linux-kernel - Re: [PATCH 4/7] drivers: serial: kgdboc: Check CON_SUSPENDED instead of CON

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Message-ID: <CAD=FV=XFeokpbMUFjAc0OkwJ97vR8aB+4GbnFxRKymvpEY3gnA@mail.gmail.com>
Date: Tue, 10 Jun 2025 16:18:22 -0700
From: Doug Anderson <dianders@...omium.org>
To: Marcos Paulo de Souza <mpdesouza@...e.com>
Cc: Petr Mladek <pmladek@...e.com>, Steven Rostedt <rostedt@...dmis.org>, 
	John Ogness <john.ogness@...utronix.de>, Sergey Senozhatsky <senozhatsky@...omium.org>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jiri Slaby <jirislaby@...nel.org>, 
	Jason Wessel <jason.wessel@...driver.com>, Daniel Thompson <danielt@...nel.org>, 
	Richard Weinberger <richard@....at>, Anton Ivanov <anton.ivanov@...bridgegreys.com>, 
	Johannes Berg <johannes@...solutions.net>, linux-kernel@...r.kernel.org, 
	linux-serial@...r.kernel.org, kgdb-bugreport@...ts.sourceforge.net, 
	linux-um@...ts.infradead.org
Subject: Re: [PATCH 4/7] drivers: serial: kgdboc: Check CON_SUSPENDED instead
 of CON_ENABLED

Hi,

On Tue, Jun 10, 2025 at 1:03 PM Marcos Paulo de Souza
<mpdesouza@...e.com> wrote:
>
> On Mon, 2025-06-09 at 13:13 -0700, Doug Anderson wrote:
> > Hi,
> >
> > On Fri, Jun 6, 2025 at 7:54 PM Marcos Paulo de Souza
> > <mpdesouza@...e.com> wrote:
> > >
> > > All consoles found on for_each_console are registered, meaning that
> > > all of
> > > them are CON_ENABLED. The code tries to find an active console, so
> > > check if the
> > > console is not suspended instead.
> > >
> > > Signed-off-by: Marcos Paulo de Souza <mpdesouza@...e.com>
> > > ---
> > >  drivers/tty/serial/kgdboc.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/tty/serial/kgdboc.c
> > > b/drivers/tty/serial/kgdboc.c
> > > index
> > > 85f6c5a76e0fff556f86f0d45ebc5aadf5b191e8..af6d2208b8ddb82d62f33292b
> > > 006b2923583a0d2 100644
> > > --- a/drivers/tty/serial/kgdboc.c
> > > +++ b/drivers/tty/serial/kgdboc.c
> > > @@ -577,7 +577,8 @@ static int __init kgdboc_earlycon_init(char
> > > *opt)
> > >         console_list_lock();
> > >         for_each_console(con) {
> > >                 if (con->write && con->read &&
> > > -                   (con->flags & (CON_BOOT | CON_ENABLED)) &&
> > > +                   (con->flags & CON_BOOT) &&
> > > +                   ((con->flags & CON_SUSPENDED) == 0) &&
> >
> > I haven't tried running the code, so I could easily be mistaken,
> > but...
> >
> > ...the above doesn't seem like the correct conversion. The old
> > expression was:
> >
> > (con->flags & (CON_BOOT | CON_ENABLED))
> >
> > That would evaluate to non-zero (true) if the console was _either_
> > "boot" or "enabled".
> >
> > The new expression is is:
> >
> > (con->flags & CON_BOOT) && ((con->flags & CON_SUSPENDED) == 0)
> >
> > That's only true if the console is _both_ "boot" and "not suspended".
>
> My idea here was that the users of for_each_console would find the
> first available console, and by available I would expect them to be
> usable. In this case, is there any value for kgdboc to use a console
> that is suspended? Would it work in this case?
>
> I never really used kgdboc, but only checking if the console was
> enabled (which it's always the case here) was something that needed to
> be fixed.
>
> Maybe I'm missing something here as well, so please let me know if I
> should remove the new check.

So it's been 5 years since I wrote the code, but reading that I was
checking for:

  (con->flags & (CON_BOOT | CON_ENABLED))

Makes me believe that this was the case when I wrote the code:
1. Early boot consoles (earlycon) were not marked as CON_ENABLED but
were instead marked as CON_BOOT.
2. Once consoles became non-early they were moved to CON_ENABLED.

...and the code was basically looking for any consoles with a matching
name that were either boot consoles or normal/enabled consoles.

Is that a plausible theory? It's also possible that I just was
confused when I code things up and that I really meant to write:

((con->flags & (CON_BOOT | CON_ENABLED)) == (CON_BOOT | CON_ENABLED))

...AKA that I wanted consoles that were BOOT _and_ ENABLED.

In any case, I booted up the current mainline and I put a printout here. I saw:

[    0.000000] kgdboc: DOUG: console qcom_geni has flags 0x0000000f

So that means that both BOOT and ENABLED were set. That makes me feel
like it's plausible that I was confused and really meant BOOT _and_
ENABLED. I didn't spend time trying to figure out how to build/boot an
old kernel to check, though.

In any case, given my test then I think your change should be fine.
Given that it does change the boolean logic, it seems like that
deserves a mention in the commit message.

-Doug