x86: add force_hpet boot option

add force_hpet boot option.

(this will be useful to make the forced-enable quirks depend on.)
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

Documentation/kernel-parameters.txt

@@ -431,8 +431,10 @@ and is between 256 and 4096 characters. It is defined in the file
 			over the 8254 in addition to over the IO-APIC. The
 			kernel tries to set a sensible default.
 
-	hpet=		[X86-32,HPET] option to disable HPET and use PIT.
-			Format: disable
+	hpet=		[X86-32,HPET] option to control HPET usage
+			Format: { enable (default) | disable | force }
+			disable: disable HPET and use PIT instead
+			force: allow force enabled of undocumented chips (ICH4, VIA)
 
 	com20020=	[HW,NET] ARCnet - COM20020 chipset
 			Format:

arch/x86/kernel/hpet.c

@@ -69,12 +69,15 @@ static inline void hpet_clear_mapping(void)
  * HPET command line enable / disable
  */
 static int boot_hpet_disable;
+int hpet_force_user;
 
 static int __init hpet_setup(char* str)
 {
 	if (str) {
 		if (!strncmp("disable", str, 7))
 			boot_hpet_disable = 1;
+		if (!strncmp("force", str, 5))
+			hpet_force_user = 1;
 	}
 	return 1;
 }

include/asm-x86/hpet.h

@@ -64,6 +64,7 @@
 /* hpet memory map physical address */
 extern unsigned long hpet_address;
 extern unsigned long force_hpet_address;
+extern int hpet_force_user;
 extern int is_hpet_enabled(void);
 extern int hpet_enable(void);
 extern unsigned long hpet_readl(unsigned long a);