linux-kernel - Re: [PATCH v3 1/7] Documentation: dynamic-debug: Add description of level bitmask

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Date:   Wed, 10 Jun 2020 13:29:20 +0300
From:   Stanimir Varbanov <stanimir.varbanov@...aro.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-media@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-btrfs@...r.kernel.org, linux-acpi@...r.kernel.org,
        netdev@...r.kernel.org, Joe Perches <joe@...ches.com>,
        Jason Baron <jbaron@...mai.com>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH v3 1/7] Documentation: dynamic-debug: Add description of
 level bitmask

Hi Greg,

On 6/9/20 2:16 PM, Greg Kroah-Hartman wrote:
> On Tue, Jun 09, 2020 at 01:45:58PM +0300, Stanimir Varbanov wrote:
>> This adds description of the level bitmask feature.
>>
>> Cc: Jonathan Corbet <corbet@....net> (maintainer:DOCUMENTATION)
>>
>> Signed-off-by: Stanimir Varbanov <stanimir.varbanov@...aro.org>
>> ---
>>  Documentation/admin-guide/dynamic-debug-howto.rst | 10 ++++++++++
>>  1 file changed, 10 insertions(+)
>>
>> diff --git a/Documentation/admin-guide/dynamic-debug-howto.rst b/Documentation/admin-guide/dynamic-debug-howto.rst
>> index 0dc2eb8e44e5..c2b751fc8a17 100644
>> --- a/Documentation/admin-guide/dynamic-debug-howto.rst
>> +++ b/Documentation/admin-guide/dynamic-debug-howto.rst
>> @@ -208,6 +208,12 @@ line
>>  	line -1605          // the 1605 lines from line 1 to line 1605
>>  	line 1600-          // all lines from line 1600 to the end of the file
>>  
>> +level
>> +    The given level will be a bitmask ANDed with the level of the each ``pr_debug()``
>> +    callsite. This will allow to group debug messages and show only those of the
>> +    same level.  The -p flag takes precedence over the given level. Note that we can
>> +    have up to five groups of debug messages.
> 
> As was pointed out, this isn't a "level", it's some arbitrary type of
> "grouping".

Yes, it is grouping of KERN_DEBUG level messages by importance (my
fault, I put incorrect name).  What is important is driver author
decision.  Usually when the driver is huge and has a lot of debug
messages it is not practical to enable all of them to chasing a
particular bug or issue.  You know that debugging (printk) add delays
which could hide or rise additional issue(s) which would complicate
debug and waste time.

For the Venus driver I have defined three groups of KERN_DEBUG - low,
medium and high (again the driver author(s) will decide what the
importance is depending on his past experience).

There is another point where the debugging is made by person who is not
familiar with the driver code. In that case he/she cannot enable lines
or range of lines because he don't know the details. Here the grouping
by importance could help.

> 
> But step back, why?  What is wrong with the existing control of dynamic
> debug messages that you want to add another type of arbitrary grouping
> to it?  And who defines that grouping?  Will it be
> driver/subsystem/arch/author specific?  Or kernel-wide?
> 
> This feels like it could easily get out of hand really quickly.
> 
> Why not just use tracepoints if you really want to be fine-grained?
> 
> thanks,
> 
> greg k-h
> 

-- 
regards,
Stan