On Tuesday, February 25, 2025 11:19:04 AM EST Petr Mladek wrote: > Hi Adam, > > please add printk maintainers into Cc as already suggested by Andy > at https://lore.kernel.org/r/CAHp75VeBaetiQBykfLk_weBHdzZF1nWp=k8BJu+OKNp6iYRRTg@xxxxxxxxxxxxxx > OK, I have added the other printk maintainers in this thread, and will update my git send email script. > 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 > OK. I can change the option name. > > 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: > OK, I had this in an #ifdef before, it was the cleanest way to set it to -1 that I could think of, other than the ifdef... If I still need this, I will try to think of something else to set it to -1 when the option is enabled > 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. > OK, I was duplicating what I saw in other consoles. I can try to remove it > > }; > > > > 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 > OK, actually in earlier revisions locally, I did actually have diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index dddb15f48d59..c1554a789de8 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -3712,6 +3712,11 @@ void __init console_init(void) initcall_t call; initcall_entry_t *ce; +#ifdef CONFIG_NULL_TTY_CONSOLE + if (!strstr(boot_command_line, "console=")) + add_preferred_console("ttynull", 0, NULL); +#endif + /* Setup the default TTY line discipline. */ n_tty_init(); Which worked as far as I could tell, at least on x86. Not sure if that was the right place, and yeah, I was trying to better copy how CONFIG_VT_CONSOLE worked because I thought that was more correct.
