Subject: Re: power management
To: Garrett D'Amore <>
From: Jachym Holecek <>
List: tech-kern
Date: 06/24/2006 00:29:42
Hello Garrett,

# Garrett D'Amore 2006-06-22:
> We also need a way for a device driver that wants to access a device to
> indicate so to the framework.  Solaris has pm-busy-component and
> pm-raise-power, etc.  Take a close look at it.

Can you provide links to documentation/code? I didn't bookmark the
manpages and can't find them now.

> For example, if I'm about to write data to disk, I mark it
> pm-busy-component, and then pm-raise-power.  This tells the power
> management framework in kernel that the device needs to be powered up,
> and the framework powers up the device if it is not already (and maybe,
> for example, busses or controllers to which the device is attached!) 

The "mark device as needed/about-to-be-busy" part sounds reasonable,
but I think "raise power" should be implemented separately. Just
indicate you want the device busy, queue the request (at least ifnet
drivers are already separated from the network stack by buffers),
and if someone decides you should have the device powered up, it will
happen. It may also be reasonable to deny the request, in which case
you'd start getting/returning errors as soon as buffers fill up/you
make calls to powered-down device (you can check for yourself if
device is operational).

> This can result in a recursive callback into a different power
> management entry in the device driver, btw.

I'd prefer to avoid recursion, if reasonable possible (see my note
about kernel thread handling requests that can sleep).

> Then when the write is done, the driver does pm-idle-component to tell
> the framework that the device is no longer in use.

Shouldn't higher layer really do this, instead of device itself?

> As far as "power states", I'd look closely at the PCI and USB power
> management specs to see what they offer.  It would be nice to have
> support for fully using the power features supplied by the most common
> busses.

True. The PCI power management specification seems to be getting many
concepts right, hope I can digest it over the weekend. I particularly
like the idea of having pm-state/request semantics precised by the
kind of device (network/disk/...) and having some kind of type-specific

Anyway, this & the above already cover point (3). For now, it's fine
with me to keep powerhook request semantics, so that powerhooks can
go away early.

	-- Jachym