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

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Message-ID: <CAD=FV=WFWviPPR6VWmsN2-+vzRDoU6oTNH=EP6z1usG4EDR3+w@mail.gmail.com>
Date: Thu, 12 Jun 2025 16:16:09 -0700
From: Doug Anderson <dianders@...omium.org>
To: Petr Mladek <pmladek@...e.com>
Cc: Marcos Paulo de Souza <mpdesouza@...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 Thu, Jun 12, 2025 at 6:57 AM Petr Mladek <pmladek@...e.com> wrote:
>
> > > > > @@ -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:
>
> It is easy to get confused by the register_console() code. And
> it has been even worse some years ago.
>
> Anyway, the current code sets CON_ENABLED for all registered
> consoles, including CON_BOOT consoles. The flag is cleared only
> by console_suspend()[*] or unregister_console().
>
> IMHO, kgdboc_earlycon_init() does not need to care about
> console_suspend() because the kernel could not be suspended
> during boot. Does this makes sense?

Yeah, makes sense to me.


> Also it does not need to care about unregister_console() because
> the unregistered console won't be listed by for_each_console().
>
> [*] The 1st patch in this patchset updates console_suspend()
>     to set CON_SUSPENDED instead of clearing CON_ENABLED.
>     It helps to make it clear that it is suspend-related.
>
>
> Resume:
>
> I would remove the check of CON_ENABLED or CON_SUSPENDED
> from kgdboc_earlycon_init() completely.
>
> IMHO, we should keep the check of CON_BOOT because we do not
> want to register "normal" console drivers as kgdboc_earlycon_io_ops.
> It is later removed by kgdboc_earlycon_deinit(). I guess
> that the code is not ready to handle a takeover from normal
> to normal (even the same) console driver.

I'm not sure I understand your last sentence there. In general the
code handling all of the possible handoff (or lack of handoff) cases
between kgdboc_earlycon and regular kgdboc is pretty wacky. At one
point I thought through it all and tried to test as many scenarios as
I could and I seem to remember trying to model some of the behavior on
how earlycon worked. If something looks broken here then let me know.


> To make it clear, I would use:
>
>         for_each_console(con) {
>                 if (con->write && con->read &&
>                     (con->flags & CON_BOOT) &&
>                     (!opt || !opt[0] || strcmp(con->name, opt) == 0))
>                         break;
>         }
>
> And I would do this change as the 1st patch in the patchset
> before we change the flag modified by console_suspend().

Seems OK to me.

-Doug