PM Suspend resume overview

Objective

Suspend enables the system to enter low power state to reduce power consumption whenever user desires to do so.

Read this first:

• This document is applicable for DM814x release 04.01.00.06 onwards, which include suspend/resume support.
• This document is applicable for DM8137 release 04.04.00.01 onwards, which include suspend/resume support.
• DDR self-refresh support is added from 04.01.00.07 release onwards, for more info refer DDR Self-refresh
• Suspend/resume is not supported on PG 1.x revisions of the chip.
• Lowest 1 KB(1024 Bytes from the end) of OCMC RAM is used by Suspend code to execute from OCMC when DDR is placed in Self-refresh mode during suspend/resume.This memory area will be over-written by Suspend code when kernel is loaded, during SRAM initialization so drivers should avoid using the lowest 1KB of OCMC RAM

Introduction

Following two low power modes are supported as part of suspend:

* StandBy
* DeepSleep - wake up is not supported yet

DeepSleep mode is an extra power saving state in addition to what is being saved in StandBy mode.
The default suspend mode is StandBy.
This can be changed using the "enable_deep_sleep" flag provided in the debugfs, refer to section " Enable DeepSleep".

Implementation

Entering StandBy

1. PM Suspend Framework calls all drivers' suspend calls.Drivers release the clocks.
2. DDR placed in self-refresh
3. All switchable(on board power switch) Power domains are switched off.(based on flag "turnoff_idle_powerdomains")
4. WFI is executed on A8

Exit StandBy:

1. Any key press on the serial console
2. Timer interrupt (timer count can be set through debugfs)

Entering DeepSleep:Not Tested

1. Same as steps 1 to 6 of Entering StandBy mentioned above
2. Enable DeepSleep logic, this will gate the oscillator output which results in gating of all clocks thus reduces further active power

Exit DeepSleep:

• Toggle OSC_WAKE pin <Not supported>

Assumptions

1. All module clocks are disabled by drivers.
2. All PHYs and I/Os will be handled by drivers.
   

• • Refer the respective driver documentation for suspend/resume support

1. DeepSleep doesn't mandate that all the PDs are off

• • DeepSleep is entered when DeepSleep mode is enabled (debugfs entry) and in this case, the modules which do not have corresponding kernel driver supporting suspend operation will not even be notified and DeepSleep state will be entered.

OUT OF SCOPE:

1. Suspend hooks/drivers for controlling slave processors.

• • Current suspend implementation does not provide any drivers or hooks for drivers to manage above modules during suspend and resume.

StandBy/DeepSleep Comparison

System Status

Module	StandBy	DeepSleep
Clocks	Clock domains not in use are forced to sleep, drivers disable the clock enabled by them	-same-
DDR	Self Refresh(Not Supported)	Self Refresh(Not Supported)
Power Domains	All switchable(on board power switch) Power domains are switched off.(if "turnoff_idle_powerdomains" enabled) ALWON remains on.	-same-
Voltage Domains	--No Change--	--No Change--
DPLL	--No Change--	--No Change--
Peripherals	All peripherals in ALWON domain remain on as they dont have power switches. UART stays clk enabled, rest all are disabled	-same-
Entry	ARM executes WFI	DeepSleep logic is enabled, OSC0 is gated
Wake-up	UART interrupt/key bord and Timer interrupt	OSC WAKE(Not Supported)

User Guide

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 ti814x_pm_suspend().

Configuration

The default EVM configuration (ti8148_evm_defconfig) has "Suspend to RAM and standby" support enabled.

To disable/enable Suspend to RAM support, start the Linux Kernel Configuration tool.

$ make menuconfig

Select Power management options from the main menu.

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

Power Management support should be enabled by default. In addition, select Suspend to RAM and standby by pressing 'y' or SPACE key. To disable, press 'n' key.

-*- Power Management support
[*]   Power Management Debug Support
[ ]     Extra PM attributes in sysfs for low-level debugging/testing
[*]     Verbose Power Management debugging
[*] Suspend to RAM and standby
< > Advanced Power Management Emulation
-*- Run-time PM core functionality

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
[ ]     Extra PM attributes in sysfs for low-level debugging/testing
[*]     Verbose Power Management debugging
[*] Suspend to RAM and standby
< > Advanced Power Management Emulation
-*- Run-time PM core functionality

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  --->

Debug Filesystem is already selected

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

User Interface

Suspend

Notes:

• You must determine and configure a supported wakeup source before proceeding to suspend otherwise system won't come out of sleep. To use UART as a wake-up source "no_console_suspend" needs to be added to bootargs, refer to Uart Wakeup.
• If MMC/SD card boot mode is used then following kernel config option must be enabled to prevent hang during suspend to memory:

Device Drivers-->
    MMC/SD/SDIO Card support -->
          [*]   Assume MMC/SD cards are non-removable (DANGEROUS)

• Drivers that do not support suspend may cause DDR corruption during suspen to Memory, Refer to DDR Self-refresh to avoid DDR corruption.

