RE: [iPAQ] blinking square

Nicolas Pitre writes:
> > 1) active devices turned on, full speed CPU clock
> > 2) StandbyH: processor clock slowed, LCD off
> > 3) StandbyL: all peripherals turned off
> > 4) Suspend: active DRAM flushed to flash or other storage,
> > CPU and DRAM halted, all peripherals off
> > - requires firmware support, lack of real-time-clock an issue
> > 5) Off: kernel must reboot
> >
> > I think we can do (1) and (5) now. I would like us to implement (2) and
> > (3). I'm not sure (4) is worth the effort -- we'll have to measure power
> > consumption in the standby states. WinCE gets at least a week of battery
> > life without implementing 4.
>
> The SA1110 sleep mode (for 4) doesn't need DRAM backup.

And further, it sounds like #5 can't be implemented in software, since
the DRAM preserves itself. So the interesting power modes are:

1) All devices currently in use powered on, all devices not in use
powered off. CPU clock running only as fast as it needs to, to keep
the load average down.
2) All devices powered off, CPU halted, timer interrupt waking up the
CPU as needed.

The i386 Linux halts the processor when it has nothing better to do.
Does the ARM Linux do the same thing? If so, then so as long as no
programs are running, state #2 would JUST shut down always-open
devices like the digitizer, buttons, LCD, and frontlight.

The interesting problem is how to power devices down even though
they're held open by the program. Also, some programs will probably
want to control their own power. For example, a GPS program might
want to keep the GPS interface powered up so it could continue to
track movement even though it's not displaying it on the screen.

You could send a suspend signal to all programs to tell them to
suspend themselves. This would work for programs that don't hold
devices open, or that understand the meaning of the signal, trap it,
and close their devices. But how do you deal with other programs?
Perhaps the answer is "just deal".

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