Re: So, what's the status on the recent patches here?

Matthew Locke
2006-08-17T05:20:42Z

On Aug 15, 2006, at 12:04 PM, Dave Jones wrote:

> On Tue, Aug 15, 2006 at 01:35:15PM +0300, Amit Kucheria wrote:
>
>> Here is my shot at providing a 10,000ft view:
>>
>> a. For embedded platforms, cpufreq just does not cut it when
>> specifying
>> an operating point for the device. These platforms use tuples such as
>>
>> <voltage, pll_freq, core1_freq, core2_freq, interconnect_freq>
>>
>> to signify an 'operating point'. Note that core1 and core2 can be
>> totally different processors e.g. ARM and DSP. x86 platforms hide this
>> complexity behind ACPI.
>
> If there are dependancies inherently linking core1 and core2, cpufreq
> should already be programming both parts. For example, the SA1100
> driver programs both CPU and SDRAM controller. If there isn't any
> dependancy
> between them, I don't see the attraction of creating an artificial one
> in the way suggested for no real purpose.

Are you arguing against the operating point concept because it creates
an artificial dependency? I assume your definition of dependency means
a physical dependency.

The operating point represents both a physical and operational
dependency. It is a collection of parameters that can/will be adjusted
to reduce power consumption. However, adjusting these parameters can
have a severe impact to performance and operational state of the
system. The parameters can not be adjusted individually and still
achieve the goal of an operational and power efficient system. SoC's
have a fixed number of values in a fixed number of combinations that
keep the system operational and power efficient. Using power op, a
piece of controlling software can tell the system to go to specific
instance of the power parameters that provide the best combination of
power savings and performance/operational integrity according to the
current state of the system. This instance is represented by a string.

PowerOP is needed to do advanced power management on embedded mobile
devices.

>
> Things like voltage and frequency are closely tied together, so
> offering
> any means of controlling them independantly makes no sense afaics.

It's not about controlling parameters independently. We need to be
able to control them as described above.

>
>> b. The clocking and voltage dependency tree in embedded devices can be
>> summarised in the clock framework ( find ./arch -name clock* ) and
>> soon-to-be-available voltage framework. These is again done to large
>> extent by ACPI on x86.
>
> Of the 14 x86 cpufreq drivers, 3 of them _optionally_ use ACPI.
> powernow-k7 for example doesn't use it, and is possibly one of the most
> stable cpufreq drivers we've had in the tree (for x86 at least).
>
>> d. In the end, all this is leading to an interface for a user-space
>> policy manager that will control _system_ power state based on
>> constraints imposed by HW peripherals or on policies implemented by
>> device manufacturer/distro maintainer.
>
> How does that interface look from a userspace point of view ?
> Hopefully not anything like the tuple described above.
> Why would userspace ever care about "interconnect freq" ?
>
> Userspace cares about "save power" or "go fast".
> Historically, I wish we had never exposed frequencies, but instead
> a performance percentage, so that the various userspace tools
> didn't have to care about things like 'what frequencies are available'.
> Adding the same mistake for voltages doesn't strike me as a fantastic
> idea.

I'm not sure I follow your comments here. We are not making the same
mistake. In fact we are fixing it with PowerOP. The power parameters
are represented by a name and you create whatever name makes sense for
your system. In fact the names can all be the same for the various x86
platforms if you so desire. The abstraction allows userspace to use
the name and not know anything about the frequencies or voltages. As
Scott pointed out, some power managers will need to know lots of
architecture and board specific details to be able to reduce power
consumption and keep the system operational. The abstraction enables
this as well.

>
>> At this point, PowerOP is an optional component in mainline tree.
>> Cpufreq drivers can _choose_ to use it or not. But now embedded
>> platforms can do the PM dance in a consistent way.
>
> That's about the only part I really like so far. The option to opt-out
> where it makes absolutely no sense to pointlessly abstract stuff
> (which for x86 seems to be the case). For ARM, I'm going to leave
> Russell to comment/review.

I'm not following why you think PowerOP isn't needed for x86. It seems
to address the issues with cpufreq that you point out above. The
conclusion we reached at the PM summit was that cpufreq/PowerOP
integration was useful and desired.

If we need to, I'm happy to put the integration of cpufreq/PowerOP
aside and just work on getting PowerOP accepted.

>
> Dave
>
> --
> http://www.codemonkey.org.uk
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