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: <CAHp75VdogqwA2qJBp5Sp-tuJbKvmj9mLuop8GZP+vLVeJNg2DQ@mail.gmail.com>
Date: Thu, 6 Mar 2025 09:10:29 +0200
From: Andy Shevchenko <andy.shevchenko@...il.com>
To: Adam Simonelli <adamsimonelli@...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 Thu, Mar 6, 2025 at 6:22 AM Adam Simonelli <adamsimonelli@...il.com> wrote:
>
> 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.

Thanks. Make sure the summary of this gets into the commit message.
Also consider updating the relevant documentation under
Documentation/, if any.

> -------------------------------------------------------------------------------
> 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.

I leave this to Petr to comment as I'm not that expert in the area.

-- 
With Best Regards,
Andy Shevchenko