power management experiences with joyride-1572

[david at lang.hm]

On Fri, 25 Jan 2008, Richard A. Smith wrote:

> Chris Ball wrote:
>
>>   >> Can I wake up 10 seconds from now?  Is there a timer in any of the
>>   >> hardware that is left running?
>>
>>   > Yes, but the software does not support this yet.  See bug #4606:
>>   > http://dev.laptop.org/ticket/4606
>>
>> We don't *use* the southbridge RTC wakeup, but it's not strictly true
>> that we don't support it.  You can set your own wakeups easily:
>>
>>    # rtcwake -s 120
>>    <after 30s, the laptop should suspend due to idleness>
>>    <after another 90s, the laptop should wake itself>
>>
>> rtcwake is in the OLPC build already.
>>
>> - Chris.
>
> RTC wakeups have a chance of hitting #1835 because the EC cannot prevent
> the short cycle of the control line to the voltage regulator so we don't
> use them.  Andres has discussed prohibiting RTC wakeups in kernel space
> but I suggested we put that in the "don't do that" category since he has
> higher priority stuff to worry about.
>
> The safe way to schedule a future wake up will be to use a EC timer.
>
> The framework for this exists but I don't have the kernel facing EC
> command plumbed yet.  This timer will allow you to schedule a wakeup
> with about 10ms resolution up to 24 days in the future.

what is the shortest time that a sleep (followed by a wakeup from the EC 
timer) can be programmed?

would it make sense to hack the kernel so that if all timers are set to 
fire more than this far in the future it wakes a user task that can decide 
to sleep

this sort of sleep should not dim the backlight, when the wakeup gets fast 
enough this should be completely transparent to the user.

if the wakeup was faster we could dispense with the user 
process, but aa 1-2 sec we need a hook to decide if it's busy or not

if a less power efficiant sleep is available that wakes up faster it 
probably makes sense to use it for this (with a userspace process checking 
for idle at a longer time interval to move things to a deeper sleep if 
desired. personally I wouldn't want to machine to go to a deeper sleep 
unless I hit the power button, I don't even want closing the lid to do 
more then turn off the backlight, I want software to keep running)

David Lang