Re: [linux-pm] [PATCH] Custom power states for non-ACPI systems

[David Brownell <david-b@pacbell.net>]

On Thursday 03 March 2005 6:55 am, Pavel Machek wrote:
> Hi!
> 
> > In most of the cases I'm thinking of, it wouldn't be a user 
> > requesting a state but rather software (say, a cell phone 
> > progressively entering lower power states due to inactivity).  I 
> > haven't noticed a platform with more than 3 low-power modes so far, 
> 
> Are not your power states more like cpu power states?

For System-on-Chip devices it can be a fine line.  Maybe six
of the most important devices (including CPU) are in a low
power state, but some others are still active.


> These are expected to be system states, and sleeping system
> does not take calls, etc...

Pavel, remember that great big "wakeup" shaped hole in the
current PM framework... ?  Even ACPI sleep states support
wakeup mechanisms, although not well under Linux (yet).

One way a sleeping system could take a call is if some
external chip raised a wakeup-enabled IRQ to wake up the
system.  And if going from deep sleep to normal operational
state has a low cost, why shouldn't the system routinely
enter deep sleep instead of going to CPU idle state?

It's certainly the case that connecting the USB device
to a host can un-gate that peripheral's 48 MHz clock and
wake the system up from deep sleep.

- Dave