PM / hibernate: Make passing hibernate offsets more friendly

Currently the only way to specify a hibernate offset for a
swap file is on the kernel command line.

Add a new /sys/power/resume_offset that lets userspace
specify the offset and disk to use when initiating a hibernate
cycle.

Signed-off-by: Mario Limonciello <mario.limonciello@dell.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/ABI/testing/sysfs-power

@@ -287,3 +287,17 @@ Description:
 		Writing a "1" to this file enables the debug messages and
 		writing a "0" (default) to it disables them.  Reads from
 		this file return the current value.
+
+What:		/sys/power/resume_offset
+Date:		April 2018
+Contact:	Mario Limonciello <mario.limonciello@dell.com>
+Description:
+		This file is used for telling the kernel an offset into a disk
+		to use when hibernating the system such as with a swap file.
+
+		Reads from this file will display the current offset
+		the kernel will be using on the next hibernation
+		attempt.
+
+		Using this sysfs file will override any values that were
+		set using the kernel command line for disk offset.

Documentation/power/swsusp.txt

@@ -24,8 +24,16 @@ Some warnings, first.
  * see the FAQ below for details.  (This is not true for more traditional
  * power states like "standby", which normally don't turn USB off.)
 
+Swap partition:
 You need to append resume=/dev/your_swap_partition to kernel command
-line. Then you suspend by
+line or specify it using /sys/power/resume.
+
+Swap file:
+If using a swapfile you can also specify a resume offset using
+resume_offset=<number> on the kernel command line or specify it
+in /sys/power/resume_offset.
+
+After preparing then you suspend by
 
 echo shutdown > /sys/power/disk; echo disk > /sys/power/state
 

kernel/power/hibernate.c

@@ -1061,6 +1061,29 @@ static ssize_t resume_store(struct kobject *kobj, struct kobj_attribute *attr,
 
 power_attr(resume);
 
+static ssize_t resume_offset_show(struct kobject *kobj,
+				  struct kobj_attribute *attr, char *buf)
+{
+	return sprintf(buf, "%llu\n", (unsigned long long)swsusp_resume_block);
+}
+
+static ssize_t resume_offset_store(struct kobject *kobj,
+				   struct kobj_attribute *attr, const char *buf,
+				   size_t n)
+{
+	unsigned long long offset;
+	int rc;
+
+	rc = kstrtoull(buf, 0, &offset);
+	if (rc)
+		return rc;
+	swsusp_resume_block = offset;
+
+	return n;
+}
+
+power_attr(resume_offset);
+
 static ssize_t image_size_show(struct kobject *kobj, struct kobj_attribute *attr,
 			       char *buf)
 {
@@ -1106,6 +1129,7 @@ power_attr(reserved_size);
 
 static struct attribute * g[] = {
 	&disk_attr.attr,
+	&resume_offset_attr.attr,
 	&resume_attr.attr,
 	&image_size_attr.attr,
 	&reserved_size_attr.attr,