linux-kernel - Re: [PATCH] printk: Document that CONFIG_BOOT_PRINTK_DELAY required for boot

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Date:   Mon, 23 Jan 2023 14:44:19 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH] printk: Document that CONFIG_BOOT_PRINTK_DELAY required
 for boot_delay=

On Mon, Jan 23, 2023 at 06:18:26PM +0000, Matthew Wilcox wrote:
> On Mon, Jan 23, 2023 at 12:04:40PM -0600, Bjorn Helgaas wrote:
> > From: Bjorn Helgaas <bhelgaas@...gle.com>
> > 
> > Document the fact that CONFIG_BOOT_PRINTK_DELAY must be enabled for the
> > "boot_delay" kernel parameter to work.  Also mention that "lpj=" may be
> > necessary.
> > 
> > Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>
> > ---
> >  Documentation/admin-guide/kernel-parameters.txt | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 6cfa6e3996cf..b0b40b6a765c 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -480,8 +480,9 @@
> >  			See Documentation/block/cmdline-partition.rst
> >  
> >  	boot_delay=	Milliseconds to delay each printk during boot.
> > -			Values larger than 10 seconds (10000) are changed to
> > -			no delay (0).
> > +			Enable CONFIG_BOOT_PRINTK_DELAY and also specify
> > +			"lpj=".  Boot_delay values larger than 10 seconds
> > +			(10000) are changed to no delay (0).
> 
> That's counterintuitive.  I'd understand clamping it to 10 seconds,
> but not setting it to zero.

I'm not opposed to setting it to 10 sec instead of 0, but that came
from bfe8df3d314b ("slow down printk during boot"); added Randy in
case he wants to comment.

> Also, there are two ways of reading this.  One is that by specifying
> boot_delay=, lpj= is set to a sane value.  The other (intended?) is
> that in order to use this option, you must also specify lpj=.

Maybe this is better?

 	boot_delay=	Milliseconds to delay each printk during boot.
-			Values larger than 10 seconds (10000) are changed to
-			no delay (0).
+			Only works if CONFIG_BOOT_PRINTK_DELAY is enabled,
+			and you may also have to specify "lpj=".  Boot_delay
+			values larger than 10 seconds (10000) are changed
+			to no delay (0).
 			Format: integer
 
 	bootconfig	[KNL]