linux-kernel - Re: [PATCH v3 1/1] soc: fujitsu: Add A64FX diagnostic interrupt driver

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Date:   Tue, 19 Apr 2022 08:36:42 +0000
From:   "hasegawa-hitomi@...itsu.com" <hasegawa-hitomi@...itsu.com>
To:     Daniel Thompson <daniel.thompson@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
CC:     Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        SoC Team <soc@...nel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        Sumit Garg <sumit.garg@...aro.org>,
        Olof Johansson <olof@...om.net>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Jason Wessel <jason.wessel@...driver.com>,
        Doug Anderson <dianders@...omium.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "kgdb-bugreport@...ts.sourceforge.net" 
        <kgdb-bugreport@...ts.sourceforge.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Mike Travis <mike.travis@....com>
Subject: Re: [PATCH v3 1/1] soc: fujitsu: Add A64FX diagnostic interrupt
 driver

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