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

[Sascha Hauer]

On Mon, Oct 20, 2025 at 02:36:11PM +0200, Jonas Rebmann wrote:
> 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.

At least clearing all messages would be in line with dmesg -c in Linux.

> 
> 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.

I rather meant while using dmesg interactively. For the long-running
test I agree, we shouldn't lose log messages in this case, but Ahmad
provided a way to archieve this.

Sascha

-- 
Pengutronix e.K.                           |                             |
Steuerwalder Str. 21                       | http://www.pengutronix.de/  |
31137 Hildesheim, Germany                  | Phone: +49-5121-206917-0    |
Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |