Re: [linux-pm] [PATCH 0/2] RFC: CPU frequency max as PM QoS param

[Kevin Hilman <khilman@ti.com>]

Hello,

Antti P Miettinen <amiettinen@nvidia.com> writes:

> This is a continuation to "RFC: CPU frequency min as PM QoS param"
> patchset. This patchset adds CPU frequency maximum as a PM QoS
> parameter and modifies CPU frequncy core to enforce the limit. CPU
> frequency ceiling can be used to improve the energy efficiency of
> workloads that would cause the cpufreq governors to enforce an
> unnecessarily high operating point. In other words, CPU frequency
> maximum can act as an energy efficiency level request.
>
> Tested on Dell E6420 with the ACPI cpufreq driver against Ubuntu
> 3.2. Patches are against linux-next, compile tested.

I know there were some earlier discussions about the usefulness of a max
frequency QoS parameter, so I wanted to throw in a reason to include max
as well as min frequency parameters.

IMO, having a max frequency QoS parameter would be very useful from a
thermal perspective.  

There are some ongoing projects in the PM working group at Linaro that
are exploring plugins to the thermal framework that implment a "cooling
device" by capping CPU frequency.  Having a QoS parameter do do this
would be the logical interface.

I also agree with some earlier requests that these should probably be
per-CPU instead of global.  That would make it simple to cap frequency
of one cluster while leaving another alone.

Kevin