Re: [PATCH] Console: fall back to /dev/null when no console is availlable

Geert Uytterhoeven
2004-10-06T13:56:15Z

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On Wed, 6 Oct 2004, [iso-8859-1] Jörn Engel wrote:
> On Wed, 6 October 2004 15:07:05 +0200, Geert Uytterhoeven wrote:
> > On Wed, 6 Oct 2004, [iso-8859-1] Jörn Engel wrote:
> > > Point is that above patch is simpler and empiria didn't give me a
> > > reason to worry about anything else.
> >
> > I'll give you another reason :-)
> >
> > If I do have multiple active struct consoles registered (e.g. normal tty0 or
> > ttyS0 and a debug console without a real tty), and the /dev/console demux
> > thinks the debug console is the real one (the one opened if you open
> > /dev/console), printk() messages will appear on both active consoles, but
> > /dev/console cannot be opened.
> >
> > To avoid this problem, the /dev/console demux should walk the list of active
> > consoles until it finds one that can be opened, or fall back to /dev/null if
> > none is found.
> >
> > Does that sound reasonable?
>
> Not to me, no. But I was wrong before.
>
> Having no console at all is a valid design. It used to cause

One problem is that `console' means multiple things:
1. The output device for printk() (multiple consoles are allowed, cfr.
multiple console= kernel parameters and debug-only consoles)
2. The tty (both input and output) for /sbin/init (only one instance, cfr.
the last console= kernel parameter)

I suggested to change the logic for 2 not to use the last console= kernel
parameter if it turns out not to support input (cfr. the return value of struct
console.device()), but try the other registered struct consoles.

> problems, my patch fixes them. A command-line option like
> "console=/dev/null" doesn't fix it because it doesn't do what it
> appears to do at first glance, so the patch is needed.

Indeed. Because the /dev/null driver doesn't call register_console().
It could be made to work that way (`console=null'), though, but make sure to
register your null-console _first_.
But I think it's simpler to just check console->device() and take appropriate
actions.

> Having a non-working console, esp. for debug, is a rather odd design.
> My approach would be to either explicitly tell the kernel to use the
> other as default console via "console=/dev/ttyS0" or not have the
> debug thing in the kernel in the first place. Either way, no patch is
> needed.

It was not `designed' to be that way. But due to how `the console' (nr. 2 from
above) works, registration order matters. If people make the mistake (or just
forget) to say `console=ttyS0', a debug console registered later causes
problems.

And the reason the debug consoles (read: capturers) use register_console() is
to avoid code duplication.

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds