[PATCH] swsusp documentation updates

From: Pavel Machek <pavel@ucw.cz>

Documentation/power/swsusp.txt

@@ -123,10 +123,61 @@ server is not hotplug capable. What do you do? Suspend to disk,
 replace ethernet card, resume. If you are fast your users will not
 even see broken connections.
 
-Any other idea you might have tell me!
+Q: Maybe I'm missing something, but why doesn't the regular io paths
+work?
 
-Contacting the author
-If you have any question or any patch that solves the above or detected
-problems please contact me at seasons@falcon.sch.bme.hu. I might delay
-answering, sorry about that.
+A: (Basically) you want to replace all kernel data with kernel data saved
+on disk. How do you do that using normal i/o paths? If you'll read
+"new" data 4KB at a time, you'll crash... because you still need "old"
+data to do the reading, and "new" data may fit on same physical spot
+in memory.
 
+There are two solutions to this:
+
+* require half of memory to be free during suspend. That way you can
+read "new" data onto free spots, then cli and copy
+
+* assume we had special "polling" ide driver that only uses memory
+between 0-640KB. That way, I'd have to make sure that 0-640KB is free
+during suspending, but otherwise it would work...
+
+Q: Does linux support ACPI S4?
+
+A: No.
+
+When swsusp was created, ACPI was not too widespread, so we tried to
+avoid using ACPI-specific stuff. ACPI also is/was notoriously
+buggy. These days swsusp works on APM-only i386 machines and even
+without any power managment at all. Some versions also work on PPC.
+
+That means that machine does not enter S4 on suspend-to-disk, but
+simply enters S5. That has few advantages, you can for example boot
+windows on next boot, and return to your Linux session later. You
+could even have few different Linuxes on your box (not sharing any
+partitions), and switch between them.
+
+It also has disadvantages. On HP nx5000, if you unplug power cord
+while machine is suspended-to-disk, Linux will fail to notice that.
+
+Q: My machine doesn't work with ACPI. How can I use swsusp than ?
+
+A: Do reboot() syscall with right parameters. Warning: glibc gets in
+its way, so check with strace:
+
+reboot(LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2, 0xd000fce2)
+
+(Thanks to Peter Osterlund:)
+
+#include <unistd.h>
+#include <syscall.h>
+
+#define LINUX_REBOOT_MAGIC1     0xfee1dead
+#define LINUX_REBOOT_MAGIC2     672274793
+#define LINUX_REBOOT_CMD_SW_SUSPEND     0xD000FCE2
+
+int main()
+{
+    syscall(SYS_reboot, LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2,
+            LINUX_REBOOT_CMD_SW_SUSPEND, 0);
+    return 0;
+}