[PATCH] commands: dmesg: clear logbuffer fully if not requested otherwise

[Jonas Rebmann]

Hi Sascha,

On 2025-10-20 11:36, Sascha Hauer wrote:
>> If loglevels are selected using -l or -p in addition to cleaning being
>> selected using -c, only clean those messages shown by the loglevel
>> selection.
> 
> I find this behaviour rather surprising. The only useful thing I could
> think of we can do with this is "Discard unimportant messages", but only
> by printing them. It would be more useful if we had a -C (uppercase)
> option to discard messages without printing them. But even with that,
> there are many things we can do with this behaviour that are not useful
> and only a few that are actually useful. We could delete for example all
> "info" messages and keep the more and less important messages, but why
> would we want to do this?
> > I think you should rather use the filter options -l and -p to limit the
> output to the messages you are interested in and finally use -c to clear
> the buffer.

I think both behaviors could come as a surprise depending on how you
look at it but if no one raises another position, I can make sure that
in v2, -c will always clear all messages of all loglevels which is also
much closer to the current behavior. I'd then add a warning that in
these cases, filtered-out messages are cleared without being shown.

However I disagree about the -C option and the idea of clearing in a
separate step.

My concept of -c is that it 'consumes' the ringbuffer, a 'dequeue'
operation, if you will. This needs to be a single step to ensure that no
messages get lost or duplicated.

For example in some long-running test, you might run `dmesg -c`
periodically and store the output. It should be guaranteed that when
doing this, you never loose a single log message (if you clear often
enough to never fill/overflow the buffer). I think the prior
implementation couldn't guarantee that, but my patch changes that. From
that perspective, the option to clear the buffer in a separate step
would not help.

In a 'consume and filter' scenario, one should just use dmesg -c -r and
perform the loglevel filtering elsewhere instead of 'consuming' per
loglevel by mixing -c with -l/-p which is why I'm ok with the change.

Regards,
Jonas

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