[power] Add flag to control suspend-to-disk behavior.

Suspend-to-disk can be handled in numerous ways, some we have control over,
and others we don't. The biggest difference is whether or not the firmware
is responsible for entering a low-power state or if the platform driver is.
The two modes are incompatible, so we enable the platform driver tell the 
PM core when they register their pm_ops (via the ->pm_disk_mode) field. 

If the firmware is responsible, then it will also write memory to disk, 
while the kernel is otherwise responsible. However, a user may choose to 
use the in-kernel suspend mechanism, even if the system supports only 
the firmware mechanism. Instead of entering a low-power state, the system
will turn off (or reboot for testing). 

A sysfs file -- /sys/power/disk -- is available to set the mode to one of:

  'firmware' 
  'platform'
  'shutdown'
  'reboot'

The latter two are settable any time, and assume that one is using swsusp. 
The other two are only settable to what the platform supports.

drivers/acpi/sleep/main.c

@@ -165,9 +165,13 @@ static int __init acpi_sleep_init(void)
 			sleep_states[i] = 1;
 			printk(" S%d", i);
 		}
-		if (i == ACPI_STATE_S4 && acpi_gbl_FACS->S4bios_f) {
-			sleep_states[i] = 1;
-			printk(" S4bios");
+		if (i == ACPI_STATE_S4) {
+			if (acpi_gbl_FACS->S4bios_f) {
+				sleep_states[i] = 1;
+				printk(" S4bios");
+				acpi_pm_ops.pm_disk_mode = PM_DISK_FIRMWARE;
+			} else if (sleep_states[i])
+				acpi_pm_ops.pm_disk_mode = PM_DISK_PLATFORM;
 		}
 	}
 	printk(")\n");

include/linux/pm.h

@@ -201,7 +201,17 @@ enum {
 	PM_SUSPEND_MAX,
 };
 
+enum {
+	PM_DISK_FIRMWARE = 1,
+	PM_DISK_PLATFORM,
+	PM_DISK_SHUTDOWN,
+	PM_DISK_REBOOT,
+	PM_DISK_MAX,
+};
+
+
 struct pm_ops {
+	u32	pm_disk_mode;
 	int (*prepare)(u32 state);
 	int (*enter)(u32 state);
 	int (*finish)(u32 state);

kernel/power/main.c

@@ -20,6 +20,9 @@ static DECLARE_MUTEX(pm_sem);
 
 static struct pm_ops * pm_ops = NULL;
 
+static u32 pm_disk_mode = PM_DISK_SHUTDOWN;
+
+
 /**
  *	pm_set_ops - Set the global power method table. 
  *	@ops:	Pointer to ops structure.
@@ -29,6 +32,8 @@ void pm_set_ops(struct pm_ops * ops)
 {
 	down(&pm_sem);
 	pm_ops = ops;
+	if (ops->pm_disk_mode && ops->pm_disk_mode < PM_DISK_MAX)
+		pm_disk_mode = ops->pm_disk_mode;
 	up(&pm_sem);
 }
 
@@ -224,6 +229,74 @@ static struct subsys_attribute _name##_attr = {	\
 	.store	= _name##_store,		\
 }
 
+
+static char * pm_disk_modes[] = {
+	[PM_DISK_FIRMWARE]	= "firmware",
+	[PM_DISK_PLATFORM]	= "platform",
+	[PM_DISK_SHUTDOWN]	= "shutdown",
+	[PM_DISK_REBOOT]	= "reboot",
+};
+
+/**
+ *	disk - Control suspend-to-disk mode
+ *
+ *	Suspend-to-disk can be handled in several ways. The greatest 
+ *	distinction is who writes memory to disk - the firmware or the OS.
+ *	If the firmware does it, we assume that it also handles suspending 
+ *	the system.
+ *	If the OS does it, then we have three options for putting the system
+ *	to sleep - using the platform driver (e.g. ACPI or other PM registers),
+ *	powering off the system or rebooting the system (for testing). 
+ *
+ *	The system will support either 'firmware' or 'platform', and that is
+ *	known a priori (and encoded in pm_ops). But, the user may choose
+ *	'shutdown' or 'reboot' as alternatives.
+ *
+ *	show() will display what the mode is currently set to. 
+ *	store() will accept one of
+ *
+ *	'firmware'
+ *	'platform'
+ *	'shutdown'
+ *	'reboot'
+ *
+ *	It will only change to 'firmware' or 'platform' if the system
+ *	supports it (as determined from pm_ops->pm_disk_mode).
+ */
+
+static ssize_t disk_show(struct subsystem * subsys, char * buf)
+{
+	return sprintf(buf,"%s\n",pm_disk_modes[pm_disk_mode]);
+}
+
+
+static ssize_t disk_store(struct subsystem * s, const char * buf, size_t n)
+{
+	int i;
+	u32 mode = 0;
+
+	for (i = PM_DISK_FIRMWARE; i < PM_DISK_MAX; i++) {
+		if (!strcmp(buf,pm_disk_modes[i])) {
+			mode = i;
+			break;
+		}
+	}
+	if (mode) {
+		if (mode == PM_DISK_SHUTDOWN || mode == PM_DISK_REBOOT)
+			pm_disk_mode = mode;
+		else {
+			if (pm_ops && (mode == pm_ops->pm_disk_mode))
+				pm_disk_mode = mode;
+			else
+				return -EINVAL;
+		}
+		return n;
+	}
+	return -EINVAL;
+}
+
+power_attr(disk);
+
 /**
  *	state - control system power state.
  *
@@ -251,10 +324,6 @@ static ssize_t state_store(struct subsystem * subsys, const char * buf, size_t n
 	u32 state;
 	struct pm_state * s;
 	int error;
-	char * end = strchr(buf,'\n');
-	
-	if (end)
-		*end = '\0';
 
 	for (state = 0; state < PM_SUSPEND_MAX; state++) {
 		s = &pm_states[state];
@@ -272,6 +341,7 @@ power_attr(state);
 
 static struct attribute * g[] = {
 	&state_attr.attr,
+	&disk_attr.attr,
 	NULL,
 };