On Tuesday, March 4, 2025 9:06:17 PM EST Adam Simonelli wrote:
> On Tuesday, March 4, 2025 1:51:52 AM EST Andy Shevchenko wrote:
> > On Tue, Mar 4, 2025 at 5:55 AM <adamsimonelli@xxxxxxxxx> wrote:
> > >
> > > From: Adam Simonelli <adamsimonelli@xxxxxxxxx>
> > >
> > > If CONFIG_NULL_TTY_DEFAULT_CONSOLE is enabled, and CONFIG_VT is disabled,
> > > ttynull will become the default primary console device, based on the link
> > > order.
> > >
> > > Many distributions ship with CONFIG_VT enabled. On tested desktop hardware
> > > if CONFIG_VT is disabled, the default console device falls back to
> > > /dev/ttyS0 instead of /dev/tty.
> > >
> > > This could cause issues in user space, and hardware problems:
> > >
> > > 1. The user space issues include the case where /dev/ttyS0 is
> > > disconnected, and the TCGETS ioctl, which some user space libraries use
> > > as a probe to determine if a file is a tty, is called on /dev/console and
> > > fails. Programs that call isatty() on /dev/console and get an incorrect
> > > false value may skip expected logging to /dev/console
> >
> > Missing period at the end.
> >
> > > 2. The hardware issues include the case if a user has a science instrument
> > > or other device connected to the /dev/ttyS0 port, and they were to upgrade
> > > to a kernel that is disabling the CONFIG_VT option, kernel logs will then be
> > > sent to the device connected to /dev/ttyS0 unless they edit their kernel
> > > command line manually.
> > >
> > > The new CONFIG_NULL_TTY_CONSOLE option will give users and distribution
> > > maintainers an option to avoid this. Disabling CONFIG_VT and enabling
> > > CONFIG_NULL_TTY_CONSOLE will ensure the default kernel console behavior
> > > is not dependant on hardware configuration by default, and avoid
> > > unexpected new behavior on devices connected to the /dev/ttyS0 serial
> > > port.
> >
> > ...
> >
> > > 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-$(CONFIG_NULL_TTY_DEFAULT_CONSOLE) \
> > > + += ttynull.o
> >
> > Here is the question: are you sure that all console drivers that exist
> > in the kernel happen to be here? Have you grepped the source tree for
> > checking this?
> >
> Grepping for console_initcall, the only other places I see outside of
> drivers/tty/ is
>
and if I, as a wacky test try
diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index 05c5aa951da7..bcd248c44fc8 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -1159,6 +1159,8 @@ void __init setup_arch(char **cmdline_p)
e820__setup_pci_gap();
+ add_preferred_console("ttyS", 0, NULL);
+
#ifdef CONFIG_VT
#if defined(CONFIG_VGA_CONSOLE)
if (!efi_enabled(EFI_BOOT) || (efi_mem_type(0xa0000) != EFI_CONVENTIONAL_MEMORY))
just as a test, and do
cat /proc/consoles
I see
ttynull0 --- (EC ) 242:0
ttyS0 -W- (E p a) 4:64
where ttynull0 is at the top of the list
> arch/mips/fw/arc/arc_con.c
> arch/mips/sibyte/common/cfe_console.c
> arch/powerpc/kernel/legacy_serial.c
> arch/powerpc/kernel/udbg.c
> arch/powerpc/platforms/powermac/setup.c
> arch/um/drivers/stderr_console.c
> arch/xtensa/platforms/iss/console.c
> drivers/s390/char/con3215.c
> drivers/s390/char/con3270.c
> drivers/s390/char/sclp_con.c
> drivers/s390/char/sclp_vt220.c
>
> > ...
> >
> > > +# If ttynull is enabled, but not as a boot console, it is linked and used later
> > > +# after the real ones.
> > > +ifneq ($(CONFIG_NULL_TTY_DEFAULT_CONSOLE),y)
> >
> > Also can be written as
> > ifeq ($(...),)
> > but it might be less explicit. Up to you.
> >
> > > obj-$(CONFIG_NULL_TTY) += ttynull.o
> > > +endif
> >
> >
>
>
