On Tue, 11 Aug 2020 at 20:28, Greg Kroah-Hartman
[off-list ref] wrote:

On Tue, Aug 11, 2020 at 07:59:24PM +0530, Sumit Garg wrote:

quoted

Hi Greg,

Thanks for your comments.

On Tue, 11 Aug 2020 at 19:27, Greg Kroah-Hartman
[off-list ref] wrote:

quoted

On Tue, Aug 11, 2020 at 07:20:26PM +0530, Sumit Garg wrote:

quoted

On Tue, 21 Jul 2020 at 17:40, Sumit Garg [off-list ref] wrote:

quoted

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:

quoted

quoted

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

:)

quoted

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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