x86 / ACPI / sleep: Provide registration for acpi_suspend_lowlevel.

Which by default will be x86_acpi_suspend_lowlevel.
This registration allows us to register another callback
if there is a need to use another platform specific callback.

Signed-off-by: Liang Tang <[e-mail]>
Signed-off-by: Konrad Rzeszutek Wilk <[e-mail]>
Tested-by: Ben Guthro <[e-mail]>
Acked-by: "H. Peter Anvin" <[e-mail]>
Signed-off-by: Rafael J. Wysocki <[e-mail]>

arch/x86/include/asm/acpi.h

···
 }
 
 /* Low-level suspend routine. */
-extern int acpi_suspend_lowlevel(void);
+extern int (*acpi_suspend_lowlevel)(void);
 
 /* Physical address to resume after wakeup */
 #define acpi_wakeup_address ((unsigned long)(real_mode_header->wakeup_start))

arch/x86/kernel/acpi/boot.c

···
 #include <asm/mpspec.h>
 #include <asm/smp.h>
 
+#include "sleep.h" /* To include x86_acpi_suspend_lowlevel */
 static int __initdata acpi_force = 0;
 u32 acpi_rsdt_forced;
 int acpi_disabled;
···
 
 int (*__acpi_register_gsi)(struct device *dev, u32 gsi,
 			   int trigger, int polarity) = acpi_register_gsi_pic;
+
+#ifdef CONFIG_ACPI_SLEEP
+int (*acpi_suspend_lowlevel)(void) = x86_acpi_suspend_lowlevel;
+#else
+int (*acpi_suspend_lowlevel)(void);
+#endif
 
 /*
  * success: return IRQ number (>=0)

arch/x86/kernel/acpi/sleep.c

···
 #endif
 
 /**
- * acpi_suspend_lowlevel - save kernel state
+ * x86_acpi_suspend_lowlevel - save kernel state
  *
  * Create an identity mapped page table and copy the wakeup routine to
  * low memory.
  */
-int acpi_suspend_lowlevel(void)
+int x86_acpi_suspend_lowlevel(void)
 {
 	struct wakeup_header *header =
 		(struct wakeup_header *) __va(real_mode_header->wakeup_header);

arch/x86/kernel/acpi/sleep.h

···
 extern void wakeup_long64(void);
 
 extern void do_suspend_lowlevel(void);
+
+extern int x86_acpi_suspend_lowlevel(void);

drivers/acpi/sleep.c

···
 		break;
 
 	case ACPI_STATE_S3:
+		if (!acpi_suspend_lowlevel)
+			return -ENOSYS;
 		error = acpi_suspend_lowlevel();
 		if (error)
 			return error;