Date:	Tue, 24 Jul 2007 17:40:29 -0700 (PDT)
From:	david@...g.hm
To:	Jerome Glisse <j.glisse@...il.com>
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 Wed, 25 Jul 2007, Jerome Glisse wrote:

> On 7/24/07, david@...g.hm <david@...g.hm> wrote:
>>  On Tue, 24 Jul 2007, Jerome Glisse wrote:
>> 
>> >  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.
>>
>>  will each plugin have it's own interface? or will you have one interface
>>  to access the plugins and then the plugins do things behind the scenes?
>>
>>  I'll bet that the API for the plugins is common, and if so then it could
>>  be similar to the API that I suggested.
>
> I take here ohm as a reference (this come from my limited understanding of
> this daemon so there might be inaccuracy) driver export through HAL
> there power management tunning capacity, Then an ohm plugin would use
> HAL to give a higher
> view of this capacity and also manage policy, preference, permission, ...
>
> Last consumer in power management food chain would be an user interface which
> will communicate with ohm (and with all ohm plugin) so desktop writter 
> (gnome,
> kde, ...) can write some kind of power management center where each ohm 
> plugin
> can have its own panel. So in the end the user got one place to do all its
> power management which is the goal i think you are trying to aim.

no. I am talking about the interface to the drivers that things like HAL 
would use

>> >  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.
>>
>>  sure you can, just enumerate all the options the driver writer wants to
>>  offer as options. yes this could be a lengthy list, so what?
>> 
>
> My point was that your interface by trying to fit square pegs into round hole
> will fail to expose all subtility of each device which might in the end bring
> to wrong power management decision. So i believe we can't sum up
> power management to list of mode whose attribute are power consumption
> & capacity.

it's possible (which is part of the reason I started the thread), but so 
far there hasn't been anything identified that is a really bad fit.

> And there is no way to design an abstraction given that all hw we will have
> to deal with are too much different and do not follow any standard things
> (beside ACPI there is other way to save power brightness, gpu/memory
> clock, pll, ...) so i don't see how one might give a common view of things
> which are fundamentally different in how they affect consumption (same end
> result with many different paths leading to it).

so you are saying that the power management software must know the details 
of each and every driver, and if you add a new driver you must change the 
power management software before it can do anything (including allowing 
manual control of the modes)

seems to me I heard similar arguments several years ago about the CPU 
speed settings, it turns out that the cpufreq interface works really well 
for them and the software that's controlling things no longer needs to 
know the details of every CPU.

why did it work there but can't work anywhere else?

David Lang
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