A set of updates for the 86 reboot code:

- Limit the Dell Optiplex 990 quirk to early BIOS versions to avoid the
     full 'power cycle' alike reboot which is required for the buggy BIOSes.
 
   - Update documentation for the reboot=pci command line option and
     document how DMI platform quirks can be overridden.
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Merge tag 'x86-misc-2021-08-30' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull misc x86 updates from Thomas Gleixner:
 "A set of updates for the x86 reboot code:

   - Limit the Dell Optiplex 990 quirk to early BIOS versions to avoid
     the full 'power cycle' alike reboot which is required for the buggy
     BIOSes.

   - Update documentation for the reboot=pci command line option and
     document how DMI platform quirks can be overridden"

* tag 'x86-misc-2021-08-30' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/reboot: Limit Dell Optiplex 990 quirk to early BIOS versions
  x86/reboot: Document how to override DMI platform quirks
  x86/reboot: Document the "reboot=pci" option

Documentation/admin-guide/kernel-parameters.txt

@@ -4794,7 +4794,7 @@
 
 	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]

Documentation/x86/x86_64/boot-options.rst

@@ -126,7 +126,7 @@ Idle loop
 Rebooting
 =========
 
-   reboot=b[ios] | t[riple] | k[bd] | a[cpi] | e[fi] [, [w]arm | [c]old]
+   reboot=b[ios] | t[riple] | k[bd] | a[cpi] | e[fi] | p[ci] [, [w]arm | [c]old]
       bios
         Use the CPU reboot vector for warm reset
       warm
@@ -145,6 +145,8 @@ Rebooting
         Use efi reset_system runtime service. If EFI is not configured or
         the EFI reset does not work, the reboot path attempts the reset using
         the keyboard controller.
+      pci
+        Use a write to the PCI config space register 0xcf9 to trigger reboot.
 
    Using warm reset will be much faster especially on big memory
    systems because the BIOS will not go through the memory check.
@@ -155,6 +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
 =======================
 

arch/x86/kernel/reboot.c

@@ -388,10 +388,11 @@ static const struct dmi_system_id reboot_dmi_table[] __initconst = {
 	},
 	{	/* Handle problems with rebooting on the OptiPlex 990. */
 		.callback = set_pci_reboot,
-		.ident = "Dell OptiPlex 990",
+		.ident = "Dell OptiPlex 990 BIOS A0x",
 		.matches = {
 			DMI_MATCH(DMI_SYS_VENDOR, "Dell Inc."),
 			DMI_MATCH(DMI_PRODUCT_NAME, "OptiPlex 990"),
+			DMI_MATCH(DMI_BIOS_VERSION, "A0"),
 		},
 	},
 	{	/* Handle problems with rebooting on Dell 300's */