@....net>, Leo Yan <leo.yan@....com>
Subject: Re: [PATCH v4 13/13] coresight: docs: Document etm4x timestamp
 interval option



On 14/11/2025 5:02 am, Randy Dunlap wrote:
> 
> 
> On 11/12/25 7:22 AM, James Clark wrote:
>> Document how the new field is used, maximum value and the interaction
>> with SYNC timestamps.
>>
>> Signed-off-by: James Clark <james.clark@...aro.org>
>> ---
>>   Documentation/trace/coresight/coresight.rst | 15 +++++++++++++--
>>   1 file changed, 13 insertions(+), 2 deletions(-)
>>
>> diff --git a/Documentation/trace/coresight/coresight.rst b/Documentation/trace/coresight/coresight.rst
>> index 806699871b80..80b5ed09d69b 100644
>> --- a/Documentation/trace/coresight/coresight.rst
>> +++ b/Documentation/trace/coresight/coresight.rst
>> @@ -613,8 +613,19 @@ They are also listed in the folder /sys/bus/event_source/devices/cs_etm/format/
>>        - Session local version of the system wide setting: :ref:`ETM_MODE_RETURNSTACK
>>          <coresight-return-stack>`
>>      * - timestamp
>> -     - Session local version of the system wide setting: :ref:`ETMv4_MODE_TIMESTAMP
>> -       <coresight-timestamp>`
>> +     - Controls generation and interval of timestamps.
>> +
>> +       0 = off, 1 = maximum interval .. 15 = minimum interval.
>> +
>> +       Values 1 - 14 use a counter that decrements every cycle to generate a
>> +       timestamp on underflow. The reload value for the counter is 2 raised to
>> +       the power of timestamp interval - 1. If the value is 1 then the reload
>> +       value is 1, if the value is 11 then the reload value is 1024 etc.
> 
> 	  value = 11: 2^11-1 = 2047
> 
> Maybe add some parens?
> 
> 

Will do. I had them but I removed them because it was in a sentence so 
it made it look like a side note. I think I'll change from words to symbols:

   The reload value for the counter is 2 ^ (interval - 1).

Thanks
James