[PATCH] powerpc/xmon: add debugfs entry for xmon

[Nicholas Piggin]

On Mon, 13 Feb 2017 19:00:42 -0200
"Guilherme G. Piccoli" <gpiccoli at linux.vnet.ibm.com> wrote:

> Currently the xmon debugger is set only via kernel boot command-line.
> It's disabled by default, and can be enabled with "xmon=on" on the
> command-line. Also, xmon may be accessed via sysrq mechanism, but once
> we enter xmon via sysrq,  it's  kept enabled until system is rebooted,
> even if we exit the debugger. A kernel crash will then lead to xmon
> instance, instead of triggering a kdump procedure (if configured), for
> example.
> 
> This patch introduces a debugfs entry for xmon, allowing user to query
> its current state and change it if desired. Basically, the "xmon" file
> to read from/write to is under the debugfs mount point, on powerpc
> directory. Reading this file will provide the current state of the
> debugger, one of the following: "on", "off", "early" or "nobt". Writing
> one of these states to the file will take immediate effect on the debugger.
> 
> Signed-off-by: Guilherme G. Piccoli <gpiccoli at linux.vnet.ibm.com>
> ---
> * I had this patch partially done for some time, and after a discussion
> at the kernel slack channel latest week, I decided to rebase and fix
> some remaining bugs. I'd change 'x' option to always disable the debugger,
> since with this patch we can always re-enable xmon, but today I noticed
> Pan's patch on the mailing list, so perhaps his approach of adding a flag
> to 'x' option is preferable. I can change this in a V2, if requested.
> Thanks in advance!

xmon state changing after the first sysrq+x violates principle of least
astonishment, so I think that should be fixed.

Then the question is, is it worth making it runtime configurable with xmon
command or debugfs tunables?

Thanks,
Nick