[PATCH v2] commands: dmesg: clear logbuffer fully if not requested otherwise

[Sascha Hauer]

On Wed, Oct 29, 2025 at 02:18:27PM +0100, Jonas Rebmann wrote:
> Previously, `dmesg -c` would clear the logbuffer except for the last 10
> lines. This comes with a fair amount of surprise given linux `dmesg -c`
> clears the whole logbuffer.
> 
> Clear the complete logbuffer given `dmesg -c` but with an optional
> argument `dmesg -c [<num>]` to keep <num> lines.
> 
> While at it, add documentation for log_print(), and switch it from
> ctrlc() to ctrlc_non_interruptible(). This ensures that the complete
> execution of the dmesg command has no resched() point so that no other
> thread can intermittently modify the logbuffer, and that no message
> could arrive from another thread during log_print() and be interleaved
> with the dmesg output.
> 
> If loglevels are selected using -l or -p in addition to clearing being
> selected using -c, then messages of any loglevel are cleared.
> 
> Signed-off-by: Jonas Rebmann <jre at pengutronix.de>
> ---
> Changes in v2:
> - Switch from ctrlc() to ctrlc_non_interruptible() instead of clearing
>   in log_print() (Thanks, Ahmad)
> - Improve documentation
> - Always clear all loglevels regardless of -l/-p (Thanks, Sascha)
> - Style improvements (Thanks, Ahmad)
> - Link to v1: https://lore.barebox.org/barebox/20251017-dmesg-v1-1-ac9c2fb021f6@pengutronix.de
> ---
>  .../migration-guides/migration-2025.11.0.rst       |  9 ++++++++
>  commands/dmesg.c                                   | 25 ++++++++++++----------
>  common/console_common.c                            | 12 ++++++++++-
>  3 files changed, 34 insertions(+), 12 deletions(-)
> 
> diff --git a/Documentation/migration-guides/migration-2025.11.0.rst b/Documentation/migration-guides/migration-2025.11.0.rst
> index a9c7abb66e..2649923bbc 100644
> --- a/Documentation/migration-guides/migration-2025.11.0.rst
> +++ b/Documentation/migration-guides/migration-2025.11.0.rst
> @@ -18,3 +18,12 @@ Karo TX6X
>  The barebox update handler for this SoM no longer unconditionally updates
>  /dev/mmc3.boot0, but instead it now updates the inactive boot partition
>  on /dev/mmc3 and then sets it as active allowing for power-fail safety.
> +
> +Shell
> +-----
> +
> +* An optional parameter was added to the `-c` option of `dmesg` allowing
> +  configuration of the number of lines to remain in the log buffer after
> +  clearing. When no parameter is provided to `-c`, zero is assumed, and no
> +  lines are retained. Earlier versions always left 10 lines of logs remain in
> +  the log buffer.

Moved this to the migration guide for v2025.12.0. while applying.

Sascha

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