Re: Runtime device power management in userspace

[Pavel Machek <pavel@suse.cz>]

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On Po 26-12-05 12:58:02, Patrick Mochel wrote:
> On Sat, 24 Dec 2005, Holger Macht wrote:
> > On Fr 23. Dez - 07:12:35, Patrick Mochel wrote:
> 
> > > How do you determine the idleness of a device? Or, is it based purely on
> > > user direction?
> >
> > That's one of the main problems. For some devices, for instance network
> > cards, we can query the NetworkManager (as far as available) via DBus if
> > the device is currently in use or not. But we don't have that possiblity
> > for all devices. The even more problematic point is the other way
> > round. How to figure out if we have to resume a device as soon as another
> > application wants to access it? I have currently no clue how to do this.
> 
> That is definitely a devlish detail (or rather, several of them). In
> general, you want a device to resume as soon as it's needed by an
> application. There's no way of knowing this from a 3rd party app, and
> there is no way of changing every application to first power up a device.
> So, it must happen in the kernel.

While we want to implement that someday, we should fix the easy stuff. 

> Where in the kernel is dependent on the type (class) of device. For some,
> it will be on open(2), others will be connect(2), and others will be the
> set of read(2)/write(2)/select(2)/poll(2). To this effect, I think that
> the individual classes need to be changed to do e.g.

And for some, it is not possible to keep them functional and powered
down. Take ethernet interface: if you power it down, you'll not see
incoming packets.

> Implementing all of this will take time and collaboration with the
> different device classes, but I think that this is a direction which most
> distros would like to head. Is that even remotely correct?

Would like to head is correct... but that's year+ away. For now, we'd
like to get the simpler "power this piece of hardware down, return
errors to userspace", first.

Example usage is... you are traveling by train, so you know you are
not using ethernet and wifi... so you just force it down. 
								Pavel
-- 
Thanks, Sharp!

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