ARM Hibernation

Introduction
The swsusp framework provides hibernation support on mainline kernels. This page discusses adding that functionality to ARM.

Motivation
Providing hibernation support for ARM provides several advantages:
 * Zero power consumption sleep
 * Snapshot boot ability
 * Shared effort with DDR self-refresh only suspend modes (such as RTC-only on am335x)

Utilizing the mainline swsusp framework also provides to uswsusp, allowing flexibility with where and how hibernation images are stored.

Challenges
The primary challenge to bringing hibernation to ARM is the complex state of PM within ARM, the large amount of CPU registers related to PM, and the complexity of saving and restoring those registers properly.

Patches
A set of patches is available for am335x hibernation, it is currently based on 3.8-rc5 plus some TI PM and am33xx patches.


 * Base of patchset: https://github.com/russdill/linux/commits/arm-hibernation-am33xx
 * am33xx WKUP domain save/restore: https://github.com/russdill/linux/commits/arm-hibernation-am33xx-wkup
 * Generic hibernation support for ARM: https://github.com/russdill/linux/commits/arm-hibernation
 * am33xx hibernation support: https://github.com/russdill/linux/commits/arm-hibernation-am33xx

U-Boot restore
Rather than utilizing a boot kernel to reload a hibernation image, U-Boot can be used directly. Patchset available against v2013.01 here:

https://github.com/russdill/commits/hibernation

This has the advantage of potentially being faster as a boot kernel does not need to be run, but it has the disadvantage that loading and decompressing pages from disk can be slower under U-Boot.

To function, it requires the kernel to pass a cpu_resume function and also restore it's nosave section.

Articles and presentations

 * Previous work based on 2.6.11: Suspend To Disk For ARM
 * Extending the swsusp Hibernation Framework to ARM
 * To be presented by Russ Dill at ELC 2013
 * Slides available at: https://docs.google.com/file/d/0B-JeSjLTabpdOFdwVlhVRjNTUUU/edit?usp=sharing