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Date:   Wed, 12 Aug 2020 11:18:23 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.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, 11 Aug 2020 at 20:28, Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> On Tue, Aug 11, 2020 at 07:59:24PM +0530, Sumit Garg wrote:
> > Hi Greg,
> >
> > Thanks for your comments.
> >
> > On Tue, 11 Aug 2020 at 19:27, Greg Kroah-Hartman
> > <gregkh@...uxfoundation.org> wrote:
> > >
> > > 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...
> > >
> >
> > Okay, I understand and I can definitely wait for your feedback.
>
> My feedback here is this:
>
> > > I suggest you work to flesh this out first and submit something that you
> > > feels works properly.
>
> :)
>
> > IIUC, in order to make this approach substantial I need to make it
> > work with 8250 UART (major serial driver), correct? As currently it
> > works properly for amba-pl011 driver.
>
> Yes, try to do that, or better yet, make it work with all serial drivers
> automatically.

I would like to make serial drivers work automatically but
unfortunately the interrupt request/ handling code is pretty specific
to the corresponding serial driver.

BTW, I will look for ways how we can make it much easier for serial
drivers to adapt.

-Sumit

>
> thanks,
>
> greg k-h

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