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Date:   Fri, 8 Apr 2022 14:32:46 +0100
From:   Daniel Thompson <daniel.thompson@linaro.org>
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Greg KH <gregkh@linuxfoundation.org>,
        Hitomi Hasegawa <hasegawa-hitomi@fujitsu.com>,
        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,
        Peter Zijlstra <peterz@infradead.org>,
        Mike Travis <mike.travis@hpe.com>
Subject: Re: [PATCH v3 1/1] soc: fujitsu: Add A64FX diagnostic interrupt
 driver
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On Thu, Mar 31, 2022 at 05:44:55PM +0200, Arnd Bergmann wrote:
> On Thu, Mar 31, 2022 at 1:49 PM Greg KH <gregkh@linuxfoundation.org> wrote:
> > > +
> > > +static irqreturn_t a64fx_diag_handler(int irq, void *dev_id)
> > > +{
> > > +     handle_sysrq('c');
> >
> >
> > Why is this calling this sysrq call?  From an interrupt?  Why?
> >
> > And you are hard-coding "c", are you sure?
> 
> This is an actual sysrq driver in the traditional sense, where you can send
> a single interrupt to the machine from the outside over a side channel.
> 
> I suggested sysrq instead of just panic() to make it a bit more flexible.
> Unfortunately there is no additional data, so it comes down to always
> sending the same character.
> 
> It would be possible to make that character configurable with a module
> parameter or something like that, but I'm not sure that is an improvement.
> Maybe you have another idea for this.

Given the interrupt can be dismissed then offering non-fatal actions in
response the chassis command seems reasonable.

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.


Daniel.