Hi Adam, please add printk maintainers into Cc as already suggested by Andy at https://lore.kernel.org/r/CAHp75VeBaetiQBykfLk_weBHdzZF1nWp=k8BJu+OKNp6iYRRTg@xxxxxxxxxxxxxx The motivation is that the console registration code is in kernel/printk/printk.c. It is historically pretty tricky. Some ordering is defined rather by chance than by design. And we should be careful when adding new rules and hacks. On Mon 2025-02-24 07:39:14, adamsimonelli@xxxxxxxxx wrote: > From: Adam Simonelli <adamsimonelli@xxxxxxxxx> > > The new config option, CONFIG_NULL_TTY_CONSOLE will allow ttynull to be > initialized by console_initcall() and selected as a possible console > device. > > Signed-off-by: Adam Simonelli <adamsimonelli@xxxxxxxxx> > --- > drivers/tty/Kconfig | 15 ++++++++++++++- > drivers/tty/ttynull.c | 20 +++++++++++++++++++- > 2 files changed, 33 insertions(+), 2 deletions(-) > > diff --git a/drivers/tty/Kconfig b/drivers/tty/Kconfig > index 63a494d36a1f..b4afae8b0e74 100644 > --- a/drivers/tty/Kconfig > +++ b/drivers/tty/Kconfig > @@ -383,7 +383,20 @@ config NULL_TTY > available or desired. > > In order to use this driver, you should redirect the console to this > - TTY, or boot the kernel with console=ttynull. > + TTY, boot the kernel with console=ttynull, or enable > + CONFIG_NULL_TTY_CONSOLE. > + > + If unsure, say N. > + > +config NULL_TTY_CONSOLE It makes sense to enable this behavior by a CONFIG_ setting but the name is misleading. > + > + bool "Support for console on ttynull" > + depends on NULL_TTY=y && !VT_CONSOLE > + help > + Say Y here if you want the NULL TTY to be used as a /dev/console > + device. > + > + This is similar to CONFIG_VT_CONSOLE, but without the dependency on > + CONFIG_VT. It uses the ttynull driver as the system console. It is true that CONFIG_VT_CONSOLE causes that the virtual terminal will get associated with /dev/console. But it works only "by chance". It works because "register_console(&vt_console_driver)" in con_init() is the first register_console() call. And it also works only by chance because of the linking order. Anyway, there are more similar CONFIG_ options, for example, CONFIG_LP_CONSOLE, or CONFIG_VIRTIO_CONSOLE. And they are not default when CONFIG_VT_CONSOLE is enabled. They are registered only when the related console= option is defined on the command line. I want to say that CONFIG_<BLA>_CONSOLE does not mean that the BLA console will be registered by default. And we should us a better descriptive name, for example, NULL_TTY_DEFAULT_CONSOLE NULL_TTY_DEV_CONSOLE > If unsure, say N. > > diff --git a/drivers/tty/ttynull.c b/drivers/tty/ttynull.c > index 6b2f7208b564..ec3dd3fd41c0 100644 > --- a/drivers/tty/ttynull.c > +++ b/drivers/tty/ttynull.c > @@ -57,6 +57,13 @@ static struct tty_driver *ttynull_device(struct console *c, int *index) > static struct console ttynull_console = { > .name = "ttynull", > .device = ttynull_device, > + > + /* > + * Match the index and flags from other boot consoles when CONFIG_NULL_TTY_CONSOLE is > + * enabled, otherwise, use the default values for the index and flags. > + */ > + .index = IS_ENABLED(CONFIG_NULL_TTY_CONSOLE) ? -1 : 0, This should not be needed. "con->index" is always initialized to "0" for the default console, see: static void try_enable_default_console(struct console *newcon) { if (newcon->index < 0) newcon->index = 0; [...] } > + .flags = IS_ENABLED(CONFIG_NULL_TTY_CONSOLE) ? CON_PRINTBUFFER : 0, This does not make much sense to me. CON_PRINTBUFFER prevents duplicated output when the same device has already been registered as a boot console. But ttynull does not have a boot console variant. Also it is a "null" device. It never prints anything. The output could never be duplicated by definition. > }; > > static int __init ttynull_init(void) > @@ -90,11 +97,22 @@ static int __init ttynull_init(void) > } > > ttynull_driver = driver; > - register_console(&ttynull_console); > + if (!console_is_registered(&ttynull_console)) > + register_console(&ttynull_console); > > return 0; > } > > +#ifdef CONFIG_NULL_TTY_CONSOLE > +static int __init ttynull_register(void) > +{ > + if (!console_is_registered(&ttynull_console)) > + register_console(&ttynull_console); > + return 0; > +} > +console_initcall(ttynull_register); > +#endif This looks strange. I guess that you needed to move this into console_initcall() because it is called earlier together with the other console_initcall() calls for serial ports. Otherwise, the hack with the linking order (2nd patch) did not work. But you needed to keep it in ttynull_init() so that ttynull did not get registered prematurely when CONFIG_NULL_TTY_CONSOLE was not enabled. Sigh, it looks like a dirty hack which works rather by chance than by design. Thinking loudly: The register_console() code is a historic mess. I dream about having time to clean it up. Anyway, there are basically two modes: 1. try_enable_default_console(newcon) is called only when there is no @preferred_console and there is no registered console with tty binding (valid con->device). The first register_console() caller wins. The order is defined by the __con_initcall section. Which is defined by the linking order. IMHO, it is quite fragile and non-intuitive. 2. try_enable_preferred_console() is called when some console is preferred via console_cmdline[]. The entries are added by __add_preferred_console() calls. This approach was created to handle console= command line parameters. But it was later used to define default consoles also via SPCR and device tree, see add_preferred_console() callers. It is also a bit tricky because the last added entry is preferred. Plus the .user_specified entries are preferred over the entries added via SPCR or device tree. Anyway, I think that the preference and ordering defined by console_cmdline[] array is a more intuitive approach. My proposal is to call: #ifdef CONFIG_NULL_TTY_DEFAULT_CONSOLE add_preferred_console("ttynull", 0, NULL); #endif somewhere in the kernel code. The question is where. I wonder if the following would work: #ifdef CONFIG_NULL_TTY_DEFAULT_CONSOLE static int __init ttynull_default_console(void) { add_preferred_console("ttynull", 0, NULL); return 0; } console_initcall(ttynull_register); #endif Best Regards, Petr