• To suspend system to RAM execute following command:

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

Mount debugfs

• Create a directory to mount debugfs or use /sys/kernel/debug, execute following command
  

$mount -t debugfs debugfs /sys/kernel/debug

Turn OFF Power domains

To turn off power domains with power switches if they are idle(no clock is active) during suspend enable following flag:

$echo 1 > /sys/kernel/debug/pm_debug/turnoff_idle_powerdomains

To disable this:

$echo 0 > /sys/kernel/debug/pm_debug/turnoff_idle_powerdomains

NOTE:If any of the power domains with switches(mentioned above) is active during suspend(any clock(s) running) then the power domain will not be powerd down i.e. the transition to OFF(0) state will not be successful, this will be reported during resume as shown below. e.g.if dsp clock was running then you will see following print:

 gem_pwrdm did not enter OFF mode, current state = 3

• state 3 stands for ON.

DDR Self-refresh

This section is applicable to 04.01.00.07 and above releases only.
In order to save power and reduce leakage current DDR is put in to self-refresh during suspend and brought out of self-refresh during resume. As part of self-refresh sequence, control of execution moves to OCMC, DDR is put in to self-refresh, EMIF clocks are gated,DMM is configured for No emif access and DDR PLL is pleced in bypass mode.During DDR self-refresh there should not be any access to DDR contents.
Drivers must ensure to suspend all activity before entering suspend(or as part of drivers suspend)and must not make any access to DDR when system is in suspend.

Devices without suspend support

Devices without linux drivers/drivers that do not support suspend may try to access DDR (dma transfers) when DDR in self-refresh, this leads to DDR data corruption and cause system instabilities.Hence the user must ensure to suspend all activities to be suspended/stopped that might result in accessing DDR in turn cause data corruption.

Wake-up sources

• Uart
• Timer

Uart Wakeup

• To use Uart as a wake-up source add "no_console_suspend" to boot args as shown below

set bootargs 'console=ttyO0,115200n8 root=/dev/nfs nfsroot=172.24.190.72:/srv/nfs,nolock rw ip=dhcp mem=128M earlyprintk no_console_suspend'

To wake up, tap any key on the serial console(teraterm/hyperterminal).

Timer Wakeup

To resume system after certain time use following debugfs entries

 1. timer_wakeup_seconds
 2. timer_wakeup_milliseconds

To set these variables debugfs needs to be mounted first,

 $mount -t debugfs debugfs /sys/kernel/debug

eg: To set 5 seconds and 100 milliseconds

 $echo 5 > /sys/kernel/debug/pm_debug/wakeup_timer_seconds

 $echo 100 > /sys/kernel/debug/pm_debug/wakeup_timer_milliseconds

To enter suspend:

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

Timer sends an interrupt once the count is reached and the system will be resumed.

Enable DeepSleep

NOTE: DeepSleep mode is not supported currently as this feature is not tested.

To enable DeepSleep mode, disabled by default, first mount the debugfs and do the following<bre>

$echo 1 > /sys/kernel/debug/pm_debug/enable_deep_sleep

To disable again:

$echo 0 > /sys/kernel/debug/pm_debug/enable_deep_sleep

DeepSleep wake pin polarity

Deepsleep wake up source pin (OSC_WAKE) polarity can be configured using the debugfs entry "deepsleep_polarity" as shown below:

* To config polarity to active high
 $echo 1 > /sys/kernel/debug/pm_debug/deepsleep_polarity
* To config polarity to be active low
 $echo 0 > /sys/kernel/debug/pm_debug/deepsleep_polarity

NOTE: This configures the DEEPSLEEPZ bit in DEEPSLEEPCTRL register. Default configuration is "active high", should be configured as per the board's OSC_WAKE pin default status (high/low). If default status is LOW then polarity must be configured to be Active high in order to enter DeepSleep and vice-versa.

Limitations

1. Voltage Management during suspend is not available
2. DeepSleep mode is not supported

Future Work

1. Voltage management using regulator framework

Adding suspend/resume support to Drivers

• For successful suspend and maximum power savings Device drivers must provide suspend/resume functionality
• In suspend, driver must disable all the clocks that were enabled during probe(as part of driver code).

