[power] Register PM subsystem, and create power/ directory in sysfs.

- Add file 'state' which provides single point of entry for all PM 
  transitions.
  File accepts a string specifying what state to enter, which is one of:
  "standby": Power-on Suspend (aka S1)
  "suspend": Suspend-to-RAM (aka S3)
  "hibernate": Suspend-to-disk (aka S4)

  The names do suck, because they are lifted from the ACPI spec. Better 
  naming suggestions are welcome, though these names are relatively well-
  known. 

- Handler for file does little now, but will validate the passed string and
  call the appropriate functions. 

- Needs to be integrated with swsusp and ACPI S3 code.

kernel/power/Makefile

-obj-y				:= process.o console.o pm.o
+obj-y				:= main.o process.o console.o pm.o
 obj-$(CONFIG_SOFTWARE_SUSPEND)	+= swsusp.o
 
 obj-$(CONFIG_MAGIC_SYSRQ)	+= poweroff.o

kernel/power/main.c

+/*
+ * kernel/power/main.c - PM subsystem core functionality.
+ *
+ * Copyright (c) 2003 Patrick Mochel
+ * Copyright (c) 2003 Open Source Development Lab
+ * 
+ * This file is release under the GPLv2
+ *
+ */
+
+#include <linux/kobject.h>
+#include <linux/string.h>
+#include <linux/errno.h>
+#include <linux/init.h>
+#include <linux/pm.h>
+
+
+
+static int standby(void)
+{
+	return 0;
+}
+
+static int suspend(void)
+{
+	return 0;
+}
+
+static int hibernate(void)
+{
+	return 0;
+}
+
+#define decl_state(_name) \
+	{ .name = __stringify(_name), .fn = _name }
+
+struct pm_state {
+	char * name;
+	int (*fn)(void);
+} pm_states[] = {
+	decl_state(standby),
+	decl_state(suspend),
+	decl_state(hibernate),
+	{ NULL },
+};
+
+
+static int enter_state(struct pm_state * state)
+{
+	return state->fn();
+}
+
+
+
+decl_subsys(power,NULL,NULL);
+
+
+#define power_attr(_name) \
+static struct subsys_attribute _name##_attr = {	\
+	.attr	= {				\
+		.name = __stringify(_name),	\
+		.mode = 0644,			\
+	},					\
+	.show	= _name##_show,			\
+	.store	= _name##_store,		\
+}
+
+/**
+ *	state - control system power state.
+ *
+ *	show() returns what states are supported, which is hard-coded to
+ *	'standby' (Power-On Suspend), 'suspend' (Suspend-to-RAM), and
+ *	'hibernate' (Suspend-to-Disk).
+ *
+ *	store() accepts one of those strings, translates it into the 
+ *	proper enumerated value, and initiates a suspend transition.
+ */
+
+static ssize_t state_show(struct subsystem * subsys, char * buf)
+{
+	struct pm_state * state;
+	char * s = buf;
+
+	for (state = &pm_states[0]; state->name; state++)
+		s += sprintf(s,"%s ",state->name);
+	s += sprintf(s,"\n");
+	return (s - buf);
+}
+
+static ssize_t state_store(struct subsystem * s, const char * buf, size_t n)
+{
+	struct pm_state * state;
+	int error;
+	char * end = strchr(buf,'\n');
+	
+	if (end)
+		*end = '\0';
+
+	for (state = &pm_states[0]; state; state++) {
+		if (!strcmp(buf,state->name))
+			break;
+	}
+	if (!state)
+		return -EINVAL;
+	error = enter_state(state);
+	return error ? error : n;
+}
+
+power_attr(state);
+
+static struct attribute * g[] = {
+	&state_attr.attr,
+	NULL,
+};
+
+static struct attribute_group attr_group = {
+	.attrs = g,
+};
+
+
+static int pm_init(void)
+{
+	int error = subsystem_register(&power_subsys);
+	if (!error)
+		error = sysfs_create_group(&power_subsys.kset.kobj,&attr_group);
+	return error;
+}
+
+core_initcall(pm_init);