[PATCH v3 1/1] soc: fujitsu: Add A64FX diagnostic interrupt driver

[hasegawa-hitomi at fujitsu.com]

Hi Greg, Arnd, and Daniel,

> > > > > There is some prior art for this sort of feature. AFAICT SGI UV has a
> > > > > similar mechanism that can send an NMI-with-no-side-channel to the
> > > > > kernel. The corresponding driver offers a range of actions using a
> > > > > module parameter:
> > > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/platform/uv/uv_nmi.c#n180
> > > > >
> > > > > I don't think a hardcoded 'c' makes any sense. With a hardcoded argument
> > > > > it is just obfuscation. However it is certainly seems attractive to be
> > > > > able to reuse handle_sysrq() to provide a more powerful set of actions.
> > > >
> > > > How about a module parameter that allows picking a sysrq character then?
> > >
> > > Module parameters are so 1990, as this is a platform device, why not get
> > > it from DT?
> >
> > This machine doesn't use DT. I suppose the same could be done with an EFI
> > variable, but with a module parameter you get the added benefit of having both
> > a boot time kernel command line argument, and the option to override it at
> > run time.
> 
> Pushing the decision on what action to take into firmware (whether that
> is DT or ACPI) implies that the firmware is well positioned to make a
> decision.  I don't think that is true here.
> 
> To me, it seems more like an admin choice... and admins are conditioned
> to use kernel arguments.
> 
> If these type of diagnostics request were more common then perhaps we'd
> be looking at a sysctl and call to handle_diagnostic_sysrq().

I understand that it is not appropriate to hardcode c.
How about using __setup() to add a new kernel parameter and allow the admin
to specify the sysrq command when booting?

Thank you
Hitomi Hasegawa