linux-kernel - Re: [PATCH v6 3/3] tty: Change order of ttynull to be linked sooner if enabled as a console.

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Message-ID: <4451040.8hb0ThOEGa@nerdopolis2>
Date: Wed, 05 Mar 2025 23:22:35 -0500
From: Adam Simonelli <adamsimonelli@...il.com>
To: Andy Shevchenko <andy.shevchenko@...il.com>
Cc: linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
 Jiri Slaby <jirislaby@...nel.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Steven Rostedt <rostedt@...dmis.org>,
 John Ogness <john.ogness@...utronix.de>,
 Sergey Senozhatsky <senozhatsky@...omium.org>, Petr Mladek <pmladek@...e.com>
Subject:
 Re: [PATCH v6 3/3] tty: Change order of ttynull to be linked sooner if
 enabled as a console.

On Wednesday, March 5, 2025 1:52:00 PM EST Andy Shevchenko wrote:
> On Wed, Mar 5, 2025 at 4:06 AM Adam Simonelli <adamsimonelli@...il.com> wrote:
> > On Tuesday, March 4, 2025 1:51:52 AM EST Andy Shevchenko wrote:
> > > On Tue, Mar 4, 2025 at 5:55 AM <adamsimonelli@...il.com> wrote:
> 
> ...
> 
> > > >  obj-y                          += vt/
> > >
> > > + blank line.
> > >
> > > > +# If ttynull is configured to be a console by default, ensure that it is linked
> > > > +# earlier before a real one is selected.
> > > > +obj-$(CONFIG_NULL_TTY_DEFAULT_CONSOLE) \
> > > > +                               += ttynull.o
> > >
> > > Here is the question: are you sure that all console drivers that exist
> > > in the kernel happen to be here? Have you grepped the source tree for
> > > checking this?
> > >
> > Grepping for console_initcall, the only other places I see outside of
> > drivers/tty/ is
> >
> > arch/mips/fw/arc/arc_con.c
> > arch/mips/sibyte/common/cfe_console.c
> > arch/powerpc/kernel/legacy_serial.c
> > arch/powerpc/kernel/udbg.c
> > arch/powerpc/platforms/powermac/setup.c
> > arch/um/drivers/stderr_console.c
> > arch/xtensa/platforms/iss/console.c
> > drivers/s390/char/con3215.c
> > drivers/s390/char/con3270.c
> > drivers/s390/char/sclp_con.c
> > drivers/s390/char/sclp_vt220.c
> 
> Which means you need to test your stuff on those cases, to see how the
> linker order is done there. It might be that your change wouldn't work
> there as expected (quick workaround is to mark the new option as
> depends on !S390 && !PPC and so on.
> 
> 
It will be difficult to test other arches, I mean I guess it is possible with
QEMU, and cross-building, though I did do an experimental test on x86:

Making it temporarily adding an architecture specific console like
powerpc/some mips/s390/arches with Xen enabled.
-------------------------------------------------------------------------------
diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index 05c5aa951da7..bcd248c44fc8 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -1159,6 +1159,8 @@ void __init setup_arch(char **cmdline_p)

        e820__setup_pci_gap();

+       add_preferred_console("ttyS", 0, NULL);
+
 #ifdef CONFIG_VT
 #if defined(CONFIG_VGA_CONSOLE)
        if (!efi_enabled(EFI_BOOT) || (efi_mem_type(0xa0000) != EFI_CONVENTIONAL_MEMORY))
-------------------------------------------------------------------------------

to see what /proc/consoles will look like, to pretend that x86 is an arch that
sets a console somewhere, and I get:

ttynull0             --- (EC    )  242:0
ttyS0                -W- (E  p a)    4:64

and I got console messages to ttyS0 with no issue.

which in my mind is acceptable I would think. ttynull is first in the list,
which is desired effect of CONFIG_NULL_TTY_DEFAULT_CONSOLE, it doesn't have to
be _exclusive_ AFAIK, especially if there are long-time default consoles that.
users or the hardware expects.

The only arch that seems to _unconditionally_ add a console without some other
circumstance, like boot loader env var, and command line option, or firmware
flag, or suboption (like CONFIG_SERIAL_PMACZILOG_CONSOLE) is Jazz. 

Like platforms/powernv adds it if CONFIG_HVC_OPAL is disabled, or the firmware
is missing "FW_FEATURE_OPAL". I would assume that a user of this situation
turning on CONFIG_NULL_TTY_DEFAULT_CONSOLE in addition, will just get ttynull
and hvc in /proc/consoles instead of just hvc. Could that cause something to
break?

Correct me if I am wrong, I could very very very well be wrong.