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

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Message-ID: <20200811135801.GA416071@kroah.com>
Date:   Tue, 11 Aug 2020 15:58:01 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Sumit Garg <sumit.garg@...aro.org>
Cc:     Daniel Thompson <daniel.thompson@...aro.org>,
        Douglas Anderson <dianders@...omium.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 Tue, Aug 11, 2020 at 07:20:26PM +0530, Sumit Garg wrote:
> On Tue, 21 Jul 2020 at 17:40, Sumit Garg <sumit.garg@...aro.org> wrote:
> >
> > Make it possible for UARTs to trigger magic sysrq from an NMI. With the
> > advent of pseudo NMIs on arm64 it became quite generic to request serial
> > device interrupt as an NMI rather than IRQ. And having NMI driven serial
> > RX will allow us to trigger magic sysrq as an NMI and hence drop into
> > kernel debugger in NMI context.
> >
> > The major use-case is to add NMI debugging capabilities to the kernel
> > in order to debug scenarios such as:
> > - Primary CPU is stuck in deadlock with interrupts disabled and hence
> >   doesn't honor serial device interrupt. So having magic sysrq triggered
> >   as an NMI is helpful for debugging.
> > - Always enabled NMI based magic sysrq irrespective of whether the serial
> >   TTY port is active or not.
> >
> > Currently there is an existing kgdb NMI serial driver which provides
> > partial implementation in upstream to have a separate ttyNMI0 port but
> > that remained in silos with the serial core/drivers which made it a bit
> > odd to enable using serial device interrupt and hence remained unused. It
> > seems to be clearly intended to avoid almost all custom NMI changes to
> > the UART driver.
> >
> > But this patch-set allows the serial core/drivers to be NMI aware which
> > in turn provides NMI debugging capabilities via magic sysrq and hence
> > there is no specific reason to keep this special driver. So remove it
> > instead.
> >
> > Approach:
> > ---------
> >
> > The overall idea is to intercept serial RX characters in NMI context, if
> > those are specific to magic sysrq then allow corresponding handler to run
> > in NMI context. Otherwise, defer all other RX and TX operations onto IRQ
> > work queue in order to run those in normal interrupt context.
> >
> > This approach is demonstrated using amba-pl011 driver.
> >
> > Patch-wise description:
> > -----------------------
> >
> > Patch #1 prepares magic sysrq handler to be NMI aware.
> > Patch #2 adds NMI framework to serial core.
> > Patch #3 and #4 demonstrates NMI aware uart port using amba-pl011 driver.
> > Patch #5 removes kgdb NMI serial driver.
> >
> > Goal of this RFC:
> > -----------------
> >
> > My main reason for sharing this as an RFC is to help decide whether or
> > not to continue with this approach. The next step for me would to port
> > the work to a system with an 8250 UART.
> >
> 
> A gentle reminder to seek feedback on this series.

It's the middle of the merge window, and I can't do anything.

Also, I almost never review RFC patches as I have have way too many
patches that people think are "right" to review first...

I suggest you work to flesh this out first and submit something that you
feels works properly.

good luck!

greg k-h