UserGuidePowerMgmtAM35xx

Power Management In AM35xx

Introduction

AM3517 and AM3505 silicon provides a limited set of power management features. These features are described in detail in the AM3517/05 TRM. In summary:

• Clock control at the module and clock domain level.
• Single core power domain
• Support for transitioning power domains to retention and wakeup on event.

References

1. OMAP Power Management
   1. Power Management features are being developed on pm branch of the linux-omap git tree. This page provides latest status of PM features on this branch.

Features

The power management features available in this release are based on the proposed PM interface for OMAP. This interface is described in the filename Documentation/arm/OMAP/omap_pm.

Supported

This is list of features supported in this release:

• Supports Dynamic Tick framework.
• Supports the Suspend/Resume capability, thus allowing the system to enter into a standby state.

Architecture

Dynamic Tick Suppression

The dynamic tick suppression is achieved through generic Linux framework for the same. A 32K timer (HZ=128) is used by the tick suppression algorithm.

Suspend & Resume

The suspend operation results in the system transitioning to the lowest power state being supported. The drivers implement the suspend() function defined in the LDM. When the suspend for the system is asserted, the suspend() function is called for all drivers. The drivers release the clocks to reach the desired low power state. The actual transition to suspend is implemented in the function omap3_pm_suspend().

Configuration

To enable/ disable power management start the Linux Kernel Configuration tool.

$ make menuconfig

Select Power management options from the main menu.

...
...
Kernel Features  --->
Boot options  --->
CPU Power Management  --->
Floating point emulation  --->
Userspace binary formats  --->
Power management options  --->
[*] Networking support  --->
Device Drivers  --->
...
...

Select Power Management support to toggle the power management support.

[*] Power Management support
[ ]   Power Management Debug Support
[*] Suspend to RAM and standby
< > Advanced Power Management Emulation

Power Management Debug Support

To enable power management debugging capabilities, debug file system and power management debug support need to be enabled in the configuration.

Enabling Debug Filesystem

Start the Linux Kernel Configuration tool.

$ make menuconfig

Select Kernel hacking from the main menu.

File systems  --->
Kernel hacking  --->
Security options  --->
-*- Cryptographic API  --->

Select Debug Filesystem from the next menu.

[ ] Enable unused/obsolete exported symbols
[*] Debug Filesystem
[ ] Run 'make headers_check' when building vmlinux
[*] Kernel debugging

Debugging Support In Power Management

Start the Linux Kernel Configuration tool.

$ make menuconfig

Select Power management options from the main menu.

...
...
Floating point emulation  --->
Userspace binary formats  --->
Power management options  --->
[*] Networking support  --->
Device Drivers  --->
...
...

Select Power Management Debug Support from the next menu.

[*] Power Management support
[*]   Power Management Debug Support
[*] Suspend to RAM and standby
< > Advanced Power Management Emulation

Software Interface

Suspend & Resume

The suspend for device can be asserted as follows:

$ echo -n "mem" > /sys/power/state

To wakeup, tap any key on the serial console. Automatic wakeup can be done by programming the wakeup timer before asserting suspend. The wakeup timer is accessed via debug filesystem interface and this requires both debug file system support and debugging support in power management enabled in the kernel configuration as described earlier. For example, to have an auto wakeup after 3 seconds, do the following before suspend is asserted Mount debug filesystem. This is a one time configuration.

$ mkdir /debug
$ mount -t debugfs debugfs /debug

Now program the wakeup timer.

$ echo -n 3 > /debug/pm_debug/wakeup_timer_seconds

To disable auto wakeup, do the following

$ echo -n 0 > /debug/pm_debug/wakeup_timer_seconds