Re: [PATCH 2/5] [pm] Add state field to pm_message_t (to hold actual state device is in)

[Andrew Morton <akpm@osdl.org>]

Patrick Mochel <mochel@digitalimplant.org> wrote:
>
> 
> On Fri, 17 Feb 2006, Andrew Morton wrote:
> 
> > Patrick Mochel <mochel@digitalimplant.org> wrote:
> > >
> > >
> > > Signed-off-by: Patrick Mochel <mochel@linux.intel.com>
> > >
> > > ---
> > >
> > >  include/linux/pm.h |    1 +
> > >  1 files changed, 1 insertions(+), 0 deletions(-)
> > >
> > > applies-to: 1ac50ba99ca37c65bdf3643c4056c246e401c18a
> > > 63b8e7f0896ce93834ac60c15df954b1e6d45e56
> > > diff --git a/include/linux/pm.h b/include/linux/pm.h
> > > index 5be87ba..a7324ea 100644
> > > --- a/include/linux/pm.h
> > > +++ b/include/linux/pm.h
> > > @@ -140,6 +140,7 @@ struct device;
> > >
> > >  typedef struct pm_message {
> > >  	int event;
> > > +	u32 state;
> > >  } pm_message_t;
> >
> > I don't quite understand.  This is a message which is sent to a driver
> > saying "go into this state", isn't it?
> 
> .event is one of these:
> 
> #define PM_EVENT_ON 0
> #define PM_EVENT_FREEZE 1
> #define PM_EVENT_SUSPEND 2
> 
> Which only says what to do - turn on or off (AFAICT, FREEZE and SUSPEND
> are synonymous). They are designed to work when the entire system is being
> suspended or resumed, since every device is most likely going into its
> lowest power state.
> 
> However, some devices support >1 low-power state which can be used to save
> power while the system is otherwise fully up and running. Devices that are
> not being used will most likely use the lowest power state, but devices
> that are idle (and that support >1 low power state) can use the other
> states even when idle.
> 
> In reality, there aren't many devices or drivers that support >1 low-power
> state. But, there are some and likely to be more. And, the interface had
> always supported it until some time in the 2.6.16 development cycle.
> 
> Does that help?
> 

It does, thanks.

Would it make sense to enumerate these low-power states, rather than a bare
u32?

How, from the above message, is the driver to know that it's being asked
for a low-power state rather than an `off' state?   Via `state' I guess.

I can see that the kernel would have trouble asking a device to go into a
particular low-power state because of the variation in capabilities between
devices.  Perhaps the kernel should send the driver some higher-level piece
of information informing it what's going on, let the driver choose an
appropriate power state?