Re: [PATCH v4 01/11] printk: Handle pre-enabled consoles in the top-level register_console()

[John Ogness <john.ogness@linutronix.de>]

On 2026-06-04, Petr Mladek <pmladek@suse.com> wrote:
> The function try_enable_preferred_console() currently has the
> non-obvious side effect of returning success for consoles that are
> already pre-enabled. This obscures the logic flow during console
> registration.
>
> Move the check for pre-enabled consoles directly into the top-level
> register_console(). This change makes the handling of pre-enabled
> consoles explicit and easier to follow.
>
> Furthermore, this separation lays the groundwork for future cleanups
> where try_enable_preferred_console() can be restricted to cases where
> an entry actually exists in the console_cmdline[] array.
>
> Also it fixes a possible out-of-bound access when the console_cmdline[]
> array is full and no console matched. In fact, the check of
> c->user_specified did not make much sense. The new console either
> matched and was handled in the for-cycle. Or it did not match
> and then *c pointed to an unused entry.
>
> Possible behavior change:
>
> try_enable_preferred_console() will newly be called also with
> @user_specified parameter set to "false" when it failed with the "true"
> variant. But it looks like the right way to do. It will allow to call
> newcon->setup() when the console was preferred by some platform
> specific code.

I agree that it is the right way to do it. But I am a bit confused about
the pre-enabled consoles.

I understood it that a driver manually sets CON_ENABLED because that
console is not enabled via normal methods ("console=" or "stdout" in
device-tree). There are several examples of this (netconsole, pstore,
nfcon, kgdb). But then I see drivers like:

- drivers/tty/ehv_bytechan.c
- drivers/tty/serial/ma35d1_serial.c
- drivers/tty/serial/mux.c

that are also setting CON_ENABLED, but seem that they should be
specified in the normal way.

Are these drivers falsely setting CON_ENABLED? Is there a clear
description about when a driver should manually set CON_ENABLED?

> Reported-by: Naveen Kumar Chaudhary <naveen.osdev@gmail.com> # out-of-bound access
> Closes: https://lore.kernel.org/r/7sq4tr2nmlz32tvkf6vpsghv6exvqfghsrlvywjcqihzsqqbf7@bspclmti5xg4
> Signed-off-by: Petr Mladek <pmladek@suse.com>

Despite my open questions about CON_ENABLED, this patch is simplifying
the pre-enabled path and correctly specifying the possible behavior
change.

Reviewed-by: John Ogness <john.ogness@linutronix.de>