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

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Date:   Mon, 23 Jan 2023 18:18:26 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH] printk: Document that CONFIG_BOOT_PRINTK_DELAY required
 for boot_delay=

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.

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=.