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

[Michael Ellerman]

"Guilherme G. Piccoli" <gpiccoli at linux.vnet.ibm.com> writes:
> On 14/02/2017 01:58, Pan Xinhui wrote:
>> 在 2017/2/14 10:35, Nicholas Piggin 写道:
>>> On Mon, 13 Feb 2017 19:00:42 -0200
>>>
>>> xmon state changing after the first sysrq+x violates principle of least
>>> astonishment, so I think that should be fixed.
>>>
>> hi, Nick
>> yes, as long as xmon is disabled during boot, it should still be disabled after existing xmon.
>> My patch does not fix that as it need people add one more char 'z' following 'x'.
>> I will provide a new patch to fix that.
>> 
>>> Then the question is, is it worth making it runtime configurable with xmon
>>> command or debugfs tunables?
>>>
>> They are options for people to turn xmon features on or off. Maybe people needn't this.
>> However I am not a fan  of debugfs this time as I am used to using xmon cmds. :)
>> 
>> Hi, Guilherme
>> So in the end, my thought is that: 1) cmd x|X will exit xmon and keep xmon in the original state(indicated by var xmon_off).
>> 2) Then add options to turn some features on/off. And debugfs maybe not fit for this. But I am also wondering at same time, are people needing this?
>
> Hi Nick and Xinhui, thanks very much for the feedback.
> I agree, we should keep xmon in the state it was firstly set, on boot
> time - dropping to the debugger using sysrq shouldn't change it.
>
> Now, the use case of the debugfs approach is to allow user to
> enable/disable xmon without need to drop into the debugger itself, or
> reboot the machine.
> Imagine a scenario in which we have a production machine, and:
>
>
> i) For some reason, somebody kept xmon enabled on grub.cfg and now, we
> want to let kdump work in case of crash - how to disable xmon in runtime?
>
> ii) The opposite: xmon wasn't enable on boot time in production machine,
> but we have a super-rare issue and want to drop to xmon next time it
> happens, so we need to enable it. But we don't want to drop into the
> debugger to force it gets enabled, so how do we enable it?

Yep. I think both of those are good reasons to add a debugfs file.

> Regarding the place of the xmon state file, I believe debugfs is the
> right place - where else could we add it? procfs? configfs?

debugfs, under powerpc_debugfs_root.

cheers