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From:   "hasegawa-hitomi@fujitsu.com" <hasegawa-hitomi@fujitsu.com>
To:     Daniel Thompson <daniel.thompson@linaro.org>,
        Arnd Bergmann <arnd@arndb.de>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>
CC:     Linux ARM <linux-arm-kernel@lists.infradead.org>,
        SoC Team <soc@kernel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@vger.kernel.org>,
        Sumit Garg <sumit.garg@linaro.org>,
        Olof Johansson <olof@lixom.net>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Jiri Slaby <jirislaby@kernel.org>,
        Jason Wessel <jason.wessel@windriver.com>,
        Doug Anderson <dianders@chromium.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "kgdb-bugreport@lists.sourceforge.net" 
        <kgdb-bugreport@lists.sourceforge.net>,
        Peter Zijlstra <peterz@infradead.org>,
        Mike Travis <mike.travis@hpe.com>
Subject: Re: [PATCH v3 1/1] soc: fujitsu: Add A64FX diagnostic interrupt
 driver
Thread-Topic: [PATCH v3 1/1] soc: fujitsu: Add A64FX diagnostic interrupt
 driver
Thread-Index: AQHYROFUWg6uBlnqVU2NIy0zBQeT5KzZYQIAgABBxICADG27AIAADG4AgAABLwCAAAfUgIAAA38AgBDcmG4=
Date:   Tue, 19 Apr 2022 08:36:42 +0000
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Hi Greg, Arnd, and Daniel,=0A=
=0A=
> > > > > There is some prior art for this sort of feature. AFAICT SGI UV h=
as a=0A=
> > > > > similar mechanism that can send an NMI-with-no-side-channel to th=
e=0A=
> > > > > kernel. The corresponding driver offers a range of actions using =
a=0A=
> > > > > module parameter:=0A=
> > > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.gi=
t/tree/arch/x86/platform/uv/uv_nmi.c#n180=0A=
> > > > >=0A=
> > > > > I don't think a hardcoded 'c' makes any sense. With a hardcoded a=
rgument=0A=
> > > > > it is just obfuscation. However it is certainly seems attractive =
to be=0A=
> > > > > able to reuse handle_sysrq() to provide a more powerful set of ac=
tions.=0A=
> > > >=0A=
> > > > How about a module parameter that allows picking a sysrq character =
then?=0A=
> > >=0A=
> > > Module parameters are so 1990, as this is a platform device, why not =
get=0A=
> > > it from DT?=0A=
> >=0A=
> > This machine doesn't use DT. I suppose the same could be done with an E=
FI=0A=
> > variable, but with a module parameter you get the added benefit of havi=
ng both=0A=
> > a boot time kernel command line argument, and the option to override it=
 at=0A=
> > run time.=0A=
> =0A=
> Pushing the decision on what action to take into firmware (whether that=
=0A=
> is DT or ACPI) implies that the firmware is well positioned to make a=0A=
> decision.  I don't think that is true here.=0A=
> =0A=
> To me, it seems more like an admin choice... and admins are conditioned=
=0A=
> to use kernel arguments.=0A=
> =0A=
> If these type of diagnostics request were more common then perhaps we'd=
=0A=
> be looking at a sysctl and call to handle_diagnostic_sysrq().=0A=
=0A=
I understand that it is not appropriate to hardcode c.=0A=
How about using __setup() to add a new kernel parameter and allow the admin=
=0A=
to specify the sysrq command when booting?=0A=
=0A=
Thank you=0A=
Hitomi Hasegawa=