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

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Message-ID: <4003a48f-73e0-4296-bcdc-6a975807c2d5@linaro.org>
Date: Tue, 18 Nov 2025 16:15:31 +0000
From: James Clark <james.clark@...aro.org>
To: Leo Yan <leo.yan@....com>
Cc: Suzuki K Poulose <suzuki.poulose@....com>,
 Mike Leach <mike.leach@...aro.org>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jonathan Corbet <corbet@....net>, coresight@...ts.linaro.org,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org
Subject: Re: [PATCH v4 13/13] coresight: docs: Document etm4x timestamp
 interval option



On 14/11/2025 6:05 pm, Leo Yan wrote:
> On Wed, Nov 12, 2025 at 03:22:19PM +0000, 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.
> 
> I am struggling to understand the "maximum" and "minimum" usage here.
> 
> Seems to me, you are saying:
> 
>      1 = maximum frequency .. 15 = minimum frequency
> 

Yes the documentation was the wrong way around. Will flip it.

>> +
>> +       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.
> 
> I saw your replying to Randy, yeah, the formula would be much clear.
> 
>> +
>> +       Setting the minimum interval (15) will disable the counter generated
>> +       timestamps, freeing the counter resource, leaving only ones emitted when
>> +       a SYNC packet is generated for every 4096 bytes of trace.
> 
> "every 4096 bytes" is not always true.
> 
> Isn't this defined in TRCSYNCPR.PERIOD?  Maybe just say "... for every
> period of trace defined in TRCSYNCPR.PERIOD", or even more general
> "... for every period of trace".
> 
> Thanks,
> Leo
> 

It would only not be true when using a custom config. I can say that 
it's TRCSYNCPR.PERIOD but that the default is 4096 bytes.

>> +
>>      * - cc_threshold
>>        - Cycle count threshold value. If nothing is provided here or the provided value is 0, then the
>>          default value i.e 0x100 will be used. If provided value is less than minimum cycles threshold
>>
>> -- 
>> 2.34.1
>>