From: Petr Mladek <pmladek@suse.com>
To: Doug Anderson <dianders@chromium.org>
Cc: Marcos Paulo de Souza <mpdesouza@suse.com>,
Steven Rostedt <rostedt@goodmis.org>,
John Ogness <john.ogness@linutronix.de>,
Sergey Senozhatsky <senozhatsky@chromium.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Jiri Slaby <jirislaby@kernel.org>,
Jason Wessel <jason.wessel@windriver.com>,
Daniel Thompson <danielt@kernel.org>,
Richard Weinberger <richard@nod.at>,
Anton Ivanov <anton.ivanov@cambridgegreys.com>,
Johannes Berg <johannes@sipsolutions.net>,
linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org,
kgdb-bugreport@lists.sourceforge.net,
linux-um@lists.infradead.org
Subject: Re: [PATCH 4/7] drivers: serial: kgdboc: Check CON_SUSPENDED instead of CON_ENABLED
Date: Thu, 12 Jun 2025 15:57:00 +0200 [thread overview]
Message-ID: <aErcrCKcsi9cpANY@pathway.suse.cz> (raw)
In-Reply-To: <CAD=FV=XFeokpbMUFjAc0OkwJ97vR8aB+4GbnFxRKymvpEY3gnA@mail.gmail.com>
On Tue 2025-06-10 16:18:22, Doug Anderson wrote:
> Hi,
>
> On Tue, Jun 10, 2025 at 1:03 PM Marcos Paulo de Souza
> <mpdesouza@suse.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@suse.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@suse.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:
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?
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.
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().
Best Regards,
Petr
next prev parent reply other threads:[~2025-06-12 19:42 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-06-07 2:53 [PATCH 0/7] printk cleanup - part 2 Marcos Paulo de Souza
2025-06-07 2:53 ` [PATCH 1/7] printk: Make console_{suspend,resume} handle CON_SUSPENDED Marcos Paulo de Souza
2025-06-12 11:46 ` Petr Mladek
2025-06-23 18:45 ` Marcos Paulo de Souza
2025-06-07 2:53 ` [PATCH 2/7] printk: Use consoles_suspended flag when suspending/resuming all consoles Marcos Paulo de Souza
2025-06-13 15:20 ` Petr Mladek
2025-06-20 14:43 ` John Ogness
2025-06-24 8:40 ` Petr Mladek
2025-06-24 11:04 ` John Ogness
2025-06-25 8:48 ` Petr Mladek
2025-06-23 18:53 ` Marcos Paulo de Souza
2025-06-07 2:53 ` [PATCH 3/7] drivers: tty: Check CON_SUSPENDED instead of CON_ENABLED Marcos Paulo de Souza
2025-06-12 11:48 ` Petr Mladek
2025-06-07 2:53 ` [PATCH 4/7] drivers: serial: kgdboc: " Marcos Paulo de Souza
2025-06-09 20:13 ` Doug Anderson
2025-06-10 20:03 ` Marcos Paulo de Souza
2025-06-10 23:18 ` Doug Anderson
2025-06-12 13:57 ` Petr Mladek [this message]
2025-06-12 23:16 ` Doug Anderson
2025-06-13 10:52 ` Petr Mladek
2025-06-13 16:44 ` Doug Anderson
2025-06-07 2:53 ` [PATCH 5/7] arch: um: kmsg_dump: Don't check for CON_ENABLED Marcos Paulo de Souza
2025-06-16 13:33 ` Petr Mladek
2025-06-20 15:45 ` John Ogness
2025-06-07 2:53 ` [PATCH 6/7] debug: kgd_io: " Marcos Paulo de Souza
2025-06-16 13:56 ` Petr Mladek
2025-06-30 0:31 ` Marcos Paulo de Souza
2025-06-07 2:53 ` [PATCH 7/7] printk: Don't check for CON_ENABLED on console_unblank Marcos Paulo de Souza
2025-06-16 14:02 ` Petr Mladek
2025-06-17 9:32 ` Geert Uytterhoeven
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=aErcrCKcsi9cpANY@pathway.suse.cz \
--to=pmladek@suse.com \
--cc=anton.ivanov@cambridgegreys.com \
--cc=danielt@kernel.org \
--cc=dianders@chromium.org \
--cc=gregkh@linuxfoundation.org \
--cc=jason.wessel@windriver.com \
--cc=jirislaby@kernel.org \
--cc=johannes@sipsolutions.net \
--cc=john.ogness@linutronix.de \
--cc=kgdb-bugreport@lists.sourceforge.net \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-serial@vger.kernel.org \
--cc=linux-um@lists.infradead.org \
--cc=mpdesouza@suse.com \
--cc=richard@nod.at \
--cc=rostedt@goodmis.org \
--cc=senozhatsky@chromium.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).