[PATCH RFC 2/4] printk: deprecate boot_delay in favour of printk_delay

[Andrew Murray]

On Mon, 8 Jun 2026 at 15:07, Petr Mladek <pmladek at suse.com> wrote:
>
> On Mon 2026-06-01 00:17:38, Andrew Murray wrote:
> > The boot_delay (BOOT_PRINTK_DELAY) kernel parameter and printk_delay sysctl
> > are two distinct mechanisms for providing similar functionality which add a
> > delay prior to each printed printk message.
> >
> > boot_delay provides a kernel parameter for delaying printk output from
> > kernel start through to boot (SYSTEM_RUNNING), whereas printk_delay is
> > configurable only via sysctl and thus is only used post boot.
> >
> > Let's deprecate the boot_delay feature in favour of printk_delay. In order
> > to preserve functionality, we'll also extend printk_delay such that it can
> > additionally configured via a kernel parameter.
>
> I would make it clear and say: "via an early kernel parameter".
>
> Note that there are also kernel parameters which can be modified at runtime
> via /sys/module/kernel/paramters/<parameter>

OK thanks, I will update.


>
> Also I would make it clear that this changes the behavior, for
> example:
>
> <proposal>
> Behavior change:
>
> The delay enabled by both "boot_delay" and "printk_delay" continues
> working even in SYSTEM_RUNNING state. It must be explicitly stopped
> by setting printk_delay=0 via sysctl.
>
> The delay is skipped when the message is suppressed in all system
> states. It used to skipped only for the boot_delay.
> </proposal>

Yes, I'm happy to make that clearer.


>
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -1339,11 +1327,34 @@ static void boot_delay_msec(int level)
> >       }
> >  }
> >  #else
> > -static inline void boot_delay_msec(int level)
> > +static inline void __init printk_delay_calculate(void)
> > +{
> > +}
> > +
> > +static inline void early_boot_delay_msec(void)
> >  {
>
> It would be nice to print a warning that the early boot delay
> does not work, something like:
>
>         pr_warn_once("Early boot delay does not work without CONFIG_GENERIC_CALIBRATE_DELAY enabled.\n");
>
> >  }
> >  #endif
> >
> > +static int __init printk_delay_setup(char *str)
> > +{
> > +     get_option(&str, &printk_delay_msec);
> > +     if (printk_delay_msec > 10 * 1000)
> > +             printk_delay_msec = 0;
>
> Sashiko AI warns that this code accepts negative values.
> It might cause long delays, see
> https://sashiko.dev/#/patchset/20260601-deprecate_boot_delay-v1-0-c34c187142a6%40thegoodpenguin.co.uk
>
> The problem has already been there even before. But it would be nice
> to fix it.

Thanks for pointing out Sashiko, I hadn't seen its review on my
patches. Are authors expected to get emails from it, as I didn't?

In any case, it's a good spot, so I'll address.


>
> > +
> > +     printk_delay_calculate();
> > +
> > +     return 0;
> > +}
> > +early_param("printk_delay", printk_delay_setup);
> > +
> > +static int __init boot_delay_setup(char *str)
> > +{
> > +     pr_warn("boot_delay will soon be deprecated, please use printk_delay instead");
> > +     return printk_delay_setup(str);
> > +}
> > +early_param("boot_delay", boot_delay_setup);
> > +
> >  static bool printk_time = IS_ENABLED(CONFIG_PRINTK_TIME);
> >  module_param_named(time, printk_time, bool, S_IRUGO | S_IWUSR);
>
> Otherwise, it looks good to me.
>
> Best Regards,
> Petr

Thanks,

Andrew Murray