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

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Date:   Fri, 8 Apr 2022 16:02:02 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Hitomi Hasegawa <hasegawa-hitomi@...itsu.com>,
        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,
        Peter Zijlstra <peterz@...radead.org>,
        Mike Travis <mike.travis@....com>
Subject: Re: [PATCH v3 1/1] soc: fujitsu: Add A64FX diagnostic interrupt
 driver

On Fri, Apr 08, 2022 at 04:49:31PM +0200, Arnd Bergmann wrote:
> On Fri, Apr 8, 2022 at 4:21 PM Greg KH <gregkh@...uxfoundation.org> wrote:
> >
> > On Fri, Apr 08, 2022 at 04:17:16PM +0200, Arnd Bergmann wrote:
> > > On Fri, Apr 8, 2022 at 3:32 PM Daniel Thompson
> > > <daniel.thompson@...aro.org> wrote:
> > > > On Thu, Mar 31, 2022 at 05:44:55PM +0200, Arnd Bergmann wrote:
> > > >
> > > > 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().


Daniel.