These clocks can be enabled in resume if required.

• Suspend fails when driver (with suspend support) fails to suspend a device

PM hooks for drivers

• For device driver:

struct device_driver {
        const char              *name;
        struct bus_type         *bus;
         …………………
        const struct dev_pm_ops *pm;
        struct driver_private *p;
};

• For bus driver:

struct bus_type {
   const char              *name;
   struct bus_attribute    bus_attrs;
   ……………………
    const struct dev_pm_ops *pm;
    struct bus_type_private *p;
};

similar hooks present in struct device_type , struct class

• Device PM operations structure

struct dev_pm_ops {
int (*prepare)(struct device *dev);
void (*complete)(struct device *dev);
int (*suspend)(struct device *dev);
int (*resume)(struct device *dev);
...........
int (*suspend_noirq)(struct device *dev);
int (*resume_noirq)(struct device *dev);
...
};

NOTE: Suspend() and Resume() are mandatory for successful suspend/resume of the device.All Other operations, such as Prepare() and Complete(), are Optional.
For details such as * the function calling sequence and * what each function is meant for refer to the section "Calling Drivers to Enter and Leave System Sleep States" in Kernel documentation @ <kernel_source>Documentation/power/devices.txt

Example driver code

• DSP Power Management Driver

NOTE: Refer to the attached patch for complete code File:0001-ti814x-PM-driver-for-DSP-supporting-suspend-resume.patch.gz.

static int ti814x_dsp_power_probe(struct platform_device *pdev)
{
        struct ti814x_dsp_power_dev *dev;
        int r;
.........
.........

        dev->ick = clk_get(NULL, "gem_ick");
        if (!dev->ick) {
                r = -ENODEV;
                goto err_free_mem;
        }
        clk_enable(dev->ick);
        dev->fck = clk_get(NULL, "gem_fck");
        if (!dev->fck) {
                r = -ENODEV;
                goto err_free_mem;
        }
        clk_enable(dev->fck);

..........
..........
}
                                                                                                                                                                               
#ifdef CONFIG_SUSPEND
static int ti814x_dsp_power_prepare(struct device *dev)
{
        /*XXX: Prepare DSP for suspended */
        return 0;
}

static void ti814x_dsp_power_complete(struct device *dev)
{
        /*XXX: Do post suspend operations */
}

static int ti814x_dsp_power_suspend(struct device *dev)
{
        struct platform_device *pdev = to_platform_device(dev);
        struct ti814x_dsp_power_dev *dsp_dev = platform_get_drvdata(pdev);

        clk_disable(dsp_dev->ick);
        clk_disable(dsp_dev->fck);
        return 0;
}

static int ti814x_dsp_power_resume(struct device *dev)
{
        struct platform_device *pdev = to_platform_device(dev);
        struct ti814x_dsp_power_dev *dsp_dev = platform_get_drvdata(pdev);

        clk_enable(dsp_dev->ick);
        clk_enable(dsp_dev->fck);

        return 0;
}

static const struct dev_pm_ops ti814x_dsp_power_pm = {
        .prepare        = ti814x_dsp_power_prepare,
        .complete       = ti814x_dsp_power_complete,
        .suspend        = ti814x_dsp_power_suspend,
        .resume         = ti814x_dsp_power_resume,
};

#define ti814x_dsp_power_pm_ops (&ti814x_dsp_power_pm)
#else
#define ti814x_dsp_power_pm_ops NULL
#endif
static struct platform_driver ti814x_dsp_power_driver = {
        .probe          = ti814x_dsp_power_probe,
        .remove         = ti814x_dsp_power_remove,
        .driver         = {
                .name   = "ti814x_dsp_power",
                .owner  = THIS_MODULE,
                .pm     = ti814x_dsp_power_pm_ops,
        },
};

static int __init ti814x_dsp_power_init_driver(void)
{
        return platform_driver_probe(&ti814x_dsp_power_driver,
                                        ti814x_dsp_power_probe);
}
subsys_initcall(ti814x_dsp_power_init_driver);

static void __exit ti814x_dsp_power_exit_driver(void)
{
        platform_driver_unregister(&ti814x_dsp_power_driver);
}
module_exit(ti814x_dsp_power_exit_driver);
                                                                     

References

• Refer to the attached patch for complete code File:0001-ti814x-PM-driver-for-DSP-supporting-suspend-resume.patch.gz.
• I2C driver is a good reference for implementing suspend functionality.Refer to the file : <kernel_source>drivers/i2c/busses/i2c-davinci.c.