Date:	Tue, 24 Jul 2007 10:43:20 +0200
From:	"Jerome Glisse" <j.glisse@...il.com>
To:	"david@...g.hm" <david@...g.hm>
Cc:	"Igor Stoppa" <igor.stoppa@...ia.com>,
	"ext linux-pm-bounces@...ts.linux-foundation.org" 
	<linux-pm-bounces@...ts.linux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-pm <linux-pm@...ts.linux-foundation.org>
Subject: Re: [linux-pm] Power Management framework proposal

On 7/23/07, david@...g.hm <david@...g.hm> wrote:
> On Mon, 23 Jul 2007, Igor Stoppa wrote:
> > again, HAL / OHM / Mobilin
>
> I was trying to define the lower level interfaces that these tools need.
> today they can only know what is possible by reading the source code for
> each driver and implementing the driver-specific interfaces nessasary to
> set things, I was proposing a common interface that tools like this could
> use instead of requiring all the driver-specific knowledge.
>
>
> in a nutshell (and I know this is probably not detailed to be acceptable)
>
> 1. the software needs to know what the interconnects and dependancies
>     between devices are (supposedly this is provided via sysfs)
>
> 2. the software needs to know what type of device this is (again,
>     supposedly this is provided via sysfs)
>
> 3. the software needs to know what modes exist for a driver/piece of
>     hardware. to make any decisions this infomation needs to provide some
>     information about the capability of the mode and the power consumed in
>     that mode. in addition there will need to be flags to indicate any
>     special restrictions of a mode
>
> 4. the software needs to know the cost of switching from any mode to any
>     other mode. since some transitions will interact with other devices
>     there will need to be flags to indicate such requirements for specific
>     transitions.
>
> 5. the software needs to be able to find out what mode a device is in.
>
> 6. the software needs to be able to tell the driver to switch to a
>     different mode (I think it would be a very good thing if going to a
>     particular mode was always the same command, no matter what mode it is
>     currently in)
>
> 7. the software needs to figure out the desire of the user.
>
> my proposal was addressing items #3-#6. it isn't trying to decide what to
> do, simply to allow the software that _is_ trying to decide what to do a
> way to find out what it can do.
>
> David Lang

I believe a central place where user can set/change hw state to save
power or to increase computational power is definitely a goal to pursue.
But i truly think that the OHM approach is the best one ie using plugins
so that one can make a plugin specific for each device. The point is that
i believe there is no way to do an abstract interface for this and trying to
do so will endup doing ugly code and any interface would fail to encompass
all possible tweak that might exist for all devices.

For instance on graphics card you could do the following (maybe more):
-change GPU clock
-change memory clock
-disable part of engine
-disable unit
i truly don't think you can make a common interface for all this, more
over there might be constraint on how you can change things (GPU &
memory clock might need to follow a given ratio). So you definitely
need knowledge in the user space program to handle this.

best,
Jerome Glisse
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