What determines power management policies in Android?

Question

I am just approaching to the Smart Phone and Android world and have a couple of doubts:

Which modules (and SW or FW?) decide the policies with which power is given to any component of the Smart Phone that requires it?

furthermore

Are power management modules on Smart Phones implemented in the Android OS or in some special dedicated firmware?

Obviously power management must work also if the OS is not yet loaded, but who is responsible of it?

And to what extent can I modify the power management with an application running inside of the OS?

(I hope these questions are not Off-Topic, in case can you please direct me to the correct Stack-Exchange site?)

Answer 1

Power management is determined by governors which set the CPU clock speed according to use. Each clock speed operates at a certain voltage and a lower voltage means less power. If you have root access you can change the governor and the allowed CPU clock speeds. Also there is sleep state into which the CPU will enter when the screen is off and nothing is working in the background. There are probably battery-charging functions that can't be messed with, but everything else can be changed even the cpu and dsp voltages.

Answer 2

Browsing around forums and sites I found out the following answers to my doubts. I'm not completely satisfied but it helped understand more.

Every device's power management depends on some suspend/resume policies that are implemented in the firmware controlling that particular device.

This is very device dependent: how and when each device turns off (suspends itself) and wakes up (resumes) depends on the HW specifications, you have to read the datasheets to understand which particular registries must be manipulated to suspend/resume it.

You can control this stuff via software in the drivers of the devices, inside of the kernel's source code, by accessing some funcions in the form <something>_suspend and <something>_resume.

E.g. From the source code of the kernel of the emulator "goldfish":

in file drivers/video/goldfishfb.c (the driver responsible for the frame buffer)

#ifdef CONFIG_ANDROID_POWER
static void goldfish_fb_early_suspend(android_early_suspend_t *h)
{
    struct goldfish_fb *fb = container_of(h, struct goldfish_fb, early_suspend);
    writel(1, fb->reg_base + FB_SET_BLANK);
}

static void goldfish_fb_late_resume(android_early_suspend_t *h)
{
    struct goldfish_fb *fb = container_of(h, struct goldfish_fb, early_suspend);
    writel(0, fb->reg_base + FB_SET_BLANK);
}
#endif

So the early_suspend writes a 1 in the register FB_SET_BLANK to turn off the screen, or a 0 to turn it back on.

So it seems to me that a process in the application execution environment needs to access and corrupt the devices drivers to manipulate maliciously power management of a device.