Explaining Intel Rapid Start Technology
Recent Intel-based systems often implement something called Intel Rapid Start Technology. Like many things with the word "Technology" in the name, there's a large part of this that's marketing. The relatively small amount of technical documentation available implies that it's tied to your motherboard chipset and CPU, but as far as I can tell it's entirely implemented in firmware and could work just as well on, say, a Cyrix on a circa 1996 SIS-based motherboard if someone wrote the BIOS code. But since nobody has, we're stuck with the vendors who've met Intel's requirements and licensed the code.
The concept of IRST is pretty simple. There's a firmware mechanism for setting a sleep timeout. If you suspend your computer and this timeout expires, it'll resume. However, instead of handing control back to the OS, the firmware just copies the entire contents of RAM to a special partition and turns the computer off. Next time you hit the power button, the firmware dumps the partition contents back into RAM and resumes as if nothing had changed. This takes a few seconds longer than resume from S3 but is far faster than resume from hibernation since it starts the moment the system gets power.
At a more technical level, it's a little more complicated. The first thing to know about this feature is that it's entirely invisible unless your hard drive is set up correctly. There needs to be a partition that's at least the size of your system's physical RAM. For GPT systems, this needs to have a type GUID of D3BFE2DE-3DAF-11DF-BA-40-E3A556D89593. For MBR systems, you need a partition type of 0x84. If the firmware doesn't find an appropriate partition then the OS will get no indication that the firmware supports it. Boo.
(The second thing is that it seems like it really does have to be on an SSD, and if you try to do this on spinning media your firmware will ignore it anyway)
If all the prerequisites are in place, an ACPI device with an HID of INT3392 will exist. It has four methods associated with it: GFFS, SFFS, GFTV and SFTV. GFFS returns an integer representing the events that will cause the system to wake up from S3 and suspend to SSD. The system will wake after the timeout expires if bit 0 is set, and will wake when the battery becomes critically low if bit 1 is set. The other bits appear to be unused at the moment. SFFS sets the wakeup events, using the same bit values as GFFS. GFTV returns an integer containing the current wakeup timeout in minutes. SFTV sets it. Values above 1440 (ie, 24 hours) seem to be considered invalid - if I set them the value instead ends up as 10 and the timeout flag gets cleared from the wakeup events field.
I've submitted a patch that adds a sysfs interface for setting these values, and unless anyone objects it'll probably end up in 3.11. There's still the remaining question of how userspace should make use of these, and also how installers should behave when it comes to systems that support IRST. As previously mentioned, there's no obvious indication to the OS that the feature is supported unless the appropriate partition already exists. The easiest way to deal with this is for installers to default to retaining any partitions with the magic IDs, but I'm still looking into whether it's possible to get the firmware to cough up some more information so it can be created automatically even if the drive's entirely blank.
And now, having got this working on a test machine, I just need to split my Thinkpad's swap partition in half and make sure it works here as well. Woo.
 Note: I am not going to do this.
 Conveniently, the same as the partition type that APM systems used for suspend to disk back when dubstep hadn't been invented yet
 At least, if you attempt to set them they get ignored.