printk: Document that CONFIG_BOOT_PRINTK_DELAY required for boot_delay=

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@google.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20230126225420.1320276-1-helgaas@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index f7de951..d6ae7bd 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -478,8 +478,10 @@
                        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).
+                       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 assumed
+                       erroneous and ignored.
                        Format: integer
 
        bootconfig      [KNL]