linux-kernel - Re: [PATCH] serial: kgdb_nmi: Remove unused knock code

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Message-ID: <CAD=FV=VH5YWfTggGNt2KLoSPKqVTmWArEgccok3uFQ5BCA7cNw@mail.gmail.com>
Date: Mon, 13 Jan 2025 13:53:55 -0800
From: Doug Anderson <dianders@...omium.org>
To: linux@...blig.org
Cc: jason.wessel@...driver.com, danielt@...nel.org, gregkh@...uxfoundation.org, 
	jirislaby@...nel.org, linux-serial@...r.kernel.org, 
	kgdb-bugreport@...ts.sourceforge.net, linux-kernel@...r.kernel.org, 
	Sumit Garg <sumit.garg@...aro.org>
Subject: Re: [PATCH] serial: kgdb_nmi: Remove unused knock code

Hi,

On Sun, Jan 12, 2025 at 5:58 AM <linux@...blig.org> wrote:
>
> From: "Dr. David Alan Gilbert" <linux@...blig.org>
>
> kgdb_nmi_poll_knock() has been unused since it was added in 2013 in
> commit 0c57dfcc6c1d ("tty/serial: Add kgdb_nmi driver")
>
> Remove it, the static helpers, and module parameters it used.
>
> (The comment explaining why it might be used sounds sensible, but
> it's never been wired up, perhaps it's worth doing somewhere?)
>
> Signed-off-by: Dr. David Alan Gilbert <linux@...blig.org>
> ---
>  drivers/tty/serial/kgdb_nmi.c | 101 ----------------------------------
>  include/linux/kgdb.h          |   2 -
>  2 files changed, 103 deletions(-)

I guess this functionality was supposed to be added in a later patch
in the series? I see a use of the call here:

https://lore.kernel.org/lkml/1348522080-32629-9-git-send-email-anton.vorontsov@linaro.org/

...but that never landed...

Is the answer here that we should just fully revert commit
0c57dfcc6c1d ("tty/serial: Add kgdb_nmi driver")? From a quick glance
I also don't see any architectures that implement
`arch_kgdb_ops.enable_nmi`? I guess you'd also want to revert these
two maybe?

ad394f66fa57 kdb: Implement disable_nmi command
5a14fead07bc kernel/debug: Mask KGDB NMI upon entry

I don't know all of the history here. Maybe Daniel or Sumit do?

In general I've sorta given up on the idea of trying to run a serial
driver at NMI priority. I know Sumit had some patches for that where
he wanted to interpret sysrq-g at NMI time, but it always felt a
little fragile (and I guess never landed). IMO, having the serial port
be at normal priority level is nearly always enough when combined with
a watchdog that can use NMIs to break into a locked-up CPU.