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Date:   Mon, 17 Aug 2020 10:28:30 +0100
From:   Daniel Thompson <daniel.thompson@linaro.org>
To:     Sumit Garg <sumit.garg@linaro.org>
Cc:     Doug Anderson <dianders@chromium.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-serial@vger.kernel.org, kgdb-bugreport@lists.sourceforge.net,
        Jiri Slaby <jslaby@suse.com>,
        Russell King - ARM Linux admin <linux@armlinux.org.uk>,
        Jason Wessel <jason.wessel@windriver.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>
Subject: Re: [RFC 0/5] Introduce NMI aware serial drivers
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On Mon, Aug 17, 2020 at 10:42:43AM +0530, Sumit Garg wrote:
> On Fri, 14 Aug 2020 at 19:48, Daniel Thompson
> <daniel.thompson@linaro.org> wrote:
> >
> > On Fri, Aug 14, 2020 at 05:36:36PM +0530, Sumit Garg wrote:
> > > On Thu, 13 Aug 2020 at 15:47, Daniel Thompson
> > > <daniel.thompson@linaro.org> wrote:
> > > >
> > > > On Thu, Aug 13, 2020 at 02:55:12PM +0530, Sumit Garg wrote:
> > > > > On Thu, 13 Aug 2020 at 05:38, Doug Anderson <dianders@chromium.org> wrote:
> > > > > > On Wed, Aug 12, 2020 at 8:27 AM Doug Anderson <dianders@chromium.org> wrote:
> > > > > > > One
> > > > > > > last worry is that I assume that most people testing (and even
> > > > > > > automated testing labs) will either always enable NMI or won't enable
> > > > > > > NMI.  That means that everyone will be only testing one codepath or
> > > > > > > the other and (given the complexity) the non-tested codepath will
> > > > > > > break.
> > > > > > >
> > > > >
> > > > > The current patch-set only makes this NMI to work when debugger (kgdb)
> > > > > is enabled which I think is mostly suitable for development
> > > > > environments. So most people testing will involve existing IRQ mode
> > > > > only.
> > > > >
> > > > > However, it's very much possible to make NMI mode as default for a
> > > > > particular serial driver if the underlying irqchip supports it but it
> > > > > depends if we really see any production level usage of NMI debug
> > > > > feature.
> > > >
> > > > The effect of this patch is not to make kgdb work from NMI it is to make
> > > > (some) SysRqs work from NMI. I think that only allowing it to deploy for
> > > > kgdb users is a mistake.
> > > >
> > > > Having it deploy automatically for kgdb users might be OK but it seems
> > > > sensible to make this feature available for other users too.
> > >
> > > I think I wasn't clear enough in my prior reply. Actually I meant to
> > > say that this patch-set enables NMI support for a particular serial
> > > driver via ".poll_init()" interface and the only current user of that
> > > interface is kgdb.
> > >
> > > So if there are other users interested in this feature, they can use
> > > ".poll_init()" interface as well to enable it.
> >
> > Huh?
> >
> > We appear to speaking interchangably about users (people who sit in
> > front of the machine and want a stack trace) and sub-systems ;-).
> >
> > I don't think other SysRq commands have quite such a direct relationship
> > between the sub-system and the sysrq command. For example who are you
> > expecting to call .poll_init() if a user wants to use the SysRq to
> > provoke a stack trace?
> >
> 
> Ah, I see. So you meant to provide a user-space interface to
> dynamically enable/disable NMI debug, correct? It will require IRQ <->
> NMI switching at runtime which should be doable safely.

I haven't given much thought to the exact mechanism, though I would
perhaps have started by thinking about a module parameter).

From an RFC point of view, I simple think this feature is potentially
useful on systems without kgdb (which, let's be honest, are firmly in
the majority) so making .poll_init() the only way to activate it is a
mistake.


Daniel.