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

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

[Patrick Mochel]

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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;

 /*
---
0.99.9.GIT

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

[Andrew Morton]

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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?

If so, what does the new `state' field tell us?


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

[Patrick Mochel]

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?

Thanks,


	Pat

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

[Andrew Morton]

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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?

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

[Patrick Mochel]

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On Fri, 17 Feb 2006, Andrew Morton wrote:

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

The number, name, and meaning of the power states differ on a bus-by-bus
basis. PCI has D0-D3, which are well defined by the PCI spec. ACPI defines
states of the same name, but less rigidly defined, for various devices.
I believe USB has only "on" and "off".

One generality in all buses that I've seen so far consider state '0' to be
on and functioning. The low-power states increment from there - the higher
the number, the less power is being consumed and the longer it takes to
bring the device back to a functioning state. Even buses with 2 states
fall into this category, since "on" maps to 0, and "off" maps to anything
non-zero.

To answer your question: yes, it would make sense to enumerate the number,
but only on a per-bus basis. Ideally, the bus would export the states that
it supports, either directly to userspace or via the driver core and the
per-device state file in sysfs.

But, we're not to that point yet. For now, the writer of the file needs to
know what range of values the bus and/or device is expecting. It would be
nice to have a silly little program that could make this easier on a
user..

> 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.

For PCI drivers at least, each driver's suspend function calls
pci_choose_state() to let the PCI core decide what the actual PCI power
state is that the device should enter. Before these patches, the PCI core
would always return PCI_D3hot on a suspend request. Now, it looks at
'state' and uses that as a hint - if it's set and within range, then it's
treated as a PCI D state; otherwise, the driver gets back PCI_D3hot.

> 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?

The kernel only chooses the state on a system suspend transition, and
that's exactly what happens - the driver maps a SUSPEND request,
regardless of what .state will be to the lowest power state it supports.

However, these patches are for device power transitions initated from
sysfs. With these, there is a user/utility/daemon on the other side that
knows what power states the device supports and when a good time to enter
them is. IOW, it's a policy decision that uses the sysfs interface (and
this plumbing) as the mechanism for implementing it.


	Pat

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

[Pavel Machek]

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Hi!


> 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;

We have had enough problems with u32s before... What about
char *, and pass real state names?
				Pavel
-- 
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Re: [linux-pm] [PATCH 2/5] [pm] Add state field to pm_message_t (to hold actual state device is in)

[Alan Stern]

On Sat, 18 Feb 2006, Pavel Machek wrote:

> Hi!
> 
> 
> > 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;
> 
> We have had enough problems with u32s before... What about
> char *, and pass real state names?

Hear, hear!  Exactly what I was going to say.  Now that you want to add a 
field describing an actual device state, make it meaningful.

BTW, you might want to look here:

http://lists.osdl.org/pipermail/linux-pm/2005-September/001422.html

for an example of an early implementation of just this idea.  (I still 
have the original patch file if you'd prefer to see it in plain text, 
non-html format.)

Alan Stern

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

[Patrick Mochel]

On Sat, 18 Feb 2006, Pavel Machek wrote:

> Hi!
>
>
> > 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;
>
> We have had enough problems with u32s before... What about
> char *, and pass real state names?

I certainly agree that is better in an ideal implementation. But, the
intent of the patches was not to fix the infrastructure; it was to fix the
interface to be compatible with previous behavior (while accounting for
changes made in the area that happened in the process of breaking it).

Thanks,


	Pat