linux-kernel - Re: [RFC 0/5] Introduce NMI aware serial drivers

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Message-ID: <20200814141822.opahh33nfc5yodkb@holly.lan>
Date:   Fri, 14 Aug 2020 15:18:22 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Sumit Garg <sumit.garg@...aro.org>
Cc:     Doug Anderson <dianders@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-serial@...r.kernel.org, kgdb-bugreport@...ts.sourceforge.net,
        Jiri Slaby <jslaby@...e.com>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Jason Wessel <jason.wessel@...driver.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC 0/5] Introduce NMI aware serial drivers

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@...aro.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@...omium.org> wrote:
> > > > On Wed, Aug 12, 2020 at 8:27 AM Doug Anderson <dianders@...omium.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?


Daniel.