[PATCH 2/3] x86/reboot: document how to override DMI platform quirks

[Paul Gortmaker <paul.gortmaker@windriver.com>]

In commit 5955633e91bf ("x86/reboot: Skip DMI checks if reboot set
by user") we made it so that you didn't have to recompile the
kernel in order to bypass broken reboot quirks compiled into
an image:

 * This variable is used privately to keep track of whether or not
 * reboot_type is still set to its default value (i.e., reboot= hasn't
 * been set on the command line).  This is needed so that we can
 * suppress DMI scanning for reboot quirks.  Without it, it's
 * impossible to override a faulty reboot quirk without recompiling.

However, at the time we didn't really document it outside the source
code, and so this information isn't really available to the average
user out there.

Here we tell a little white lie and invent "reboot=default" since
it is easy to remember, and documents well.  The truth is that any
random string that is *not* a currently accepted string will work.

Since that doesn't document well for non-coders, and since we don't
know what the future additions might be, we lay claim on "default"
since that is exactly what it achieves.

Cc: Borislav Petkov <bp@alien8.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Matthew Garrett <mjg@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Michael D Labriola <michael.d.labriola@gmail.com>
Cc: x86@kernel.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
---
 Documentation/admin-guide/kernel-parameters.txt | 2 +-
 Documentation/x86/x86_64/boot-options.rst       | 7 +++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 29a116b590bd..63e391d7045c 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4703,7 +4703,7 @@
 
 	reboot=		[KNL]
 			Format (x86 or x86_64):
-				[w[arm] | c[old] | h[ard] | s[oft] | g[pio]] \
+				[w[arm] | c[old] | h[ard] | s[oft] | g[pio]] | d[efault] \
 				[[,]s[mp]#### \
 				[[,]b[ios] | a[cpi] | k[bd] | t[riple] | e[fi] | p[ci]] \
 				[[,]f[orce]
diff --git a/Documentation/x86/x86_64/boot-options.rst b/Documentation/x86/x86_64/boot-options.rst
index 2d2810409cc8..8593d4f87cf9 100644
--- a/Documentation/x86/x86_64/boot-options.rst
+++ b/Documentation/x86/x86_64/boot-options.rst
@@ -157,6 +157,13 @@ Rebooting
      Don't stop other CPUs on reboot. This can make reboot more reliable
      in some cases.
 
+   reboot=default
+     There are some built-in platform specific "quirks" - you may see:
+     "reboot: <name> series board detected. Selecting <type> for reboots."
+     In the case where you think the quirk is in error (e.g. you have
+     newer BIOS, or newer board) using this option will ignore the built-in
+     quirk table, and use the generic default reboot actions.
+
 Non Executable Mappings
 =======================
 
-- 
2.25.1